So why is segmentation needed for a large gill surface? The same amount of gill surface area could be evolved and exist as one or a small number of large sheets attached to the body. But such large structures would easily fold on themselves and could also become problems during swimming by increasing drag.
How did this kind of repeated body part system come about at all? I propose that an important group of regulatory genes known as Hox genes were themselves co-opted in order to provide repeated gills.
A rich new topic of research spanning the last two decades has been at the interface of development and evolution. There are rules to the formation and growth of embryos as the genetic code of the genome becomes expressed as the protein and protoplasm of the growing individual. Newly discovered “regulatory” gene complexes, such as one called the Hox gene complex, are increasingly viewed as being nearly as important as the genetic code itself. As for segmentation, biologist S. A. Newman has postulated that it is a consequence of developmental regulation and pattern, not a feature evolved specifically for function. In other words, segments initially may have come about not because they work better in the day-to-day life of an organism with them but because they made growth and the many morphological changes occurring in these animals from embryo to adult take place more smoothly. Once in place, however, the presence of segments was molded by evolution for use in specific functions such as locomotion, respiration, feeding, and reproduction through evolutionary processes placing specific locomotory, respiratory, and reproductive appendages on various segments.
Do animals with segments really have an advantage in respiration over those that are nonsegmented? Let’s look at a measure of comparing respiratory structures, starting with passive systems. In a passive system, as mentioned in Chapter 1, the oxygen-carrying medium, in this case seawater, comes in contact with the respiratory epithelium. Oxygen diffuses across the membrane into the circulatory fluid and is moved to the various parts of the body and its cargo of cells, all needing oxygen. Carbon dioxide is exchanged across the same membrane out of the body. Three factors control the rate at which oxygen can be extracted: its concentration in the water, the rate at which it can be