584 THE INVERTEBRATA 



from the byssiis pit and by means of it the animal is firmly attached 

 to stones or other mussels. But mussels, particularly when young, 

 creep about both by using the cup at the tip of the foot as a sucker and 

 also by forming a path of threads along the surface of the substratum, 

 as can be easily seen in the laboratory. While the development of the 

 byssus is the most outstanding characteristic of the mussel, it may 

 also be mentioned that a pair of simple eyes are developed, anterior 



ten. 



Fig- 399- Pecten maximus, general anatomy, right valve and ctenidium re- 

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

 an. anus; au. 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. 



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. In the breeding season the aeration of blood in the 

 mantle is reduced and the plicate canals (Fig. 396) become the chief 

 organ of respiration. 



Pecten (Fig. 399). There are two common British species, P. 

 maximus and P. opercularis, which are commonly known under the 

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



