34 



INTRODUCTION TO CLASSIFICATION. 



essential difference between the two, that whereas, in the 

 BracMopoda, the mantle lobes corresponded with the anterior 

 and posterior regions of the body, in the Lamettibranchiata 

 they 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 well-developed heart, 

 which is much more complex than that of the Ascidians or 



Fig. 13. 



D 



Fig. 13. Sectional diagram of a freshwater mussel (Anodori). A A, mantle, the right 

 lobe of which is cut away ; B, foot ; C, branchial chamber of the mantle cavity ; D, 

 anal chamber ; /, anterior adductor muscle ; //, posterior adductor muscle ; 

 a, mouth ; b, stomach ; c, intestine, the turns of which are supposed to be seen 

 through the side walls of the foot ; d, rectum ; e, anus ; /, ventricle ; g, auricle ; A, 

 gills, except *, right external gill, largely cut away and turned back ; k, labial palpi ; 

 I, cerebral; m, pedal; n, parieto-splanchnic ganglia; o, aperture of the organ of 

 Bojiinus; p, pericardium. 



Brachiopods, being divided into distinct auricular and ven- 

 tricular chambers. Commonly, there are two auricles and one 

 ventricle, as in Anodon ; but in other Lamellibranchiata, 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 implies), are found in all Lamellibranchiata, and 

 are situated upon each side of the body, in a chamber which 



