MOLLUSC A. 351 



11. The regular sea urchins have the anus in the centre of the 

 periproct, the mouth in the peristome; the ambulacral areas are 

 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 an- 

 terior, 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 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 flatworms and leeches, 

 give the impression of parenchymatous animals. A spacious coelom 

 is absent; what was formerly regarded as a body cavity is a system 

 of sinuses surrounding the viscera and connected with the blood 

 system, and is especially developed in the Acephala. More recently 

 the view has gained ground that the molluscs have descended 

 from ccelomate animals, and from forms in which, by encroach- 

 ments of a connective tissue and muscular parenchyma, the coelom 

 has been reduced to the inconspicuous remnants of the pericardium 

 and the lumen of the gonads. 



Where the molluscan organization is well developed, as in the 

 snails, four parts may be recognized in the body (fig. 340), The 

 visceral sac forms the chief mass of the body; it is less rich in 

 muscles than the rest because it is reduced to a thin peripheral 

 layer by the alimentary canal, liver, nephridia, and gonads. In 

 front it is continuous with the head, which, according to the group, 

 is more or less marked off by a neck, and bears, besides the mouth, 

 the tentacles and eyes, the most important sense organs. Below, 

 the visceral sac passes into a muscular mass, usually used for loco- 

 motion, the foot. From the back extends the pallium or mantle, 

 a dermal fold which envelops a goodly part of the body. The 

 Acephala (fig. 340, C) have a double mantle, right and left, both 

 halves springing from the dorsal line and extending down over the 

 visceral sac and foot. The cephalopods (fig. 340, A) and the snails 

 (fig. 340, B), on the other hand, have an unpaired mantle which 

 arises from about the central part of the back and either extends 



