MOLLUSCA — PELECYPODS 2 21 



(some Veneridae, Cardiidae, etc.) the blood is bluish, owing to the 

 presence of haemocyanin. In Venus, Ostrea, Pecten, etc., it is 

 nearly colorless. 



The nervous system consists typically of four pairs of ganglia, 

 cerebral, pleural, pedal and visceral, united by nerve connec- 

 tives. The usual sense organs are otocysts and osphradia. 

 Eyes are at times present on the mantle edge; these range 

 from simple pigmented cells to the complicated eye of Pecten. 



The sexes are usually separate. The order Anatinacea is 

 hermaphroditic, as also the Cyrenidae and certain other isolated 

 genera and certain species of Pecten, Ostrea, Anodonta, etc. 



The valves are held together by one or two strong adductor 

 muscles, passing transversely from valve to valve, and their 

 opening is effected, upon relaxation of this muscular tension, 

 by external or internal ligaments. The external ligament, acting 

 on the principle of a C-spring, is exemplified in Venus (Fig. 89, A). 

 The internal ligament {resilium) functions as would a piece of 

 elastic rubber between a door and its jamb. Its triangular shape 

 (narrower at the hinge than internally) keeps the hinge portion 

 of the valves firm while causing the opposite side to open. It 

 may be single, as in Mactra and Ostrea (Fig. 95, /), or multiple, 

 lying in a series of pits, as in Inoceramus. In some forms, such 

 as Mactra, a special spoon-shaped projection from the hinge, the 

 resilifer, or ligament pit, lodges the internal ligament. The 

 ligament, whether external, or internal, is, in its origin, continu- 

 ous with the shell, being the part of the original embryonic shell 

 which remained uncalcified. 



The dorsal hinge of interlocking teeth necessitates an exact 

 closure of the valves so that even at the approach of danger, 

 when the valves are hastily closed, they will meet edge to edge 

 and thus prevent the access of any enemy to the soft and 

 tempting body portion. 



The two valves are usually similar in size and appearance, 

 one being the '^ mirror image " of the other. They are distin- 

 guished as the right and the left valve, that to the right and 



