510 COMPARATIVE ANATOMY CHAP. 



The body is surrounded by five ciliated rings, distinct from one another ; these 

 cannot be ontogenetically derived from one continuous ciliated ring. 

 The most anterior ring is interrupted on the ventral side. 



The second ring runs somewhat diagonally from above downwards and forwards, 

 the third is directed downwards and backwards, so that there is a large interval 



between the second and third rings ventrally. 



In this region, the ventral side sinks in to 

 , u|,,, form a large ciliated vestibular depression. 



A smaller depression on the ventral side 

 between the first and second ciliated ring is 

 known as the adhesive pit. The larva attaches 

 itself at this point, by means of a special 

 secretion yielded by the glandular cells of the 

 depression. 



To the left, between the third and fourth 

 rings, there is a small aperture, the primary 

 madreporite (water pore). 



The intestine lies as an entirely closed sac in 



J :-..- / the posterior part of the larva. The free-swim - 



Hpfe. ming larva has neither larval mouth nor larval 



\ anus. The definitive mouth breaks through the 



floor of the vestibular depression later. 



The whole anterior part of the larva, as far 

 FIG. 402. Free -swimming larva of as the third ciliated ring, becomes the stalk, and 

 Antedon, from the right lower side (after the posterior part the calyx of the attached 

 Bury). I- V, The five ciliated rings ; 1, the larya 

 neural tuft; 2, the adhesive pit; 3, the 



vestibular depression; d, dorsal; v, The free-swimming Crmoid larva cannot with 

 ventral. certainty be derived from the same hypothetical 



form as other Echinoderm larvae. The difficulty 



consists in the varying number and arrangement of the ciliated rings, which most 

 recall the condition in the Holothurid larva (pupa), to be described later. The 

 vestibular depression of the Antedon larva may, however, be compared without 

 forcing to the ventral depression of the other Echinoderm larvse. A thickening of 

 the ectoderm, comparable with the neural plate of the Antedon larva, also occurs, as 

 we shall see in other Echinoderm larvse. 



B. Ontogeny of the Holothurioidea. 



The segmentation of the ovum is total and equal, and leads to the formation of a 

 coeloblastula, whose unilaminar cell wall usually consists on one side of somewhat 

 larger cells. By invagination of this part of the blastula wall, a ccelogastrula is 

 formed. The invaginated part, i.e. the archenteron, is a blindly ending tube, with 

 narrow lumen (archenteric cavity), which is far from filling the segmentation cavity. 

 This latter is filled with an albuminous, fluid or semifluid, mass, the gelatinous 

 nucleus. 



The ectoderm and the endoderm are ciliated. 



During the process of invagination (occasionally even during the blastula stage) 

 cells arise by division from the ectoderm, but more especially from the endoderm, 

 which, as mesenchyme cells, wander into the enclosed jelly-like substance, multiply 

 by division and, in ever-increasing numbers, occupy the blastocoel. From them is pro- 

 duced all the connective tissue of the Holothurian body. The calcareous corpuscles 

 arise exclusively in the mesenchyme. 



