310 MOLLUSCA 



-. The Oniu'ROiPFA also ha\o disc and arms, hut ihe aiiilni!acr;il 

 groove is closed and the hcpalic ca-ca ahseul. 



S. The C'Ri-XOinrA ha\e a cup-shaped liody hearing arms, usuall)' 

 hranching, with pinuukr, and a stalk, usualh' with cirri h\- \\ liich they 

 are attached, either permanentlv or in the lar\-al stages. In these latter 

 free forms onlv tire ceiurodorsal persists as the remains of the stalk. The 

 Crinoidea are suhdi\ided into Eucrinoidea, Kdrioasleroidea, Cystidea, 

 and Blastoidea. 



0. The EciilN"Oli"ii:.\ are usualh' spherical or o\al, armored with 

 calcareous plates which extend as live pairs of amhulacral ami li\o of inter- 

 amhulacral meridioital bands from peristome to periprocl. 



10. The amlnilacral plates cud at the periproct w ilh an unpaired ocular 

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

 porite is fused with one of the geitital plates. 



11. The regular sea urchins ha\e the anus in the periproct, the 

 mouth in the peristome; the amhulacral areas handdike. 



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

 periproct in the posterior interradius; the amhulacral areas pelaloid. 



13. The Spo/ai:goi(h'a are markedly hilateral, the mouth anterior, 

 the anus posterior; amhulacral areas pietaloiik 



14. The HoLOTHfRomiu are elotigate and worm like; the skeletal 

 system greatlv reduced; thev are more or less bilaterallv sMiimetrical and 

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

 divided into Actiuopoda, with radial canals, and Taractinopoda, without. 



rilYLUM M. ^MOLLUSCA. 



At the lirst glance the molluscs, like the leeches and llatworms, appear 

 like parenchymatous animals. A spacious cadom is absent; what \\as 

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

 viscera, ami is especially well dexeloped in the snails. More recently 

 it has been shown that the molluscs ha\e descended from cadomate 

 animals, in which, by encroachment of conuecti\e tissue and muscles, 

 the cadom has been reduced to inconspicuous remnants, the pericardium 

 and the lumen of the gonads. 



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

 four parts may he recognised (lig. 31 i, A'). The visrrral s.if forms most 

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

 tract, li\er, nephridia, and gonads. In front it is continuous witli the 

 head, often separated by a lU'ck, which bears the mouth and the most 

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



