498 CONCHIFEEA. 



into a kind of little sucker, to the foreign substance whereunto it wishes 

 to adhere, and fastens it securely. Having accomplished this, the foot 

 is retracted ; and the thread, of course, being drawn out of the furrow 

 where it was secreted, is added to the bundle of byssus previously 

 existing, all of which owed its origin to a similar process. 



(1305.) Sometimes, instead of the numerous thin filaments met with 

 in the Mussel, the byssus consists of a single, thick, horny stem ; while 

 in other cases, as, for example, in Pinna, the threads are so numerous, 

 soft, and delicate, that they are not unfrequently spun like silk, and 

 manufactured into gloves and other small articles of dress not unfre- 

 quently met with in the cabinets of conchologists. 



(1306.) Taking a more general view of the Conchiferous Mollusca 

 than we have hitherto done, we shall now proceed to consider the 

 mechanism for opening and closing the valves of the shell in which they 

 reside, an operation effected in a very simple and elegant manner. 



(1307.) The shells are connected posteriorly by means of a hinge, 

 differently constructed in different species. In the Oyster we have an 

 instance of the most simple kind of junction. In these Mollusca, a 

 mass of elastic ligament, composed of perpendicular and parallel fibres, 

 is interposed between the posterior edges of the shell, and so disposed 

 that, by closing the shell, the ligamentous mass is forcibly compressed, 

 while at the same time its resiliency is such that, immediately the 

 compressing power is withdrawn, it expands, and thus forms a simple 

 spring calculated to keep the valves apart and cause their separation to 

 a greater or less extent. 



(1308.) The antagonist to this elastic force is the adductor muscle 

 (fig. 247, c), a fleshy mass of very great strength, the fibres of which 

 pass directly from one valve to the opposite. The adductor muscle, 

 although in this case single, consists of two portions of different texture 

 (fig. 248, I, m) ; so that it would appear to be formed by two muscles 

 closely approximated so as to compose a single powerful mass adapted 

 to keep the valves in contact, with a force proportioned to its massive 

 size. All those species having a single muscular mass, such as the 

 Oyster and Pecten, have been grouped together by conchologists under 

 the general name MONOMYAEIA, while another and more numerous 

 division, DIMYARIA, is characterized by having two adductor muscles, 

 distinct and widely removed from each other. The Mussel-tribe and 

 many others are examples of this arrangement, which is represented in 

 subsequent figures. 



(1309.) Simple as the structure of the hinge is in the Ostracea, in 

 other Bivalves it frequently exhibits far greater complexity, and the 

 opposed valves present prominent elevations and deep fossae which lock 

 into each other, and thus form a very secure articulation of great strength 

 and solidity. In such cases the arrangement of the elastic ligament for 

 opening the valves is slightly modified, being placed externally instead 



