PHYLUM X, MOLLUSCA (MOLLUSKS) 



The Mollusca as a group possess bilaterally symmetrical, 

 unsegmented bodies enveloped in a sac-like fold, the mantle, 

 which secretes a protective exoskeleton, the shell. The primi- 

 tive bilateral symmetry is often obscured by a secondary torsion 

 of the body. 



They are mostly free-living, able to crawl, swim and burrow. 



The general external divisions of the body are the head (ab- 

 sent in the Pelecypoda), — possessing mouth, various kinds of 

 appendages, and nearly all the organs of special sense ; the foot, 



— a ventral projection of various shapes ; and a dorsal portion, 



— including the viscera and covered by the mantle which se- 

 cretes a shell. 



The digestive system consists of three divisions : (a) an 

 anterior division, consisting of the mouth, buccal cavity (absent 

 in most pelecypods) and oesophagus ; the buccal cavity con- 

 tains the characteristic radula ; (b) a middle division, — 

 the stomach, into which pours the secretion of the liver, an im- 

 portant digestive gland ; (c) a posterior division, — the in- 

 testine, ending in an anus. 



The circulatory system consists largely of closed tubes, al- 

 though there are some open sinuses. It includes a heart con- 

 sisting of one ventricle and usually two auricles (in Nautilus, 

 four) . 



The blood is a fluid with both nutritive and respiratory func- 

 tion. It varies in color among different moUusks, being colorless, 

 bluish from the presence of hsemocyanin, or red from the pres- 

 ence of haemoglobin. 



Respiration is usually effected by ctenidia or gills, — spe- 

 cialized expansions of the ventral surface of the mantle, through 



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