76 Dynamic Theory. 



doubtless, of minor signification. Their sizes are greatlj r varied. Some 

 specimens in the Trenton period were fifteen feet long and nearly a foot 

 in diameter. 



There is one other class of Mollusks; viz. , the Lamellibranchiata 

 ( plate gilled) or conchifers. They are also called acephals or headless 

 mollusks, which only means the head is not distinct aad separate, and 

 the3 T are also commonly known as bivalves. They are divided, by Car- 

 penter, into twenty-one families. Living and dead, the species will run 

 up into the thousands. The}" include the oysters, cockles, mussels, scal- 

 lops, clams, &c. The mouth of the bivalve is at one end of the shell the 

 short end, the posterior part of the animal is at the long end, the shells 

 are right and left, his back is against the hinge, and his belly at the 

 open edge. On this side is his tough muscle, called the foot, on which 

 he stands and by which he shoves himself along.' 



* A description of afresh water mussel, Anodonta C3 T gnea, will answer 

 as a type of the Lamellibranch. The mantle or pallium almost envelops 

 the rest of the body. Inside the mantle, on each side of the body 

 toward the rear, are two gills (four in all). Between the gills, but for- 

 ward of them, is the muscular foot. Above the foot, near the hinge, 

 are the heart, stomach and other intestines. There is an orifice at the 

 bottom, when the shell is open, called the inhalent siphon, into which 

 water, laden with diatoms and other food, enters and passes on to the 

 space about the gills. From there the water, after contact with the 

 gills, passes out through the exhalent siphon, another orifice at the 

 rear, while the food matters are swept forward to the mouth by cilia, 

 which line the cavity. There is a gullet, a stomach, a long intestine 

 which is coiled and doubled up a good deal, and passes through the 

 pericardium and heart and finally reaches the rectum and anus. 



The blood is colorless and contains colorless corpuscles which resem- 

 ble those of man, and present the same amoebiform movements. There 

 is a nervous system with three pairs of yellow ganglia one in the head, 

 one in the foot and the other on the posterior of the two adductor mus- 

 cles, which are used to hold the shells shut. In the foot are developed 

 auditory vesicles ears. 



The sexes are distinct. The ova of the female are conveyed, when 

 mature, to her gills and there lodged. The male spermatozoa, which 

 are minute, rod-like bodies, are thrown off into the water and may 

 chance to fall into a current that will bring them to the feminine gills, 

 where they meet and impregnate the ova, which are spherical with a 

 tubular opening at one end. When hatched, the embryo has a bivalve 

 triangular shell, a hinge along one side, and the opposite points open 

 and curved in toward each other. When these young Glochtdia, as they 



* Huxley's Biology. 



