PELECYPODA. 29 



have neither eyes nor tentacles, nor a distinct head like Gastropods ; 

 their organs of circulation and respiration are of a rudimentary 

 kind, and they have no lieart. The sexes are separate. Their foot 

 is adapted for burrowing in sand, in which they live and obtain 

 their food, which consists of Foraminifera and minute Bivalves. 

 One species, DentaUum prefiosum, found on the shores of North- 

 West America, was until recently used as money by the Indians. 



Class IV.— PELECYPODA.* 



The Molluscs belonging to this Class have neither head, nor, Cases 

 with a few exceptions, cephalic eyes, nor jaws nor tongue like those of 1^7-204. 

 the other Classes, and are enclosed in a shell which consists of two 

 plates or valves held together on one side of the margin by a horny, 

 elastic substance, called the " ligameiity Bivalves do not creep about 

 in search of food, but find their means of existence in the shape 

 of minute particles, both animal and vegetable, which happen to be 

 contained in the water which they breathe. Some, however, are 

 capable of locomotion by means of a well-developed foot, and a 

 few swim through the water by alternately opening and shutting 

 their valves. The body is enclosed within two lobes of the mantle 

 which line the interior of the valves, and which at their base are 

 firmly attached to the shell, producing on the shell a scar or impres- 

 sion called the " pallial line." The gills are lamellar or leaf -like, and 

 placed on each side of the body. Each gill or ctenidium consists of an 

 axis which is partly attached to the body of the Mollusc. This axis 

 generally gives off two plates consisting of hollow filaments which 

 are parallel with one another, directed downwards towards the 

 ventral side, and in most cases long and refolded upon themselves,- so 

 that each plate becomes in reality a double lamella. In a few 

 instances, however, the filaments are simple and not reflected. They 

 are connected with one another sometimes by microscopic cilia, some- 

 times by vascular junctions, and the dependent and reflected portions 

 (lamellae) of each filament may be connected by " interlamellar 

 vascular junctions." The mouth is merely an oval aperture at the 

 anterior end of the body, and generally furnished on each side with 

 soft thin flaps, or labial palps, which have the function of conveying 



* This name Las priority over the term Lamellihrancliia, often applied to 

 this class of Molhisca, and is also in uniformity with the nomenclature of the 

 other classes, Gastropoda and Cephalopoda. 



