152 PHYLUM ECHESTODERMATA. 



posterior is the splanchnocoel. The splanchnocoel then divides into two 

 sacs which apply themselves to the gut, one on the right and the other on 

 the left, and give rise to the perivisceral cavity and its lining. The tube 

 connecting the combined anterior coelom and hydrocoel with the water- 

 pore elongates and a small swelling appears on its 

 5 anterior wall (Fig. 108). This is supposed to be 



the reduced anterior coelom. The vesicle itself 

 becomes lobed and forms the hydrocoel. It 

 eventually surrounds the oesophagus to form the 

 circumoral water-vascular vessel and gives off 

 five outgrowths which become the radial canals. 

 The canal connecting the hydrocoel with the 

 small anterior coelom must be regarded as the 

 stone-canal. In the forms with a so-called inter- 



Fio. 108. Diagram of the na [ madreporite, it must be supposed that the 

 hvdrocoel and anterior , . , , /. ., n 



coelom of an old Holo- canal (water-pore) distal 01 the small anterior 



thurian auricularia (after coelom breaks down and that the anterior coelom 

 Bury). 1 water-pore ; 2 .^.-^1^1 i 



pore canal ; 3 sand-canal ; acquires a tree communication with the general 



4 polian vesicle ; 5 an- perivisceral cavity by the rupture of its walls, as it 

 terior coelom. j n -j / i KO\ 



does in Urmoids (p. 158). 



Development of Crinoids. The development of Crinoids 

 differs considerably from that of the other classes and requires 

 separate treatment. The principal points of difference concern 

 (1) the form of the larva ; (2) the relation of the larval preoral 

 lobe to the adult surfaces ; (3) the fact that, though the hydro- 

 coel at first undergoes a displacement to the left side, the oral 

 surface of the adult is the posterior surface of the larva and the 

 left posterior coelom does not exceed the right in size ; (4) the 

 posterior coeloms arise from the enteron independently of the 

 common rudiment of the anterior coelom and hydrocoel ; and 

 (5) the absence of any trace of a right hydrocoel. 



In Antedon, the only Crinoid the development of which is 

 known, the egg, as in most other Echinoderms, is fertilized and 

 undergoes its whole development in the sea-water, but it remains 

 for some time within the vitelline membrane attached to the 

 pinnules of the parent. The total cleavage leads to the forma- 

 tion of a hollow blast osphere from which a gastrula arises by 

 invagination. The blastopore closes completely at or near the 

 hind end of the embryo, and the uniform ciliation gives place to 

 five ciliated bands which encircle the body transversely and 

 to a ciliated tuft at the anterior end (Fig. 109). The ciliated 

 tuft springs from a thickened patch of ectoderm which con- 

 stitutes a neural apical plate. In the deeper layers of this neural 

 plate nerve-cells and fibres are formed and constitute the larval 



