204 SYSTEMATIC ZOOLOGY. 



direction. This water brings oxygen to the gills and, 

 through them, to the blood. It also brings minute 

 animals and plants. These are passed on to the labial 

 palpi, fleshy folds, near the mouth, r which are similarly 

 ciliated, and from these organs the cilia force the food 

 into the mouth. 



In the nervous system we always find cerebral, pedal, 

 and visceral ganglia, and frequently parietal ganglia 

 between the cerebral and the visceral, the first being 

 above, the others beside or below, the alimentary canal-. 

 Ears are present, connected with the pedal ganglia; and 

 eyes may be present, either upon the edges of the mantle 

 or at the tips of the siphons. 



The alimentary canal is always provided with stomach 

 and liver. Connected with the stomach a blind sac fre- 

 quently occurs, and in this there may be a peculiar trans- 

 parent rod, the crystalline style, of uncertain use. The 

 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 with the exterior by means of a pair of con- 

 voluted kidney tubules or nephridia (organ of Bojanus) . 



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. ASIPHONIDA. 



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

 prominent of this order are the oysters. These are all 



