MARINE MOLLUSCS 217 



umbones are separated by a diamond-shaped ligament. The foot 

 of the animal is heeled that is, it has a creeping surface that extends 

 backwards as well as forwards ; the mantle is furnished with minute 

 eyes (ocelli), and the animal has two distinct hearts. We give a 

 figure of this peculiar shell, and the other British members of the 

 same genus, though varying more or less in form, may be recognised 

 at once by the same general characteristics. 



In the same family we have the small nutshells (genus Nucula), 

 which are often dredged up from deep water in large numbers ; and 

 the elongated shells of the genus Leda, also inhabitants of deep 

 water ; and, as before stated, the affinities of all may be readily 

 established by the characteristic nature of the teeth. 



We now pass on to the family of Mussels (Mytilidce), of which 

 the common Edible Mussel (Mytilus edulis} is a typical species. 

 In this interesting group the shell is oval or elongated, with equal 

 valves, and is covered with a dark-coloured epidermis which is often 

 distinctly fibrous in structure. The umbones are at the anterior 

 end of the shell, which end is usually very narrow and pointed, 

 while the posterior is broad and rounded. The hinge has small 

 teeth or none, and the ligament, which is long, is internal. The 

 shells of mussels consist of two distinct layers ; on the inner, which 

 is often of a most beautiful pearly lustre, may be traced the simple 

 pallial line and the impres- 

 sions of the small anterior 

 and the large posterior 

 muscles. 



The mantle lobes of the 

 animal are united only at 

 a point between the two 

 siphonal openings. There 

 are two elongated gills on 

 either side, and the foot is 

 thick and more or less FIG. 149. Mytilus edulis 



grooved. 



Mussels inhabit salt, brackish, and fresh waters, generally 

 attaching themselves by means of a silken byssus, but sometimes 

 concealing themselves in ready-made holes, or in burrows of their 

 own ; and some even hide themselves in a nest which they prepare 

 by binding together fragments of shells or sand. 



The edible mussel, which forms such an important article of 

 diet, especially among the poorer classes in our large towns, may 



