416 MARINE INVERTEBRATES 



The pelecypods have no head or tentacles, but the place of the 

 latter is taken by two pairs of triangular flaps upon either side of 

 the mouth. These are called the labial palps, and they no doubt 

 operate by means of their ciliated surface to keep in motion the 

 current of water over the gills and to the mouth. 



As a rule, pelecypods have no eyes, but when eyes are present 

 they are situated upon the edge of the mantle. The eyes are not 

 highly developed organs, but they nevertheless appear to be very 

 sensitive to light, for the bivalves which are so endowed will, when 

 kept in aquaria, instantly close their shells when the shadow of a 

 fish passes over them. 



The heart lies in the pericardium and is situated dorsally, just 

 in front of the posterior adductor muscle. It has a median ven- 

 tricle and two lateral auricles, each connected with the branchiae 

 upon its respective side. It seems a curious economy that passes 

 the intestine through the ventricle, but this is the case in the 

 great majority of pelecypod genera. The vascular system is closed 

 and is of a high order. It is probable that " breathing " takes 

 place upon the inner mantle surfaces as well as in the gills them- 

 selves; for these surfaces are crowded with excessively fine and 

 delicate capillaries, which bring the blood very near to the water. 

 This power of " breathing by the skin " is quite characteristic of 

 the Mollnsca in general. 



Having once familiarized one's self with the more prominent 

 features of pelecypod organization, it becomes very interesting 

 to examine anatomically every new form that is captured. Sur- 

 prises are always in store, and sometimes the peculiar juxtaposi- 

 tion and relative sizes of the organs are a trifle confusing ; but, 

 upon the whole, the bivalve organization is simple and lends 

 itself far more readily than does that of the gasteropods to satis- 

 factory examination. Some of the more striking anatomical 

 peculiarities are reserved for mention in the notes upon the 

 various genera and species common upon our shores. 



THE PELECYPOD SHELL 



As has been seen, the testaceous covering of the Pelecypoda 

 consists of two valves or shells which fit more or less closely and 



