THE PHYLA HEMICHORDATA AND ECHINODERMATA 379 



In both phyla the embryos develop into larvae (Fig. 19.18) that have 

 a tuft of sensory cilia at the anterior end, a tract of locomotor cilia on 

 the body, a ventral mouth and a posterior anus. The anterior coelomic 

 cavity opens dorsally through a pore (in a very few instances both the 

 cavity and the pore are paired). In the echinoderms the first and second 

 portions of the coelom are connected. 



Only a few hemichordates become free-swimming at this stage of 

 development. They develop rapidly to the next stage, which has a pos- 

 terior ring of stout locomotor cilia. The ciliated tract becomes more 

 elaborate and extends anteriorly on both sides, finally meeting to form 

 two tracts: a preoral circle between the mouth and the sensory tuft, and 

 a postoral circle behind the mouth and dorsal to the tuft. The rudiments 

 of several gill slits appear in the walls of the pharynx. This is the 

 tornaria larva. In some species the ciliated tracts are greatly folded over 

 the surface of the larva. 



Some of the echinoderms become free-swimming before gastrulation, 

 whereas others emerge at various times from gastrulation to metamorph- 

 osis. Development varies enormously in this phylum, depending in part 

 upon the amount of yolk in the eggs, and in part upon the taxonomic 

 group. Those with little yolk usually pass through larval stages com- 

 parable with those of the hemichordates. As in the hemichordates, the 

 ciliated tract usually extends anteriorly and fuses to form two loops. 

 Meanwhile the body becomes concave ventrally and the ciliated tract 

 becomes lobulated, extending out trom the body surface on body folds. 

 A larva of this sort, the auricularia, is found in many holothurians. In 

 the asteroids the lobulation of the ciliated tracts becomes much more 

 pronounced, while in the echinoids and ophiuroids the lobes become ex- 

 tremely long and slender with skeletal supporting rods inside (Fig. 19.19). 



The tornaria metamorphoses by a straightforward fashion into an 

 adult hemichordate. The ciliated tracts disappear and a constriction 

 separates the proboscis from the rest of the body. The pharynx elongates 

 while the middle coelomic pouches move anteriorly. A second constric- 

 tion separates the collar from the trunk, and the gill slits in the pharynx 

 open through the sides of the trunk. The anterior coelomic pouch be- 

 comes the proboscis cavity with its dorsal pore. The pair of middle 

 cavities become right and left collar cavities, with dorsal and ventral 

 mesenteries separating them above and below the pharynx. Each half 

 later develops an opening into the first gill slit. The posterior coelomic 

 pouches form the right and left cavities in the trunk, also with dorsal 

 and ventral mesenteries between them. The ciliated ring disappears, and 

 the swimming tornaria becomes a burrowing worm. (See Fig. 19.18.) 



Metamorphosis in the echinoderms involves not only a drastic 

 alteration of body parts, but also a change in symmetry. The details 

 vary considerably, and only a simplified course of events in some of the 

 starfish will be followed here. One of the first events of metamorphosis 

 is the migration of the mouth around to the left side of the body, where 

 the left middle coelomic pouch surrounds it to form the ring of the water 

 vascular system. At the same time the anus begins to migiate to the 

 right side. The region anterior to the mouth becomes a prominent 



