THE PHYLUM MOLLUSCA 



pelecypods, Pecten, the scallop (Fig. 13.4), is noteworthy for its possession of 

 numerous well-developed eyes along the margins of the mantle. These eyes 

 are not capable of image formation but are differentially sensitive to gradual 

 and to rapid changes in illumination. 



Activities related to metabolism and responsiveness in clams and mussels 

 resemble, so far as they are known, those of other animals of a comparable 

 level of complexity. Most pelecypods are sluggish animals, with a compara- 

 tively low rate of metabolism, relatively inactive habits, and a simple sensory- 

 neuro-muscular system. 



Reproductive Organs, Reproduction, and Development. In most clams 

 and fresh-water mussels the sexes are separate, although in successive breed- 

 ing seasons the same individual may function first as a male, then as a 

 female. The ovaries and testes are extensively branching structures em- 

 bedded in the visceral mass, among the coils of the intestine. Their ducts 

 open into the suprabranchial chambers near the nephridiopores. The repro- 

 ductive processes in the marine clams, which will be described first, are 

 vastly different from those of many fresh-water mussels. In Venus mercenana, 

 the common marine hard-shell clam or "quahaug," the gametes are shed into 

 the open water of the sea, reaching the exterior through the excurrent 

 siphon. Fertilization occurs externally, and the zygotes develop into free- 

 swimming, ciliated trochophore larvae. These soon transform into a second 

 larval stage, the veliger, which develops the precursors of the mantle and 

 begins to secrete the shell. Veligers by gradual changes become young clams, 

 which at first are fastened to the substratum by a thread, the byssus. They 



Fig. 13.4. A scallop, Pecten irradians, showing; sensory tentacles and eyes at the edge of the 

 mantle. (Photograph by Eugene S. Clark, Jr.) 



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