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- 44 - THE (ECOLOGY OF PORIFERA, AND POSSIBILITIES OF DEDUCTIONS AS 70 THE PALE03COLOGY OF SPONGS3 FROM THEIR FOSSIIS By M. W. de Laubenfels (Pasadeaa, California) At the present time sponges in their morphology and in. their dis- tribution show reactions to the effects of environment. Even with our pres- ent limited knowledge some things may be said as to the nature of the environ- mental effecta in question. Consequently, knowing only the environmental effects, one may deduce a little about the environment that was the causa- tive agent. Furthermore, by assuming that the procedure in bygone ages was much the same as that which prevails now, one may draw some conclusions from fossils regarding the environment which was fonaerly present. Environment may be considered under the headings of physical, chem- ical, and biological. Sponges grow only in the water. They may survive in spite of emer- gence, as for instance at low tide, but they cannot carry on. out of water. Ifhen dead they may be washed up in the wrack. They may even conceivably be blown inland. Direct observations of such occurrences are quite rare, how- ever. Occurrence of fossil sponges, therefore, constitutes almost perfect proof that during their life, the horizon in which they occur had been sub- merged. Among the more important physical items in the environment of the sponge is the matter of currents of water, their presence, absence, or de- gree; and also the nature of that which is transported ty them. It is noted, for instance, among the collections made by the "Chal- lenger" and other dredging operations, that the stations which yielded no sponges were consistently far from land, furthermore that no stations which were a great distance from the nearest island or continent yielded many sponges. One can only speculate as to what may be the nature of the mater- ial required, but certainly something or other must be brought by currents from land, otherwise sponges cannot exist. It may possibly be concluded from this that wherever fossil sponges occur there was probably a large island or continent within a few hundred kilometers. Another matter bears on the question of whatever it may be that the currents bring to the sponge. We have numerous observations of sponges growing in aquaria but having no other nourishment than what they may ob- tain from the current of water that is being introduced into said aquaria. The fact that sponges regularly die when kept in stagnant water may indi- cate that in such the oxygen supply becomes exhausted and that they perish from suffocation, but the fact that they can live in aquaria where they are not fed otherwise than by that which is brought in by the current is significant of a nutritive as well as respiratory importance.
- 45 - One of the most important effects of the current on the sponge is in connection with removal of waste products from it. In nearly every case where sponges are found growing in calm water, a high collar or "chimney" is found erected about the exhalont aperture or oscule. It is fairly evident that this has the value of separating the exhalent current from the inhal- ent, thus reducing the possibility that the material just ejected from the sponge might be at once re-admitted to its pores. The occurrence in a fossil sponge of such oscular chimneys indicates a high probability that during the life of the Individual it was surrounded by relatively stagnant water. As noted below, the deposition of silt may also account for oscular chimneys. Since such deposition is frequently also associated with relatively calm waters something can be said as to the lack of current indicated by such morphology, Sponges that live amid rapid currents always lack pronounced surface elevations. This is a relative matter. The stronger the current, the more nearly level will be the surface of the sponge. Where the currents are very violent indeed, the rule (to which there are very few exceptions) is that the sponges take what is called the encrusting form, extending laterally indef- initely but never protruding far above the substratum. If they do grow per- pendicularly it is in a hemispherical or low, rounded manner that is very characteristic. Each of these simple architectures may also occur in calm water. It would appear quite justifiable, however, to argue from the ap- pearance of anything other than encrusting or hemispherical form in a fossil sponge that there were no considerable currents about it during its lifetime. The preceding paragraph refers chiefly to conditions wherein current frequently changes direction so that the sponge is affected from nearly all sides. Wherever the current is confined to a channel so that it affects the sponge principally from just one side, or from two opposite sides, it is the rule (although there are. quite a few exceptions) that the sponge fauna ex- hibits significant shapes. Sponges that in. calm water grow up in a bush- like form, when growing in a current'tend to become flabellate or fan-shaped. Another shape common in currents is the lamellate form, frequently having all the oscules on just one side and the majority (though not quite all) of the pores on the opposite side. In this manner the current may be helpful in respect to the circulation of the sponge, the oscules or exhalent open- ings being on the downstream side and the inhalent openings on the upstream side. It is by no means certain that the role of environment in cases such as this is as simple as it seems to be at first. It may be mentioned here that specimens of sponge are found having this lamellate form and living in a current, but that it is seldom (if ever) found that similar specimens near- by, out of reach of the current, have another shape. Instead, in such ad- jacent localities,. occur what seem to be clearly different species. The as- sumption may therefore be made that lamellate sponges succeed in living in a current of comparatively uniform direction, whereas sponges of some other shapes do not thrive there. The Hyalospongiae (or Hexactinellidae) afford an interesting field for speculation. with reference to the possible effects of one-way currents' upon. the morphology of the sponge. Many varieties of this class grow in an erect, hollow, cylindrical form, with their pores dis- tributed about the convex exterior while a large internal cloaca leads to an
