PBIMABY DIVISIONS AND CLASSES. 213 



of other peculiarities are found in the structure of these 

 animals, to which we shall afterwards return ; the scolo- 

 pendra or centipede (Fig. 164), the lobster or crawfish, crabs, 

 insects, &c., are specimens of this primary division of the 

 animal kingdom. 



376. Molluscous Animals. The molluscs have, like 

 the preceding, the principal organs in pairs, and symme- 

 trical; but the body has a tendency to assume a spiral or 

 curved form, so that the mouth and anus, instead of occu- 

 pying the two extremities of the trunk of the animal, are 

 more or less contiguous. The nervous system is composed 

 essentially of ganglions, as in the annulata ; and here also a 

 portion of this system occupies the dorsal aspect, and another 

 portion the ventral aspect, of the digestive tube ; but these 

 ganglions do not form a long median chain, as in the pre- 

 ceding division. 



e d pi 



Fig. 167. Anatomy of the Snail.* 



They have no skeleton internally or externally ; their body 

 is soft, and their skin constitutes a flexible and contractile 

 envelope or mantle ; it is often covered with horny or cal- 



* pi, the foot ; t, tentacle, half contracted ; d, a sort of diaphragm sepa- 

 rating the respiratory cavity from the other viscera; e, portion of the 

 stomach ; f, the liver; o, the ovary; i, intestines; r, rectum; a, anus; e, 

 the heart, the pericardium has been opened ; ap, pulmonary artery, rami- 

 fying on the walls of the pulmonary cavity p ; ar, aorta ; v, gland secreting 

 the viscosity; c e t its excretory canal opening near the anus. 



