ECHINODERMATA. 547 



pierces an oral in Crinoidea as do others formed later. The epithelium 

 of the water-vascular system is ciliated partially or throughout. The 

 coelome is derived invariably from a right and left peritoneal vesicle, 

 either separate outgrowths of the archenteron or split off from a common 

 outgrowth (Holothurioidea). The walls of the two outgrowths fuse round 

 the archenteron, and the dorsal fused portion persists as the mesentery, 

 already mentioned, while the ventral portion is absorbed. The coelome 

 is large, its wall generally ciliated, its cavity filled by a liquid of about 

 the same specific gravity as sea-water, and containing, where it has 

 been examined, but little proteid material and relatively few corpuscles. 



The larval mouth and oesophagus are carried on into the adult in 

 Holothurioidea and Ophiuroidea, but in Echinoidea and Asteroidea the 

 oesophagus of the adult is a new formation, growing out from the arch- 

 enteron to the left side, that to which the mouth is said to be transported. 

 The larval anus is a persistent gastrula mouth. It is retained in Holo- 

 thurioidea, closed permanently in Ophiiiroidea, and a few Asteroidea, said 

 to be retained in Echinoidea and other Asteroidea, but in some instances 

 it certainly closes and a new anus is formed subsequently. ^The digestive 

 tract is typically a spirally coiled tube. The oesophagus lies in the 

 madreporic interradius, and if the Echinoderm be regarded from the 

 oral surface, the tract passes to the right, and the anus is situated in the 

 fourth interradius in Holothurioidea and Echinoidea, but in the fifth, i. e. 

 the one adjoining the madreporic, interradius in Asteroidea and Crinoidea. 

 The tract is saccular in its first section in Asteroidea, and wholly saccular 

 in Ophiuroidea. It is generally ciliated except in Holothurioidea. There 

 are radial glandular caeca in Asteroidea, and to the anal end of the 

 intestine there are appended, as also in many Holothurioidea, two caeca. 

 Respiration is carried on by external prolongations of the coelome 

 (dermal branchiae of Asteroidea, peristomal gills of some Echinoidea] ; 

 by modified tube feet (many Echinoidea, tentacles of Crinoidea) ; or by 

 the expanded anal end of the intestine with its two appended caeca or 

 respiratory trees (many Holotlmrioidea\ In all cases, however, the tube 

 feet must have a limited respiratory function, and sea-water diffuse 

 through the walls of the stone canal and water-vascular vessels into the 

 coelome. There is no excretory organ. 



The sexes are separate. The Ophiuroid, Amphiura squamata, is said, 

 however, to be hermaphrodite. The genital glands are more or less 

 branched interradial caeca, single in Holothurioidea, multiple in other 

 Echinoderms. Details vary in the different groups. Impregnation is 

 either external, or the spermatozoa pass from the water into the female 

 ducts. A few forms in all groups except Crinoidea are viviparous. 

 Development is then abbreviated and takes place internally in the genital 

 bursae (Ophiuroidea) or the coelome (a Chirodota, Phyllophorw urna 



N n 2 



