MOLLUSC A. 351 



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

 peri2)roct, the mouth in the peristome; the ambulacral areas are 

 band-like. 



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

 the periproct in the posterior iuterradius; the ambulacral areas 

 l)etaloid. 



13. The Spatangoidea are markedljr bilateral, the mouth an- 

 terior, the anus posterior; ambi^lacral areas petaloid. 



14. The IIOLOTiiuROiDEA 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 brancliial 

 trees. They are divided into Actinopoda, with radial canals, and 

 Paractinoj)oda, without. 



PHYLUM VI. MOLLUSC A. 



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 ca3lomate animals, and from forms in which, by encroach- 

 ments of a connective tissue and muscular jDarenchyma, the Cffdom 

 has been reduced to the incons2)icuous remnants of the pericardium 

 and the lumen of the gonads. 



Where the molluscan organization is well developed, as in tlie 

 .snails, four parts may be recognized in tlie body (fig. 310), 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 nuirked off by a neck, and Ijcars, besides the mouth, 

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

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

 motion, the foot. From the back extends f^o palliiiiii or maiifle, 

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

 Acephala (fig. 310, (') hiive a double mantle, right and left, both 

 halves springing from the dorsal line a)"id extending dowi"! over the 

 visceral sac and foot. The cephalopods (fig. 340. .1) and tlie snails 

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

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



