CONCHIFEKA.- 



-MOLLUSCA. CoKCHiFEBA. 



359 



(Peden), the foot is either very small or is nearly 

 wanting; in Nucula and some others it is deeply cleft, 

 and is capable of being expanded into a disc, like that 

 of the snails ; and in the burrowing species the foot is 

 strong and stout, and adapted for boring vertically into 

 the sand or mud. 



The body of the Conchifers is entirely covered by a 

 twolobed mantle, which envelopes it as a book is in- 

 closed within its boards, and is protected on each side 

 by a shelly valve. It generally incloses a pair of gills 

 which are placed on the sides of the body, and are each 

 composed of two large membranous laminre which, when 

 examined by the microscope, appear to be a complete 

 network of bloodvessels fringed with vibratile cilia. 

 The water in which they live is filtered through these 

 gills, and thus whatever particles the current brings, 

 are collected on their surface and conveyed to the 

 mouth. The mantle which covers the body of the 

 animal is usually more or less united at its margins ; 

 sometimes being produced into long siphons or tubes 

 (fig. 228, Venus verrucosa), at others merely having two 

 holes (fig. 220, Anoclon anatinus) for the passage of 

 the current of water. The various modifications which 

 the end of the mantle assumes, aflord some of the best 

 characteristics for dividing the species into orders. The 

 mantle secretes the shell (which, as we have stated, 

 consists of two valves), and lines the interior, to which 

 it is attached by a pair of adductor muscles. The 

 valves are always placed one on each side of the animal, 

 and are united together on their dorsal edges by a liga- 

 ment of greater or less strength. Within this ligament, 

 in a cavity, there is placed an elastic cartilage; and the 



two valves are articulated together by a hinge furnished 

 with interlocking teeth. The use of the ligament is to 

 keep the two valves together, whilst that of the carti- 

 lage is to keep them open without any exertion to the 

 animal while waiting for its prey, or moving from one 

 place to another by means of its foot. The valves are 

 shut, or brought together, by means of two muscles which 

 are attached to their internal surface, and are hence 

 called adductor muscles. In the greater number of 

 species these two muscles are quite distinct, one placed 

 near the anterior, the other near the posterior apertures, 

 whilst in others there is apparently only one ; the two 

 being so approximated, as to make it appear as if there 

 were only one. Lamarck and other conchologists 

 adopted this disposition of tliese muscles as characters 

 for the systematic ariangement of the bivalve shells; 

 but Cuvier and other later authors have preferred to 

 base their systems upon different characters altogether. 

 The majority of the bivalve mollusca are believed 

 to be moncecious or unisexual ; but as Dr. Johnston 

 observes, the exceptions are yearly increasing under 

 the dissections of the comparative anatomist, who has 

 proved several of them to be bisexual, "although the 

 distinction between the male and female does not appear 

 in any external character either of the animal or shell." 

 A few of the genera in this class are viviparous, the 

 ova being carried from the ovary to be deposited in 

 the interstices of the external plates of the gills, where 

 they are retained till the young are hatched and 

 arrive nearly to maturity, the ova remaining there in 

 some of the genera for several months after leaving 

 the ovaria. 



SYSTEMATIC ARRANGEMENT OF CONCHIFERA. 



The principal character adopted in the present arrange- 

 ment, that of Dr. Gray, is the structure of the mantle. 

 We have stated above that the margins of the mantle 

 are either produced into siphons, or that there are only 

 two holes in it, for the passage of the water to the gills 

 for aerating the blood. The species of this class have 



Fig. 228. 



Venus vemicosa. 



in consequence been divided into two large groups or 

 sub-classes — I. Siphoiwphora, in which the mantle 



leaves or lobes are connected, and produ'^ed into 

 two elongated siphons or tubes, through which the 

 water is admitted (fig. 228, Venus verrucosa). II. 

 Asiphonophora, in which the mantle lobes are mostly 

 free, or when united, are pierced with one or two holes, 



Fig. 229. 



Anodon anatinus. 



instead of being produced into siphons, for the admission 

 of water (fig. 229, Anodon analinus). 