- 46 - apical exhalent aperture. The â¢whole structure is so openwork that currents may sweep through it. Now it frequently occurs that upon collection. such sponges (Euplectella is an example) are found to have a sharp bend in the cylindrical form. This is so clearly ingrown that it must have been long present. Spongelogists therefore conclude that as it grew the spongelean- ed downstream with respect to a permanent or semi-permanent current which could thus pour in at the- inhalent openings and sweep out at the exhalent, in this way reinforcing the sponge's inadequate hydraulic efficiency. It is impossible to verify this conclusion by means of diving apparatus because of the great depth at rtrich such species grow, none the less, the theory is so plausible as to command respect. Such morphology in a fossil sponge very probably indicates that when still alive the animal in question was bathed with current of uniform direction, and at a depth to be measured at least in terms of hundreds of meters. Many calcareous sponges or Calcis- pongiae also have a cylindrical form with the inhalent openings on' the out- side and the exhalent openings apical. It may be noted in these which oc- cur at such shallow depths that they can be observed directly, that they are so pivoted upon their bases that regardless of the direction of the current, the oscule is always downstream. This lends additional support to the credibility of the above mentioned theory. It is rather easy to discriminate between the morphology permitting such pivoting, and that which instead implies rigidly sessile fixation. Another factor in connection with currents is that of the deposi- tion of silt. To begin with, it may be stated that where very fine silt is settling, no sponges can live, apparently because their inhalent open- ings become clogged. This is necessarily a somewhat inexact statement be- cause there is no sharp dividing line; the finer the particles in question, the fewer are the sponges that occur. An occasional member of this phylum is found living, however, even where the fineness is such that the condi- tions might appropriately be referred to as muddy. The mere occurrence of a fossil sponge does not prove that there was no silt being deposited during its lifetime at the place where it grew, but one could say roughly that the more abundant the sponges are in any given geologic horizon, that the less likelihood therefore exists that said horizon was muddy at the time of growth of the sponges. Where coarse sand is being deposited sponges frequently flourish, but invariably with a certain morphology that is quite distinctive. In them, as also in sponges that grow in very calm water, (but even more reg- ularly) there are thrown up collars or "chimneys" from the oscules, fre- quently to a height of many centimeters. The pores are of course sometimes covered up by the sand, but where this is coarse enough, the currents may still enter the sponge through the interstices between the particles. The ooze which forms very slowly on the bottom of the ocean affords a very different type of problem from the standpoint of the sponge. One may judge that there is little difficulty in the way of clogging the pores by rapid accimulation of this ooze, but that there is a problem of support lest the sponge sink into it. The rule is that sponges occurring in this
- 47 - type of locality have what are known as "root tufts" of long spicules in dense masses which run out into and down smonc the surrounding masses of ooze, thereby holding the functional portion of the sponge on the surface of it. Wherever a fossil sponge shows clear cut evidence of such root.tufts the assumption may fee made with great confidence that it was growing on ooze, hence probably at a considerable depth, and not upon any rapidly forming silt or mud. Another item that needs to be considered in connection with currents, although having only a slight paleontological connection, is the.matter of larval distribution. There are indications that sponges spread from one place to another more slowly than members of most other phyla. In Puget Sound the conditions for sponge life are very good, which is proven by the fact that enormous numbers of individuals live there; on the other hand, the number of different species is very small. The species which do occur are confined largely to those which might be carried in on the- backs of mollusks, hermit crabs, or on ship bottoms. The indications are that during the interval which has elapsed since the region that is now Puget Sound sank below the sea level, only a score or so of exceptionally migratory species has had time to work in. It is tempt- ing therefore to assume if in an horizon there are found numerous different sorts of fossil sponges, that the locality in question had been continuous- ly favorable for the growth of sponges for many thousands of years at the time 1*1 en those fossilized were still living. In further confirmation of this general idea one may note that the sponges at the Pacific side of the Isthmus of Panama, sbow a most remarkable resemblance to those occurring on the Atlantic or Caribbean side. Among the Invertebrates in general such a resemblance does not exist, and indeed is not to be expected, because in that region there has probably been no connection between the two oceans for a very long time, oven geologically speaking. The inference is that once sponges are established in a given locality they remain for many millennia. There is another matter that has very little to do with currents, but has much to do with the shape of the sponge,. and needs therefore to