coelom

Although not parasitic, sessile, or microscopic, the animal lacks a centralized nervous system, anus, coeloms, and reproductive organs.

The mid-Cambrian (~520 Mya) was characterized by burrowing worms (especially the segmented marine coeloms, compared to their soft-bodied flatworm precursors moving only on the sediment surface) that developed hydrostatic skeletons and associated muscles to exert mechanical leverage, much of it probably an evolutionary advantage to escape from predators, yet which required transport of their own [O.sub.2] reserve from the open waters and thus an elevated consumption of metabolic energy.

Collar and trunk coeloms that were present in the uncut animals were notably absent in the regenerated larvae.

Just ventral to the stomochord are the perihaemal coeloms (Fig.

A parallel situation exists for the phylum Hemichordata, wherein enteropneusts form their coeloms largely by enterocoely, but pterobranchs do so by schizocoely (Lester, 1988); however, an evolutionary explanation of the difference is hampered by current uncertainty as to whether pterobranchs are basal or derived within the phylum (Sato et al., 2008).

The origin of the adult rudiment on the left side of the larval gut in echinoids and asteroids is an obvious departure from bilateral symmetry for internal structures such as coeloms and gene expression territories.

Axons extend beneath the ciliated bands and to the muscles derived from the coeloms (Lacalli and West, 1993).

membranacea within the paternal coeloms (perivisceral and lophophoral), in seawater after spawning, and within the lophophores of conspecifics.

Afterwards, complete differentiation of the newly formed digestive system, completion of regeneration of the lost components of the postoral ciliary band, and further development of both coeloms are observed (Fig.

These observations are consistent with those of Bonch-Bruevich and Malakhov (1987), who report one unpaired preoral coelom and three pairs of trunk coeloms, although we were unable to locate with certainty the boundaries between the trunk cavities.

It is not known from where these cells originate or if they reform the larval gut, and subsequently the larval coeloms. Asexually produced 4-arm plutei appeared to have disproportionately longer posterolateral arms than those expected for primary plutei at a similar developmental stage, but without direct comparison, this remains a subjective observation.

We did not observe that coeloms develop through the typical enterocoelic phenomena characteristic of most echinoderms (Czihak, 1971).