ix PHYLUM ECHINODERMATA 429 



simple sac giving off short diverfcicula, and there is no anal 

 aperture. In the Asteroidea the alimentary canal is more complex 

 than in the other classes. The stomach is divided, as already 

 described in the account of the examples, into two portions, the 

 cardiac and the pyloric, the former giving off five large rounded 

 radial diverticula the cardiac pouches or cardiac caeca, and the 

 latter five pairs of very long branched diverticula the pyloric 

 or hepatic caeca. The intestine is short and conical, and opens, in 

 all but a few, by an anal aperture. In some Asteroidea (as in 

 Aiithenea, Figs. 308 and 310) the intestine has connected with it 

 a system of five elongated bifurcated inter-radial intestinal caeca ; 

 in others (as in Asterias, Fig. 306) these are represented only by 

 two or three lobed diverticula. In one member of the class there 

 are also ten caeca connected with the oesophagus. 



In the nervous system of the Echinodermata three distinct 

 parts, the relative development of which differs in the different 

 classes, are to be recognised. These are the epidermal or super- 

 ficial, the deep, and the aboral. The epidermal system is well 

 developed in all the classes : its principal parts are a circum- 

 oral nerve-ring and radial branches, but a plexus of nerve-fibres 

 with occasional nerve-cells extends from it through the epi- 

 dermis. In the Ophiuroids the radial nerves and the ring 

 nerve are similar in their arrangement to what is to be observed 

 in the Asteroids, but are more deeply placed, being covered over 

 by the investing calcareous plates. The deep-lying nervous system 

 is absent in the Crinoidea, very feebly developed in the Echinoidea, 

 but well developed in the Asteroidea, Ophiuroidea, and Holo- 

 thuroidea. Its general arrangement has already been described 

 in the account of the Starfish. The aboral system is best 

 developed in the Crinoidea and is absent altogether in the 

 Holothuroidea. 



The sexes are distinct in all the Echinoderms, with one or two 

 exceptions ; but there is very rarely any trace of sexual dimorphism. 

 Aster ina gibbosa, the Starfish the development of which has been 

 described (p. 388), is one of the exceptional hermaphrodite forms ; 

 the young animals of this species are male, producing sperms, but 

 at a later stage they become female and produce only ova. In the 

 family Synaptidae of the Apoda there are also numerous examples 

 of hermaphroditism, the animal at first producing ova, later only 

 sperms. In Amphiura squamata, an Ophiuroid, both ovaries and 

 testes are present at once. The gonads, ovaries or testes as the 

 case may be, are branching bodies, inter-radial in position, and 

 usually in pairs. In the Asteroidea there are five pairs, the ducts 

 from which open usually on a special plate on the aboral sur- 

 face, but in one or two species on the oral surface. In the 

 Echinoidea there are five ovaries or testes, the five ducts of 

 which open on the genital plates of the apical system. In the 



