242 



SYSTEMATIC ZOOLOGY 



*i 



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■ 



with a mantle which lines the shell. This 

 mantle develops as two folds, a right and 



a left on the dorsal 

 side of the body, and 

 «> these folds later fuse 



on the ventral side ; 

 : this condition shows 

 a relationship to the 

 succeeding class. 

 The mouth is at the 

 tip of a process called 

 the proboscis, and is 

 provided with a rad- 

 ula, in this respect 

 resembling" the Gas- 

 tropoda. On each 

 side of the proboscis 

 is a tuft of slender 

 tentacular out- 

 growths, which may 

 serve in securing 

 food and also in res- 

 piration, as there are 

 no special organs of 

 respiration. The foot 

 is nearly cylindrical, 

 with a rounded coni- 

 cal end, and the sexes 

 are separate. The 

 shells vary in length 

 from two or three to 

 ten centimeters; 

 Dentalium ( Fig. 250) 

 is the commonest 



Fig. 250. Dentalium entails. Anatomy; enlarged, i, shell 

 and mantle cut away from the right side of the body; 

 2. animal removed from shell and viewed from ventral 

 A, anus; G, germ-gland ; GC, cerebral ganglia; GO, gen- 

 italopening; GP, pedal ganglion ; GS, sympathetic ganj 

 7, intestine; A", respiratory network; L, liver; .1/, mantle; 

 A/A', oral proboscis; Mu, muscle; A', excretory organ; 

 P, foot; SD, dorsal blood-sinus; SK, pharynx; SV, ven- 

 tral blood-sinus; T, tentacles; 11, tentaculiferous lobe. 

 (Photographed from a Leuckart-Nitsche wall chart by the 

 author; permission of Dr. C. Chun.) 



genus. 



CLASS IV. LAMELLIBRANCHIA OR PELECYPODA 



The Lamellibranchia (Lat. lamella, leaf, and branchia, gill), 

 or Pelecypoda (Gr. ireXeicvs, hatchet, and irovs, foot), sometimes 