be kept in mind in deductions (as to presence or absence of currents) based on sponge morphology. When sponges are forced to grow on a small substratum they grow principally perpendicularly, at the expense of lateral growth. This may be because there is only a small area available that is stable, that is to say, not shifting, inasmuch. as sponges require a relatively fixed point of attachment. Again this stato may indicate that other sessile or- ganisms (such as other sponges) grew abundantly in the vicinity, and crowded in upon the specimen in question. The fact that there are these two inter- pretations would make it difficult to come to conclusions as to environment, and an additional even. greater difficulty is brought about by the fact that there are many species of sponges that assume the erect cylindrical form even when growing in completely uncrovfded quarters. Ono may say just this much, that the appearance of fossil sponge showing a certain amount of point of attachment, proves that at least that much of its environment was fixed and stable. The individual sponge is very sensitive as to temperature. It has,
- 48 - one may say, no margin to spare in- this regard. We find it capable of sur- viving at the maximum and minimum temperatures of the place at which it is found, but perishing at extremes colder or warmer than this range. This delicacy does not extend to species as a rule, if ever. A striking case is Aplysilla glacialis, which occurs in the Arctic, Antartic, in Intermediate regions on the Pacific coast of the United States, and also in Panama, near the equator. There is little if any evidence of ecological modifications evidenced by the specimens form these different localities. This partic- ular species is especially noteworthy in this regard as it is a sponge hav- ing no proper mineral skeleton whatever. It is frequently stated in the earlier works on sponges that such species are characteristic of the trop- ics or warmer waters, and not at all of the colder portions of the earth. Further investigation indicates that a more nearly correct statement would be as follows: in warm waters those sponges lacking mineral skeletons are more numerous in comparison to the others, whereas in colder waters the latter are more numerous in comparison to the former. Either sort may oc- cur in either place arid where so occurring shows apparently little if any modification as a result of its environment. One may conclude that it is impossible to decide from any single fossil whether or not the water in which it grew was relatively cold or warm (regarding equatorial regions as warmer and polar regions as colder). A fossil record might have just this much value in connection with deductions as to temperature, that if very numerous sponges be in evidence, and the majority appear to have been kera- tose, there would be sane cause for suzmise that the waters in question might have been tropical or sub-tropical. This is of very little help, how- ever, in view of the fact that the sponges which lack proper spicules are seldom fossilized because of their lack of hard parts. The question of light as a factor affecting sponges is of some sig- nificance from the standpoint of paleontology. We have, it is true, the situation exanplified by Acarnus erithacus of the California coast. It oc- curs nonnally where the illumination is very good, that is to say, in in- tertidal waters. One specimen, however, has been dredged from a short distance off shore where the depth was so great (700 meters) that it may be assumed no light was present. The difference between the specimens from the two localities were quite noticeable but involved only the matter of color at the time of collection; this would not be evident in fossilized specimens. Whereas we may conclude that migration away from light does not bring about morphological alteration, it nevertheless ronains true that some sorts of sponge are more likely than others so to migrate. Little may be deduced as to light from the presence or absence of Porifera (such as the above mentioned Acarnus) of the class Demospongiae. The class Cal- cispongiae (or Calaraea) occurs in relatively shallow water, however, that is to say at depths of less than one hundred meters. It may be further- more stated with confidence that nearly all of the members of this group grow where there is illumination. The occurrence of a fossil Calcisponge may be taken as a moderately 0ood indication that the immediate surroundings ir which it grew were illuminated. In contrast to this, the entire class Hyalospongiae (or Hoxactinellida) grows exclusively in the darkness. Prac- tically no specimens of this class are on record as having been collected
- 49 - at less than one hundred meters, and in the very few cases where the depth was shallower the conditions were such that one inust conclude that little or no light reached the sponge where it grew. Even if a few exceptions should still turn up in regard to this matter one may nevertheless state with considerable confidence that the occurrence of fossil hexactinellid sponges indicates that their horizon had been characterized by darkness. With regard to osmotic pressure or tonicity the individual sponge is somewhat sensitive. An increase above that which is normally true of the sponge's environment brings about immediate damage, The effect of a hy- potonic solution is not so quickly evident, there is a lag, so that re- duction in osmotic pressure does not immediately begin to show its ill effects, but if it is carried to any considerable extent the sponge ie certainly damaged. The extent in question varies considerably with the species studied. The net result of this is that a marine sponge is not ser- iously injured (even though near the surface) if a severe spell of rain con- siderably dilutes the surface of the ocean with fresh water for a snort period of time. From the standpoint of geologists it is very noteworthy, however, that an entirely different fauna of Porifera occurs in fresh water