GENERAL ZOOLOGY 



Fig. 13.8. Variously modified bivalves. A, two sedentary species: an ovster, Crassoslrea, en- 

 crusted with jingles, Anomia simplex. B, C, and Z), burrowing and boring forms: B, Mya 

 arenaria; C, Barma truncata; D, Emu directus. Forms like Mya and Ensis, burrowing in soft 

 sand and mud, possess enlarged and elongated siphons and somewhat reduced shells, with well- 

 developed feet. Barnea, which bores into harder mud or peat, has a much reduced foot, highlv 

 specialized shells, and elongate siphons. [A, C, and /), photographs bv Eugene S. Clark, Jr.; 

 /?, photograph by Charles Walcott.) 



the foot and the siphons. The anterior ends of the valves often develop 

 special cutting edges, as in Barnea (Fig. 13.8). Perhaps the most highly 

 specialized and aberrant of the boring bivalves is the shipworm type, repre- 

 sented by Teredo and related forms. This animal derives its common name 

 from its extremely elongate, worm-like appearance, and from the fact that it 

 burrows into the planking of wooden boats or any other submerged wood not 

 artificially protected. The siphons, which protrude from the burrow poste- 

 riorly, and the greatly reduced bivalve shell, which constitutes the anterior 

 boring mechanism, are marks of the pelecypod nature of this bizarre clam 

 (Fig. 13.9). 



For comparison with other mollusks, the members of the class Pelecypoda 

 are characterized by lateral compression, by the bivalve shell, and by modi- 



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