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 branchie 
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 Mollusca 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 
