548 THE ANIMAL KINGDOM. 



among Holothurioidea) ; or else externally in depressed ambulacra pro- 

 tected by the spines (certain Echinoidea) ; among the dorsal spines or 

 paxillae (the Asteroid Leptychaster] ; among the dorsal ambulacral pro- 

 cesses (the Holothurioid Cladodactyla crocca) ; or in special marsupia 

 formed by stalked plates (the Holothurioid Psolus ephippiger], or by 

 a soft skin (the Asteroids Pteraster and Hymenaster, and Echinoid 

 Anochanus). The Asteroids, Echinaster Sarsii and Asterias Millleri bend 

 their arms ventrally and so protect the ova. The female in most of 

 these instances is distinguishable from the male. A sexual difference 

 in colour has been noticed in the Asteroid Oreaster (Pentaceros) turritus 

 and Ophiuroid Ophiothrix Petersi. Otherwise the sexes are alike. 



Among the Holothtirioidea the majority mutilate themselves by dis- 

 charging their Cuvierian organs. The Aspidochirotae also break off the 

 alimentary tract behind the water-vascular ring but repair the injury. 

 If irritated, the Synaptae break the body up into sections and certain 

 species of Stichopus resolve their skin into a diffluent mucus. Asteroidea, 

 Ophiuroidea and Crinoidea regenerate an injured arm, and the two latter, 

 especially the Ophiuroidea, possess the power of throwing the arms off. 

 In the Asteroidea and Ophiuroidea the organism may be formed anew 

 from an arm with a portion of the disc or perhaps even without it. It is 

 possible indeed that asexual multiplication may thus take place normally 

 in some genera. The Crinoidea appear to have the power of regenerating 

 the visceral mass as a wrjole when removed or lost naturally as it some- 

 times is. 



Segmentation of the ovum is total. The gastrula is formed as a rule 

 by invagination. It is stated to be delaminate in the Ophiuroids Ophio- 

 thrix versicolor and Amphiura squamata. The mesoblast is derived from 

 amoeboid cells which originate from the invaginated cells (i. e. hypoblast), 

 and the epithelium of the coelome and water-vascular vessels from the 

 vaso-peritoneal vesicles, hollow outgrowths of the archenteron, solid only 

 in certain Ophiuroidea. The larval oesophagus is a stomodaeum. The larva 

 is at first uniformly ciliated, but the cilia are subsequently restricted to cer- 

 tain bands. Antedon has four transverse bands and a posterior tuft. In all 

 other Echinoderms, with the exception of a few with shortened development, 

 the free-swimming larva has a prae-oral and a prae-anal ciliated ridge. 

 These ridges either unite into a single closed longitudinal band (Holo- 

 t/mrioidea, Echinoidea, Ophiuroidea) or form two separate but closed bands 

 of which the prae-anal is longitudinal (Asterioidea}. The Holothurioid 

 larva is known as Auricularia. Its ciliated band is somewhat undulated 

 and breaks up into transverse ciliated bands, usually five in number, con- 

 stituting the so-called Ptipa-stage. The prae-oral lobe is large. The Asteroid 

 larva closely resembles the Holothurioid. Its ciliated band developes 

 soft arms, and it is then known as Bipinnaria ; or as Brachiolaria, if three 



