RESPIRATION IN METAZOAN EVOLUTION.

The delineation of almost all metazoan phyla took place in pre-Cambrian times, and there is practically no direct fossil evidence of the morphological trends or the evolutionary pathways involved in this major episode in the history of Earth's biota. Models for rational sequences of events which could have produced the diverse array of metazoans now present on Earth can be tested only by assessing their compatibility with meager geological and paleontological data from late pre-Cambrian and early Cambrian rocks, and their compatibility with features of living inetazoan groups which are now removed at least 600 million years from their first divergence. It is hardly surprising that there are several competing models for early metazoan evolution, and it is obvious that there is a need for much more careful analysis in this area. Many current models of metazoan evolution derive the major invertebrate phyla from flatworms through annelid-like worms by the acquisition of a coelom. Clark has thoroughly discussed this question, and constructs the following adaptive pathway by which the process might have occurred. He argues (1964:206-214) that the first metazoans were probably small swimming or creeping organisms employing ciliary action for movement. An increasing exploitation of detritus as a food resource would have led to the differentiation of a mouth and gut; ciliary creeping on a soft substrate would have necessitated mucous secretion, and dorso-ventral flattening might have accompanied size increase among surface creepers. Essentially, a plexus of benthic flat-worms would have resulted from these selective pressures. Clark envisages that an adaptive radiation of flat-worms on the substrate would have generated in turn a selective advantage for any faunal member which could burrow into the substrate, primarily for escape from predators. Body-wall musculature alone would have allowed a rather inefficient burrowing mechanism, but the development of any internal fluid-filled cavity as a hydrostatic skeleton would have conferred a greatly improved capacity for burrowing by peristaltic movement. Clark (1964:214-219) stresses that there have been several different ways of developing such an internal fluid cavity, and that the coelom is mostly likely polyphyletic. Early coelomate worms would have been circular in shape, rather than flat, and they would have most likely engaged in intermittent burrowing near the surface. Clark suggests that they were probably relatively sedentary, living rather like echiuroids or sipunculids, with a variety of food-gathering devices. In association with the development of peristaltic body waves for locomotion, Clark points out that respiratory currents could have been generated by a minor modification of the same mechanism, generating water flow instead of body movement through the sediment. Considerable complexity and organization of muscular and nervous systems is implied in either of these activities. There have been at least two main lines of evolution from these early coelomate worms. The oligomerous phyla have remained sedentary, and are capable of weak burrowing, if they burrow at all. The metameric body architecture developed from an adaptive strategy favoring active burrowing, in which segmentation and the development of paired external appendages