CHAPTER XXXVII. 



MOLLUSCA, MOLLUSCOIDEA AND TUNICATA. 



THE Mollusca are soft-bodied animals without internal skele- 

 ton, and without segments or joints. The body is covered loosely 

 with a soft contractile skin which usually secretes a calcareous 

 shell as a protective covering. Most of the mollusca are aquatic 

 animals or live in damp places, and many of them are perma- 

 nently fixed to solid bodies, having no organs of locomotion. A 

 mass of skin and muscular tissue called the foot is developed on 

 the ventral surface of many of this group, by means of which 

 slow movements may be accomplished. 



The alimentary canal consists of the oesophagus, stomach 

 and intestine, and the rectum. Attached to the digesting por- 

 tion of the canal there is quite a complete liver, and kidneys are 

 always present. There is a vascular system with a heart driving 

 the blood through the body, but the system is not completely 

 closed from the body cavity. Respiration is carried on through 

 the general surface of the body, also by means of branchia or 

 gills, and occasionally by means of lungs. 



The nervous system consists of a pair of dorsal ganglia lying 

 above the oesophagus, with pedal and visceral nerve cords; and 

 sometimes it would be better described as consisting of a cerebral 

 ganglion, a pedal ganglion and a ganglion supplying the walls of 

 the body. The cerebral ganglia supply the sense organs, which 

 usually consist of eyes, tactile and auditory organs. But no 

 general description can do justice to this large and interesting 

 group of animals; some notice must betaken of the more im- 

 portant subdivisions. 



The Lamellibranchiata are mollusca without distinct head, that 

 breathe by means of plate-like branchia or gills, and have bivalve 

 shells. To this class belong the oyster, the salt water and the 



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