

402 MANUAL OF ZOOLOGY. 



presents the kidney. There is one of these organs on each 

 side of the body, each composed of two sacs separated from 

 those of the opposite side by a venous sinus. Or it may be 

 looked upon as a double organ composed of two bilaterally 

 symmetrical halves. It is situated just below the "pericar- 

 dium " and communicates with it, and also with the mantle- 

 cavity. Though undoubtedly performing the functions of a 

 kidney, the organ of Bojanus is also connected in some cases 

 with reproduction, and it appears to correspond to the " pseudo- 

 hearts " of the Brachiopoda. 



The majority of the bivalves are dioecious, but in some the 

 sexes are united in the same individual. The young are 

 hatched before they leave the parent, 

 and are, when first liberated, free -swim- 

 ming, and furnished with a single or 

 double ciliated lobe, constituting what 

 is called the " velum." A long lash-like 

 filament or flagellum is also often present. 



The velum is wanting in some forms. 



Fig. ax..-Embo of Cockle The muscular system of the Lamelli- 

 (Cardiunt), after Loven. v branchs is well developed. Besides the 

 ciliated un ; ft Fiagei- muscular marg i n o f t h e mantle, and the 



muscles of the siphons (when these ex- 

 ist), there are also present other muscles, of which the most 

 important are the muscles which close the shell and those 

 which form the "foot" (figs. 208 and 209,7). The "foot" 

 is present in the majority of bivalves, though it is not such a 

 striking feature as in the Gasteropoda. It is essentially a mus- 

 cular organ, developed upon the ventral surface of the body, 

 its retractor muscles usually leaving distinct impressions or scars 

 (the "pedal impressions") in the interior of the shell. In 

 many, the foot, which is usually compressed, and often sickle- 

 shaped, subserves locomotion, but in the attached bivalves it 

 is rudimentary, and in others (as in the Scallops) locomotion 

 is effected by the alternate opening and closure of the valves. 

 In some such as the ordinary Mussel the foot is subsidiary 

 to a special gland, which secretes the tuft of silky threads 

 ("byssus") whereby the shell is attached to foreign objects. 

 This gland secretes a viscous material, which is moulded into 

 threads by grooves on its external surface. 



The valves of the shell are brought together by one or two 

 muscles, which are called the "adductor muscles" those 

 bivalves with only one being called Monomyaria, whilst those 

 which possess two are termed Dimyaria. In most there are 

 two adductor muscles (fig. 208, a a') passing between the inner 