than in the oceans. The occurrence as fossils of the type of sponge at present known to occur in fresh water constitutes a fairly good proof that the im- mediate surroundings at the time of fossilization were not marine, and vice versa. There is some warning that needs to be given in this connection however, in that marine species have been found in the very large fresh water lake Baikal in eastern Siberia. In brackish water no sponges seem to thrive and not many species of any sort occur» but those which arc found in such localities are divided a- mong the-marine and fresh water sponges, the tiro sorts occurring side by side. The relationship between bathymetric pressure and the occurrence or absence of sponge types and any effect on the morphology of individual sponges cannot be considered apart from the probably more important question of light. As already noted there are distinct orders or classes of sponges which seem to be found only in the light (Calcispongiae), or only in the darkness (Hyalospongiae). This is correlated with the occurrence of the latter class in relatively deep water. The class Demospongiae is well represented where any sponges at all occur, in shallow or very deep water. A second type of ecological reaction is to the chemical environment instead of to the physical. This would concern substances perhaps to be listed as poisons, which by their presence might be inimical to sponge growth, and also other substances which are necessary as accretions or auxiliary sub- stances if the sponge is to maintain its existence. With respect to the first item; needless to say there are numerous toxic substances Trtiich affect sponges adversely, but there does not seem to be good evidence that any of the minerals commonly occurring in the sea or
- 50 - In fresh water ere seriously injurious to Porifera. Rare occurrences can scarcely be deduced from the evidence of fossil sponge morphology. From the standpoint of substances required positively by the sponges, oxygen may be considered first. There is, as might be expected, rather good evidence as to the need for free oxygen in solution on the part of members of this phylum. In other words there is little or nothing to in- dicate that any of the manbers of this group are directly anaerobic. On the other hand it should be noticed that very numerous species of sponges commonly live in symbiosis with plant cells, and the latter possess appro- priate pigment ty means of which photosynthesis is carried on, carbon di- oxide consumed, and oxygen released. It is therefore conceivable that dur- ing the day such sponges might live in the absence of free oxygen, were suf- ficient carbon dioxide present. This would not carry over into the night, and since there seems to be no method of storing up oxygen from the day's production, we may take for granted that the presence of sponge life in- dicates that at the time the specimens were growing oxygen was available. There is much work bearing upon the requirement of sponges with re- spect to hydrogen ion concentration and the presence of metallic and non- metallic ions in general. Aside from calcium carbonate and silicon diox- ide, there is little of this material of interest for the geologist. Those salts most important to the existence of sponges are so widespread that one can do little reasoning from the occurrence in fossils to what salts were or were not present in their environment during life, other than to assume that they must have been present much as in the ocean and lakes and streayns of today. Even the questions of silica and calcium carbonate are of little help from the ecological standpoint in connection with paleontology. Sponges can satisfy their needs for either in the presence of amazingly small quan- tities. Very few places in the world lack the minimum necessary to the Porifera that might live there if other factors permitted. It is, for in- stance, noteworthy that siliceous sponges thrive in the vicinity of coral reefs where there is a considerable lack of siliceous material on the bot- tom or in the surrounding vicinity, and conversely that calcareous sponges thrive especially well on granite shores where there is no limestone very near. Since (for otherrreasons noted above) it is sometimes significant to notice whether or not the fossil sponge considered was in its lifetime calcareous or siliceous, it must be kept in mind, as is well known, that these two substances occasionally replace each other during fossilization. Also it may be remarked that either of the two roay be replaced during fos- silization by pyrite. In connection with fossil sponges and deductions that may or may not be made from them it should be noted that the skeletal material, as we find it in existing sponges, is usually in units called spicules, which (aside from perishable organic stuff) are entirely separate from each other. When such sponges die, the skeletal material usually soon breaks up com- pletely. Needless to say they are not readily susceptible to fossilization. On the other hand in each of the three main classes of sponges we find spe- cial instances as follows: anong the Calcispongiae we have the so-called Pharetrones; among the Hyalospongiae we have the so-called dictionine sponges; and among the Demospongiae we have those referred to as the
