62 HUBERT LYMAN CLARK ON 



pushing onward and downward against the thickened part of the ectoderm, becomes 

 attached to it, and the mouth breaks through at that point. The mouth may be formed 

 before the separation between the archenteron and the other vesicle is completed. Mean- 

 while the latter increases in size, and its walls become thinner. It grows backward toward 

 the blastopore especially and soon becomes constricted and divided into two vesicles, the 

 anterior of which is connected with the exterior by the water-pore, while the posterior is 

 entirely closed and lies beside the posterior part of the stomach, if we may so designate 

 the middle section of the archenteron (Fig. 13). Soon afterwards this posterior vesicle 

 grows out laterally to the right, across the dorsal surface of the larval hind-gut, and forms 

 on the right-hand side a vesicle like itself, which soon becomes entirely separate from it 

 (Fig. 14). The anterior of these three vesicles is the rudimentary hydrocoel, while the 

 pair of posterior pouches represent the right and left coelomic vesicles. Their mode of 

 formation is essentially the same as that described by Selenka ('83) for 8. digitata, but 

 the relative size of the various organs is markedly different, so that the figures of the same 

 stages show almost no resemblance. If figure 15 of Synapta vivipara be compared with 

 the auricularia of about the same age which Semon ('88, Plate 6, Fig. 2) figures, this 

 difference will appear in many ways. The rudiments of the two coelomic pouches and 

 especially of the hydrocoel are very much larger relatively in S. vivijjara than in 8. digi- 

 tata, while the difference in the digestive tract is even more noticeable. In the Eiu-opean 

 species there is a well-marked differentiation into fore-, mid-, and hind-gut, while in the 

 Jamaican form there is no such distinction evident. A remarkable difference is also to 

 be seen in the external form of the larvae. That of 8. vivipara has retained its elliptical 

 shape and shows no trace of ciliated bands, while the other has assumed the familiar 

 auricularia form. Furthermore, there is no trace of calcareous structures of any kind in 

 8. vivipara to correspond with the plates at the posterior end of auricularia. In the 

 latter also, Semon ('88) has figured and described a larval nervous system, but after care- 

 ful investigation of this point, I am convinced there is none in the larva of 8. vivipara. 

 In external form, the latter is very regular, but with the growth of the coelomic pouches 

 the ectoderm of the dorsal surface begins to become thinner and more flattened, while that 

 around the mouth, especially posterior to it, shows a tendency to. increase in thickness. 

 This difference between the dorsal and ventral surfaces may sometimes be seen in very 

 young larvae and is clearly shown in figure 22. Before the two coelomic vesicles have 

 entirely separated, the hydrocoel has become considerably larger and begins to grow 

 anteriorly and toward the right, while about the same time five outgrowths begin to appear 

 on that side which is furthest from the fore-gut (Fig. 15). Soon after their appearance 

 five other much smaller outgrowths arise, one at the right of each of the first five. The 



