34 



ON CLASSIFICATION. 



mussel (Anodon), is included within a mantle or " pallium," which 

 is formed by a prolongation of the dorsal integument, — a structure 

 in principle quite similar to that which Ave met with in the 

 Brachiopoda. But there is this essential difference between the 

 two, — that whereas, in the Brachiopoda, the mantle lobes corre- 

 sponded with the anterior and posterior regions of the body, in 

 the LamellibrancMata thev answer to the right and left halves of 

 the body. The intestine, which always terminates by a definite 

 anus between the mantle lobes, at the posterior end of the body, 

 has its first flexure neural. There is always a w T ell-developed 

 heart, which is much more complex than that of the Ascidians 

 or Brachiopods, being divided into distinct auricular and ven- 

 tricular chambers. Commonly, there are two auricles and one 

 ventricle, as in Anodon ; but in other LamellibrancMata, such as 

 the oyster, there is a single auricle and a single ventricle, and in 

 some exceptional cases there are two auricles and two ventricles, 

 forming two distinct hearts. Distinct respiratory organs, which 

 usually have the form of lamellae (as the name of the class 



Fig. 13. 



Fig. 13. — Anodon, vertical and transverse section of the body through the heart ; /, ven- 



tricle; g, auricles; c, rectum; p, pericardium; /i, inner, i, outer gil 

 of Bojanus ; B, foot; A A, mantle lobes. 



oq, 



organ 



implies), are found in all Lamellibranchiata, and are situated 

 upon each side of the body, in a chamber which extends bet \\< ■« n 



