36 



Guide to the Mollusca. 



Class IV. LAMELLIBRANCHIA. 



{Cases 137 to 204.) 



The Lamellibranchs. or Bivalves, include the Oysters, Cockles, 

 Mussels, &c. They have no jaws and no radula, and the head and 

 its associated sense-organs are little developed. The body is 

 enclosed between two deep lobes of the mantle which secrete a 

 bivalve shell, the two halves of which are connected by a horny 

 elastic structure, the ligament, and, in addition, are usually 

 articulated together by a series of interlocking ' teeth '. The 

 closing of the valves is effected by a pair of adductor muscles, 

 sometimes reduced to one. When the muscles relax, the valves 



Fig. 27. 



A. Protobranchia ; B. Filibranchia ; C. Eulamellibranchia: J). Septibranchia. 



m. mantle; v. body: /'. foot; c. outer gill-lamella; i. inner gill-lamella ; 

 e'. reflected portion of outer lamella ; /". reflected portion of inner lamella ; 

 $. gill modified to form a septum. 



From Tlit Cambridge Natural History (by permission of Messrs. Mac- 

 millan & Co.). 



gape as a result of the elasticity of the ligament. In many 

 Lamellibranchs a special gland in the foot secretes the byssus. 

 a bundle of hard threads that serve to fix the animal down on the 

 bottom. The Lamellibranchia are all aquatic and most of them 

 are marine. They usually burrow in sand or mud, or may be 

 permanently fixed ; some (Teredo, Pholas, &c.) excavate burrows 

 in wood or stone. A few crawl about (e. g. Cyclas) or progress by 

 short leaps (e. g. Tellina) or swim by abruptly opening and closing 

 the valves of the shell (e. g. Lima, Pecten). They do not seek their 

 food, but live on microscopic organisms and organic debris, which 

 are swept towards the mouth by the cilia on the gills. 



The Lamellibranchs are very uniform in structure, and this 

 makes them very difficult to classify ; the most satisfactory 

 system is based on the modifications of the gills, which are 

 suspended between the body and the mantle-lobe, one on each 