- 51 - Lithistida. In each of these three special cases the spicules during the lifetime of the sponge become interlocking or even actually cemented to- gether. These therefore are especially suitable for fossilization and loom up large in the paleontological record. This does not mean that they were formerly much more common than they are at present, but merely that the oth- er kinds are less susceptible to preservation in strata. It has been very generally assumed, particularly in works on paleontology, that each of these three constituted a particular sub-division within the class in question. This is not at all to be taken for granted, and in fact the current opinion is tending more and more to the belief that the lithistid modification may happen to sponges in almost any family of the Demospongiae; the dictyonine modification to species in almost any of the orders of Hexactinellid sponges, and the Pharetrone modification to at least several sorts of the Calcarea. One of the most interesting chemical ecological situations is that pertaining to food. The sponges can occur only where suitable materials are present and if we knew what sponges eat we might be able to carry on very interesting deductions from the occurrence of certain types in the fossil record. There is such widespread assumption that much is known about the nu- trition- of sponges that some remarks on this question may be in order at the present time. Many, perhaps the most, of the experiments bearing on the so-called nutrition of sponges have involved introducing carmine powder, indigo, India ink, and other such readily observed minute particles of material in- to the organism in question. As a result of these experimoits it is well known what occurs to such particles. They are taken in by certain cells, sometimes passed on to certain other cells, and eventually forced on out. It may at first appear to be rank heresy to criticize, but actually it is only fair to state that these experiments are nearly valueless and prove nothing as to the nutrition of the sponge. One might just as well blow bits of charcoal dust into the lungs of a human being and record the captur- ing of this dust by the mucous in the alveoli of the lungs, its passage in the ciliated epithelium upward to the nostrils, and its final discharge from the nose. Clearly the sponge could not go on being clogged up indef- initely with carmine particles and must push them on out someway or other, and equally clearly it is evident that neither carmine, indigo, nor India ink constitutes a natural diet for the Porifera. ⢠But do we find sponges ingesting such things as diatoms, or proto- zoans, which might conceivably be natural items of food? There are aany ob- servations of the ingestion of flagellates, but also other observations that these may exist as still living symbionts after their entry into the sponge cell. There are a few authentic records of the ingestion of diatoms and bits of plant material in the same way that carmine particles are taken in, but these instances are so rare and involve so few of the millions of cells in an ordinary sponge colony that they should not be taken very seriously. Another type of investigation as to possible diet of sponges has been to take specimens of this phylum and analyze the material for the presence or absence of certain enzymes which are known to act on certain types of food. Some enzymes, as a matter of fact, are thereby discovered to be pres- ent within the material studied, but it is very generally overlooked (as
- 52 - should not be the case) that IB the materials studied there are not only sponge cells, but also numerous beneficial, neutral or antagonistic sym- bionts (such as bacteria) and that the enzymes which are undoubtedly there may have teen provided by the other cells, and not by those of the sponge. Whereas photosynthesis cannot possibly play any part in the nutri- tion of the Hexactinellida (or Hyalospongiae) vihich live. at great depths in the ocean, it is very interesting to note that very many of the shallow water sponges, as mentioned above, do have abundant symbionts. Included in this category are many of the fresh water sponges. One may say that in the majority of cases where the diet of any given sponge is really known, that it is in connection with, or brought about by some Thallophyte. Passing mention should be made of an hypothesis much considered but not readily susceptible to proof (or disproof), that sponges live upon ma- terial already in solution in sea water about them. To Judge from the sta- tion records of some of the extensive oceanographical explorations, sponge individuals and species are slightly more common in waters opposite the mouths of rivers than elsewhere. This might indicate that dissolved sub- stances were brought from land to nourish the Porifera in question. On the other hand it must be noticed that the emount of data bearing upon this matter is utterly inadequate; the few items from literature here referred to are susceptible to explanation on the grounds of mere coincidence and no ex- tensive conclusions from them are warranted. In summary of the above paragraphs it must be said that at the pres- ent time our state of knowledge with respect to the food of sponges is so utterly inadequate that no safe conclusions with respect to environment may be drawn as to matters of this sort from any studies, morphological or distributional, of fossil sponges. The third item in consideration of the ecology of sponges, whether fossil or recent, is the biological or living environment, which all may be summed up under the general heading of symbiosis. Sponges come into rela- tionship with animals from all the other major phyla and these relationships may be considered as follows. The chordata have very little to do with the sponges. Almost the only ones to do so other than man' himself are a few fish, and most fish as a rule ignore all sorts of sponges. Some of the angel fish that browse a- round the tropical coral reefs are frequently found to have bits of sponge in their stomachs, but it is a matter of doubt as to whether they were tak- en intentionally or not; they may have been accidently secured along with bits of other sorts of food as (for instance) vroms or shrimps that live in the sponges. There are exceedingly numerous instances of interrelationships be- tween arthropods end sponges. Many kinds of insect larvae eat fresh water sponges, or live im them, thereby securing shelter. Very numerous marine arthropods live in the canals or other cavities within sponges. Further- more there are crabs iftiich deliberately place bits of sponge upon their shells of which the growth may lend protection to the crustacean in ques- tion.
