The evolution of the coelom and metameric segmentation

Clark, R. B.

The fact that the coelom severely reduced in all animals in which it does not serve a hydrostatic function is strong evidence that its chief and original function was mechanical and not physiological. If this is so, it can only have evolved to permit strong, antagonistic contractions of circular and longitudinal muscles, and this implies a burrowing habit. The morphological nature of the secondary body cavity is unimportant so long as it can serve this mechanical function. Consequently there is no reason why the coelom should not have been evolved more than once and from different morphological origins, as, indeed, embryological evidence suggests has been the case. Metameric segmentation, on the other hand, has been evolved at least twice, in annelids and chordates, and represents quite different adaptations in the 2 phyta. In the annelids it is an adaptation to sustained burrowing through the substratum, in chordates an adaptation to swimming with an axial notochord. In the absence of a notochord, animals without a segmented musculature can perform swimming movements, and many invertebrates do so. Metamerism, particularly of the coelom, as in annelids, carries with it the serious disadvantage that most of the internal organs must be replicated in each segment. Any change in the locomotory habit of these animals has been promptly followed by the loss of septa and a substantial reduction of the metameric organization of the body. Other types of metamerism, in the sense of a serial repetition of organ systems, exists, as, for instance, in the cestodes. There is no convincing logical justification for excluding cestode segmentation from the definition of "metameric segmentation;" as Hyman (1951) points out, zoologists are generally reluctant to regard the segmentation of cestodes in the same light as that of annelids, and rightly so. There are, in fact, several different types of metamerism. All represent adaptations of approximately, but not identically, the same type in response to different evolutionary demands. Metamerism of various sorts has been evolved as an adaptation to burrowing (annelids), swimming (chordates), reproduction (cestodes), and metamerism of particular organ systems, as an adaption to the needs of providing for excretion or nervous control in very long, thin animals like nemerteans. The evolution of the coelom has been strongly convergent and that of metamerism has been divergent, notwithstanding the superficial similarities between segmented animals of different origins.