64 HUBERT LYMAN CLARK ON 



over it from all sides, until the disc lies at the bottom of a shallow cavity, the so-called 

 " atrium," which opens on the ventral surface through a small poi'e (Fig. 16). This 

 pore does not lie directly over the mouth, but either before or behind or somewhat to 

 one side. While these external changes are taking place, the hydrocoel has continued its 

 growth, the anterior end passing across on to the right side>of the larva where it bends 

 downward and backward around the oesophagus to meet the posterior end near the middle 

 line. The primary tentacles have increased considerably in size and are growing up 

 around the floor of the atrium, while the secondary outgrowths also grow upward beside 

 them. No Polian vesicle has yet appeared, and no marked distinctions between the 

 different parts of the digestive tract are to be seen, but the latter has become very much 

 arched toward the dorsal surface, and the lumen of the middle section is larger than at 

 either end. A new anus may have been formed by the end of the hind-gut growing to 

 the body-wall on the ventral side and an opening breaking through, but in some cases 

 the definitive anus does not appear until the pentactula form is nearly attained. The 

 most important changes have been going on meanwhile in the coelomic pouches, 

 the growth of which has been very rapid. The left vesicle has grown more ante- 

 riorly than the right, and sends forward two finger-like processes, one of which passes 

 across the median line into the right side of the larva, while the other grows up to 

 the inner side of the hydrocoel ring, above the most posterior tentacle, and follows the 

 course of that ring around the oesophagus (Figs. 26-28). These anterior prolongations 

 of the left coelom were observed by Bury ('89 and '95) in 8. digitata, but have appar- 

 ently been overlooked by other investigators. The one which passes on to the right, 

 side of the body fuses so soon with the right coelom that I have been unable to confirm 

 Bury's further observations regarding it, and in S. vivipara its appearance might easily 

 be entirely overlooked. But the prolongation which passes to the hydrocoel remains 

 distinct through all the later stages of the larva, and its subsequent changes are easy to 

 trace. It soon loses its connection with the left coelom and forms a tube, lying on the 

 oral surface of the hydrocoel ring. Meanwhile the right and left coelomic pouches have 

 met and fused on the ventral side of the digestive tract, while dorsally they are still 

 separate, the right pouch extending considerably further back than the left. The larva 

 has now reached the condition shown in Fig. 16. In its subsequent growth the atrium, 

 with its thick ectodermal floor and narrow opening to the exterior, moves to the anterior 

 end of the larva, where it finally comes to lie, along with the mouth, oesophagus, and 

 hydrocoel ring. The latter has now closed, without the formation of a Polian vesicle, 

 somewhat to the left of the middle line apparently (Fig. 25), but I am not sure that the 

 point of closure is always on the left side, for it is by no means easy to determine the 



