310 MOLLUSCA 



7. The OPHIUROIDEA also have disc and arms, but the ambulacral 

 groove is closed and the hepatic caeca absent. 



8. The CRINOIDEA have a cup-shaped body bearing arms, usually 

 branching, with pinnuke, and a stalk, usually with cirri by which they 

 are attached, either permanently or in the larval stages. In these latter 

 free forms only the centrodorsal persists as the remains of the stalk. The 

 Crinoidea are 'subdivided into Eucrinoidea, Edrioasteroidea, Cystidea, 

 and Blastoidea. 



9. The ECHINOIDEA are usually spherical or oval, armored with 

 calcareous plates which extend as five pairs of ambulacral and five of inter- 

 ambulacral meridional bands from peristome to periproct. 



10. The ambulacral plates end at the periproct with an unpaired ocular 

 plate; the interambulacral with a similar genital plate. The madre- 

 porite is fused with one of the genital plates. 



11. The regular sea urchins have the anus in the periproct, the 

 mouth in the peristome; the ambulacral areas band-like. 



12. The Clypeastroidea have a central mouth, the anus outside the 

 periproct in the posterior interradius; the ambulacral areas petaloid. 



13. The Spatangoidea are markedly bilateral, the mouth anterior, 

 the anus posterior; ambulacral areas petaloid. 



14. The HOLOTHUROIDEA are elongate and worm-like; the skeletal 

 system greatly reduced; they are more or less bilaterally symmetrical and 

 have usually a single gonad and one or two branchial trees. They are 

 divided into Actinopoda, with radial canals, and Paractinopoda, without. 



PHYLUM VI. MOLLUSCA. 



At the first glance the molluscs, like the leeches and flatworms, appear 

 like parenchymatous animals. A spacious coelom is absent; what was 

 formerly regarded as such is a system of blood sinuses surrounding the 

 viscera, and is especially well developed in the snails. More recently 

 it has been shown that the molluscs have descended from ccelomate 

 animals, in which, by encroachment of connective tissue and muscles, 

 the coelom has been reduced to inconspicuous remnants, the pericardium 

 and the lumen of the gonads^ 



Where the molluscan features are well developed, as in the snails, 

 four parts may be recognised (fig. 311,5). The visceral sac forms most 

 of the body; it is less muscular'than the rest and contains the alimentary 

 tract, liver, nephridia, and gonads. In front it is continuous with the 

 head, often separated by a neck, which bears the mouth and the most 

 important sense organs, eyes and tentacles. Below, the visceral sac passes 



