260 ELEMENTS OF COMPARATIVE ZOOLOGY. 



intestine goes from the stomach first towards the foot, then 

 mounts towards the hinge-line, and frequently passes through 

 the ventricle of the heart. 



The heart consists of a single ventricle and usually two 

 auricles, but sometimes there is but one of the latter. The 

 heart is situated in a chamber (pericardium), which is 

 connected by means of a pair of convoluted kidney tubules 

 (organ of Bojanus) with the exterior. 



A thoroughly satisfactory classification of the Acephala 

 has not yet been worked out. Possibly the best is that 

 based upon the structure of the gills, but a more convenient 

 one for our purposes is based upon the presence or absence 

 of a siphon. 



ORDER I. ASIPHOKIDA. 



The edges of the mantle free ; no siphon present. Most 

 prominent of this order are the oysters. These are all 

 marine, species being found in all but the colder seas. In 

 these forms the animal lies upon one side, and there results 

 an inequality of the valves. On our east coasts oysters 

 extend from the Gulf of Mexico to Cape Cod. Further 

 north (except in the Bay of Chaleur) they are not found 

 native, but are "planted." The centre of the oyster 

 industry is Baltimore. In 1894 the oyster-fishery of the 

 United States amounted to over $16,000,000. 



In the scallops the shell is fluted, and the valves may be 

 unequal or similar in shape. These molluscs can swim 

 freely by rapidly opening and closing the valves of the shell ; 

 and they are further noticeable from the fact that around 

 the edge of the mantle are a series of rather complicated 

 eyes. The "scallops" of the markets are the adductor 

 muscles of these molluscs, In the pearl-oysters the inner 



