CONCHIFERA. 



701, 



Lamarck has given the title of Di/nyaires to all 

 the mollusca having two adductor muscles, a 

 character which he has invested with a consi- 

 derable degree of importance, because it is con- 

 stantly proclaimed by the interiors of shells, 

 upon which the impression left by these mus- 

 cles is very distinctly seen (a, b y fg. 367). One 

 of these muscles, the anterior, diminishes gra- 

 dually as we descend in the series of the Con- 

 chifera; in the family of Mytilacea it only 

 exists in a rudimentary state (a, Jig- 353); and 

 after these it disappears entirely. In propor- 

 tion as the anterior muscle disappears, the pos- 

 terior one increases in size, and approaches 

 more nearly to the middle of the valves. When 

 no farther trace of anterior muscle can be dis- 

 covered, the posterior muscle continues singly 

 (k,Jig. 348), and the mollusca having a single 

 muscle, very distinct from the former which 

 have two, have received the name of Mono- 

 myaires from M. Lamarck. 



Poli, however, has shewn that the muscle of 

 the Monomyaria consists in reality of two por- 

 tions, readily separable from one another, and 

 even differing considerably in their appearance. 

 This leads us to presume, with every show of 

 reason, that the single muscle in the Mono- 

 myaria is the result of the approximation of the 

 two muscles, which are parted in the Dimyaria. 

 This fact would incline us to regard the num- 

 ber of the muscles as a matter of but small im- 

 portance in the classification of the conqhiferous 

 mollusks, and we may suppose that it was with 

 such inductions before him that Cuvier was 

 led to attach such slight significance to the 

 division of these animals proposed by La- 

 marck. 



The organ denominated foot in the acepha- 

 lous mollusks is a part which presents very 

 different forms, and is destined to locomotion. 

 This part is particularly well developed among 

 the Dimyaria, and we shall pass in rapid re- 

 view its most general features. 



The foot (b y fig. 346) is usually situated at 

 the anterior and middle part of the abdominal 

 mass, and is directed forwards. It is so placed 

 as to hide the mouth in a deep sinus between 

 its base and the anterior adductor muscle. In 

 those conchiferous mollusks in which the lobes 

 of the mantle are united through a great por- 

 tion of their circumference, the foot is com- 

 monly very small and meiely rudimentary ; it 

 then forms a kind of little nipple projecting 

 from about the middle of the abdominal mass, 

 a form which is very distinctly seen in the 

 Mya, Saxicava, &c. In others of these mol- 

 lusks the foot, more anteriorly situated, is ex- 

 tremely short, broadly truncated, and similar to 

 a cupping-glass; this configuration is observed 

 in the Pholadia. In proportion as the foot be- 

 comes more free, the lobes of the mantle are 

 distinct from one another, the foot becomes 

 flattened and elongated in the form of the 

 human tongue, and is subservient to motion 

 by digging a hole or furrow in the sand into 

 Mhich the animal sinks. This form of the 

 locomotory organ is met with more especially 

 in the Tellina, the Donata, and a very great 



number of other genera, the shells of which are 

 more or less flattened. Lamarck had attached 

 some consequence to the shape of the organ of 

 locomotion, and Goldfuss has proposed a clas- 

 sification based upon the modifications pre- 

 sented by this organ ; but the groups establish- 

 ed in accordance with such considerations are 

 in reality of no importance ; the several forms 

 proper to the organ pass too insensibly one into 

 another to make it possible to say where one 

 terminates and another begins ; the boundary 

 between one family and another, with a few 

 rare exceptions, is altogether indefinite. In the 

 present day, consequently, naturalists no longer 

 admit into their methods of arrangement the 

 groups established by Lamarck under the 

 names of Tenuipeda, Crassipeda, &c. 



The foot exists developed in a greater or less 

 degree in the whole of the Dimyaria. If in 

 some species it is found merely rudimentary, it 

 is yet never altogether wanting in any member 

 of this first division of the Conchifera. The 

 organ is also met with in a very considerable 

 number, but by no means in the whole of 

 the Monomyaria, and the presence or absence 

 of the foot might be taken as the basis of a 

 division of this great family into two series, in 

 the one of which the foot was rudimentary but 

 present, whilst in the other it was no longer to 

 be found. 



Whatever the form of the locomotory organ, 

 and whatever the degree of its development, it 

 is always organized in the same manner. It is 

 essentially composed of several planes of mus- 

 cular fibres (1, 2, 3,//g. 347), which by their 

 various courses and interlacements enable it to 

 perform a great variety of different motions, 

 either in part or as a whole. When the foot is 

 short or vermiform, its mass is entirely muscu- 

 lar from the apex to the base. It is at the base 

 that the fleshy fibres separate into two fasciculi 

 (4, 5, Jig. 347), which, after having circum- 

 scribed the visceral mass, proceed backwards, 

 where they are attached to each valve of the 

 shell near the implantation of the posterior ad- 

 ductor muscle in the Dimvaria; and towards 



M 



the superior part of the valves, and occasion- 

 ally in the interior of the hook, or incurved 

 part of the shell in the Monomyaria. 



In the Conchifera denominated Lamellipeds 

 and Creu$iped by Lamarck, in a word, in 

 the whole of the Conchiferous mollusks in 

 which the foot constitutes a principal part 

 of the body, this organ presents remarkable 

 differences in its composition and its rela- 

 tions with the internal organs. It is then 



<j 



formed of two lateral planes of fibres, uniting 

 and blending together near the free edge. 

 These two planes, more or less separate ac- 

 cording to the general form of the animal, 

 have between them an internal space, within 

 which is included a considerable portion of the 

 visceral mass. In the generality of conchife- 

 rous mollusks furnished with a large foot, it is 

 here that a portion of the liver is situated, the 

 greater part of the intestinal canal, and a 

 notable portion of the ovary. These organs are 

 bound down in the place they occupy, and tho. 