- 53 - Fairly numerous sponges of all the classes have sieves over their ex- halent openings. These in seme cases are reticulations of fibers. In many of the deep-sea species they.are elaborate screens of siliceous strands. It is certainly known that the cavities beneath the screens in a considerable number of sponges contain symbionts, principally shrimps. In regard to the others, our lack of information as to conditions during the life of the sponge is noteworthy. In them also, shrimps may have once lived. Do the sponges profit by the animals they thus hold prisoner? Does the imprisoned commensal profit more by the protection he receives than he loses by his confinement? These questions are not easily answered. Nor can one be too sure of the significance of cloacal or oscular sieves in fossil sponges. They probably, but by no means certainly, kept symbiont animals within. their cavities while still alive. The matter of symbiosis with mollusks has been rather less studied, but is probably more important. There are fairly numerous individuals of the nudibranch gastropods TJhicfo have as their regular diet the tissues of sponges. These often closely resemble the sponges on which they customarily feed. Furthermore, there are sponges which regularly grow on the shells of mollusks. In some instances they employ these merely as a basis of support, but even in this case they may smother the mollusk. Again we have the bor- ing sponges for consideration; by their excavation of galleries within the calcareous material of the molluskTs shell they may do him irreparable dam- age. Their work is frequently evident in fossil shells. Although sponges and echinoderms frequently live in very similar en- vironments their relationships with each other appear to be in most cases of 1iie most casual nature. Ophiuroids very commonly seek shelter in coarse- ly cavernous sponges. More important matters concern various of the different phyla of worms. Nematodes are occasionally found in sponge colonies, and are probably liv- ing the.re at the expense of the surrounding flesh, but may be merely com- mensal. That the latter is the case with regard to the very numerous annelids found in the canals of sponges seems fairly evident. One cannot find upon dissecting such sponges that the annelid has been damaging the tender lin- ing of the canals in which it occurred. One may even imagine that the sponge derives some benefit from chemicals given off by the worms in question. Some of the Platyhelminthes of the class Turbellaria are indirectly of great benefit to the sponges, in that they destroy the above mentioned nudibranchs which feod upon sponge tissues. * With the phylum Coelenterata the relationship is largely one of at- tachment. There are fairly numerous anthozoans which grow perched upon sponge tissues, as for instance those on the rooting tufts of the hexactinellid sponge Hyalonema, and the anemones of the genus Parazoanthus which may grow upon shallow water ramose sponges. Porifera, moreover, very frequently grow upon dead coral skeletons. Symbioses between different species of sponge are usually of a nature similar to that mentioned above; one species may grow upon the framework of another that has died, or may even perhaps be still living. Mutual crowding
- 54 - is of considerable importance. One species may be so overgrown by another as to be smothered. Some sponges invade the galleries of Cliona, either destroying the flesh of the burrowed sponge, or merely inhabiting passages from which the defunct Cliona had decayed. It must therefore be noted that the occurrence of a sponge in such burrows does not prove that it did the excavating. Fossil sponges are often found associated with the fossils of other phyla, as for example, at Solenhofen (Jurassic). The nature of these other fossils affords clews as to the paleoecology of said sponges. Such in- stances are very little discussed in the treatises on fossil Porifera. All too often the animals of one group are discussed by one expert, those of another group by another expert, and each treats of almost nothing save his particular specimens. little evidence is available from references to polyphyletic fossil faunas bears out the theory that the ages have witnessed little change in the methods ifaereby sponges react to their environments. It ap- pears that in the past Porifera occurred in much the same environments as at present, and lived similar lives.
