While some are but a few inches long, there are also veritable battleships, long straight shells passing backward through the water. Some are immense. Six footers are common, but one is easily double that size, and from the large round opening of its conical tubular shell a great head, with saucer eyes, watches, searches, looks. A mass of tentacles surrounds the head, and we can see that propulsion is from great volumes of water being jetted from a funnel-like fleshy tube situated beneath the mass of tentacles. With shell and flesh, the entire animal must weigh several hundred pounds in air, but in this sea it is weightless. Although invisible, we know the inside of the massive shell is composed of numerous chambers, filled with air, but with a thin tube containing a strand of blood vessels passing through the middle of each chamber. The nautiloids have discovered the secret of achieving neutral buoyancy. But they have done far more than this. They have climbed to the top of the trophic pyramid, and it may be that it was the best respiratory system on the planet that got them there.
We move from the sea and on a whim take one more dive—into the brackish water of a large estuary connected to one of the extensive braided river systems. Here we see in large numbers some familiar shapes. There are fish in this fresh water. But they are fish very foreign to us. They are called ostracoderms, and they have small sucking mouths without jaws. Most have extensive dermal body armor in the form of massive scales and larger bony plates. Their tail is “reversed heterocercal,” like an upside-down shark’s tail. And no wonder, most of these early fish are swimming at the surface, thanks to the action of these tails whose design naturally forces the body upward, and they are eating the prolific algae and vegetable scum at the lagoon’s surface. A few other types are seen as well, hugging the bottom, and these are even more heavily armored than their surface-living brethren.
We look more closely at some of these primitive fish for evidence of a respiratory structure. One of these fish, Arandaspis, shows 15 plates covering a primitive gill system called gill pouches. The gills themselves are relatively large compared to the size of the fish, and swimming seems part of the respiratory cycle. Forward motion passes water through the gills, making this the functional equivalent of a pump gill.