LAMELLIBRANCHIATA 



509 



seen in the laboratory. While the development of the byssus is the 

 most outstanding characteristic of the mussel, it may also be men- 

 tioned that a pair of simple eyes are developed, anterior to the inner 

 ctenidial lamella ; these are an inheritance from the larval mussel. The 

 invasion of the mantle by the generative organs is another peculiar 

 point. 



Pecten (Fig. 374). There are two common British species, P. 

 maximus and P. opercularisy which are commonly known under the 



Fig. 374. Pecten maximus, general anatomy, right valve and ctenidium re- 

 moved. After Dakin. add.u. unstriped and add.s. striped adductor muscle; 

 an. anus; an. auricle; b.gr. byssal groove; ct.' descending and ascending 

 lamella of left ctenidium; e. eye;/, foot; int. intestine; l.p. labial palp; 

 M. mouth; o. ovary; oe. oesophagus; st. stomach; t. testis; ten. tentacles of 

 mantle; ven. ventricle; vm. velum. 



name of "scallops". The animal is found free and it moves not by 

 the ordinary lamellibranch method but by swimming. The two valves 

 are unequal, the right being larger and more convex, and the animal 

 rests on this valve; in P. opercularis the valves are almost equal. In 

 swimming the valves open and close very rapidly, forcing out the 

 water between them. Usually the water is forced out dorsally on each 

 side of the hinge line and the animal moves with the free ventral 



