PHYLUM 6. 

 MOLLUSCA/ 



Bilaterally sj^mmetrieal, unsegmented animals in which the body is 

 usually encased in a calcareous shell, and is made up of four parts, the 



visceral mass, the head, the mantle, 

 and the foot (Fig. 740). 



External Structure.— The vis- 

 ceral mass (1), which forms the 

 greater part of the body, contains 

 the viscera, closely compacted to- 

 gether. At its anterior end is the head 

 (13), with the mouth and the most 

 important organs of special sense, and 

 from its dorsal side the mantle falls 

 as an extensive integumental fold 

 over its sides and more or less com- 

 pletely envelopes it (2), while from 

 its ventral side extends the foot 

 (11), a muscular mass by means of 

 which the animal moves. 



In the pelecypods and scapho- 

 pods the head is wanting, the mouth 

 being at the forward end of the 

 visceral mass. In the same ani- 

 mals also the mantle is a double 

 fold, one half falling to the right 

 and the other to the left of the 



Fig. 740 — Diagrams of the types of 

 molluscan structure iHertwig). A, the 

 cephalopod type ; B, the gastropod type ; 

 C, pelecypod type. 1, visceral mass ; 2, 

 shell and mantle ; 3, mantle cavity ; 

 4, anus ; 5, siphon ; 6, visceral gan- 

 glion ; 7, pedal ganglion ; 8, arms on 

 head ; 9, mouth ; 10. cerebral ganglion ; 

 11, foot ; 12, tentacles ; 13, head. 



* See "Manual of Conchology," by G. W. Tryon and H. A. Pilsbry, Philadelphia, 

 1878 to date. "Manual de Conchologie," etc., by P. Fischer, Paris, 1880-1887. 

 "Structural and Systematic Conchology," by G. W. Tryon, Jr., 1882. "Mollusca," 

 by J. S. Kingsley, Stand. Nat. Hist., Vol. 1, 1888. "A Preliminary Catalogue of the 

 Shell-bearing Mollusks," etc., by W. H. Dall, Bull. No. 37, U. S. Nat. Mus., 1889. 

 "Mollusca," by H. Simroth, Bronn's Kl. u. Ord., Vol. 3, 1892 to date. "Mollusks," 

 by A. II. Cooke, Camb. Nat. Hist., Vol. 3, 189.''». "Molluskan Fauna of Philadelphia," 

 by M. Schick, Nautilus, Vol. 8, p. 133, 1895. "Molluscan Fauna of Western New 

 York," by F. C. Baker, Trans. Acad. Sci., St. Louis, Vol. 8, p. 71, 1898. "West Amer- 

 ican Shells," by J. Keep, San Francisco, 1904. "Mollusca," by Paul Pelseneer in "A 

 Treatise on Zoology," edited by E. R. Lankester, 1906. "The Shell-book," by Julia E. 

 Rogers, 1909. "A Biological Survey of Woods Hole and Vicinity," by F. B. Sumner 

 and others. Bull. Bur. Fish., Vol. 31, 1911. 



478 



