60 THE APODOUS HOLOTHUEIANS 



and the mouth is subsequently formed at that point. In this growth of the 

 archenteron the portion connected with the water-pore comes to lie at one side 

 (the left) and soon is separated as a pouch by itself. This pouch grows back- 

 ward and constricts, so that its posterior part soon becomes a distinct pouch, 

 which grows out laterally to the right across the dorsal surface of the archen- 

 teron onto the right sido. The right and left halves of the pouch then sepa- 

 rate and become entirely distinct. At this stage, then, we have a larva with a 

 mouth, gut, and anus (the blastopore), a hydrocrel connected with the exterior 

 by a water-tube, and two~ccelomic~pouches, one on each side of the gut. Scat- 

 tered throughout the segmentation cavity are more or less numerous mes- 

 enchyme cells, which have arisen from the endodermal cells of the archenteron. 

 In those Synaptids which have free-living larval forms, the external appear- 

 ance of the larva at this stage is very characteristic, and is very similar to the 

 auricularia larva of other holothurians. The Synaptid auricularia is a dor- 

 sally-flattened, wide, squarish larva with a deep depression on the ventral side 

 (oral vestibule), at the bottom of which opens the mouth. The uniform ciliated 

 coat of the gastrula has given place to a complicated symmetrical series of cili- 

 ated bands. A larval nervous system, muscles, and wheel-shaped calcareous 

 bodies are also present. After a time the auricularia becomes barrel-shaped, 

 and the ciliated bands form five encircling rings around the body; this stage 

 is sometimes called the pupa, sometimes barrel-shaped larva. At the upper or 

 anterior end of the pupa is a very deep, constricted, funnel-shaped space, the 

 vestibule, in the floor of which lies the mouth; the hydrocoel completely en- 

 closes the esophagus, forming the water-vascular ring from which arise the 

 polian vessel and the tentacles; of the latter, five are already indicated, and 

 between each pair are five other outgrowths corresponding to the radial water- 

 canals of pedate holothurians; these may be quite well-marked in the pupa, 

 running back toward the rear of the body; the ccelomic pouches have become 

 fused ventrally, but on the opposite side are still separated by the dorsal mes- 

 entery ; the rudiments of the calcareous ring have appeared, and the positional 

 organs are well formed. With the growth of the five tentacles the vestibule 

 disappears and the larva assumes the pentactula form (Plate VI, fig. 3), 

 characterized by the short, oval body, the absence of ciliated rings and the five 

 prominent tentacles without digits. The muscular -and nervous system are 

 fairly distinct and are rapidly assuming the adult condition. The transforma- 

 tion from the pentactula to the adult form is chiefly marked by the appearance 

 of the blood-system, ciliated funnels, and reproductive organs and the increase 

 in the number of the tentacles, with accompanying development of digits. In 

 Tceniogyrus contortus the sixth and seventh tentacles appear simultaneously in 

 the lateral dorsal interradii (Plate VII, fig. 13), and subsequently the eighth, 

 ninth, and tenth appear, in the other three interradii, but in Synaptula hydri- 



