LESSON 46.] NUTRITION IN THE LAMELLIBRANCHIATA. 



161 



FIG. 260. 



Nutrimental organs, Oyster. 



tween the lobes of the mantle, being almost entirely occupied with 

 the branchial laminae (gills), which 

 are four in number, equally divided, 

 and placed on each side of the vis- 

 ceral mass. The mouth is furnished 

 with two long, tapering, fleshy tentac- 

 ula (a, Fig. 260), and is continued 

 by a short oesophagus to an expanded 

 stomach, into which numerous rami- 

 fied hepatic follicles empty them- 

 selves (6). The intestine (c), after 

 describing a remarkable convolution, 

 ascending to the upper part of the 

 stomach, which it crosses, is contin- 

 ued along the interspace of the bran- 

 chiae towards their extremities which 

 are farthest from the mouth, where it terminates. The ovarium sur- 

 rounds the intestinal convolutions, and forms, with the liver, the chief 

 part of the visceral mass. 



PTEEOPODA. 



703. In all the Mollusca provided with a head, the capability of 

 locomotion is always considerable ; whereas in the acephalous (head- 

 less) molluscs it forms the exception not the rule. 



704. The pteropods* are provided with two fin-like muscular ex- 

 pansions, attached to the sides of the neck, which, from their resem- 

 blance to wings, suggested pteropoda (iving-foot) as the name of the 

 class. Some of these animals are provided with a light and delicate 

 seinitransparent shell. 



705. In the Hyalcea, it resembles a bivalve shell, of which the 

 two valves have been cemented together at the hinge, leaving a nar- 

 row fissure in front, and at the sides. In Cleodora, the two plates 

 of the shell are united together along the sides, as well as at the 

 base, leaving an opening only in front. Some (Clio, and others) are 

 naked, or unprovided with shells. 



706. All the pteropods are of small size ; they float in the open 

 sea, often at a great distance from shore, and in latitudes that suit 

 them they swarm in such incredible numbers, as to discolor the sea. 

 The huge whale, with its remarkably small oesophagus, chiefly sub- 

 sists on the Clio borealis, and Limarcina^ both of which species 

 abound in the northern seas. 



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