CONCHIFERA. 



705 



exist, nervous branches arc likewise discovered, 

 destined for them, one for each of the retractor 

 muscles, and one for each of the siphons. 

 The posterior part of the nervous system of 

 the Dimyaria is so considerable in comparison 

 with the anterior part, that some anatomists 

 have maintained that the title of brain should 

 be given to the posterior ganglions, conceiving 

 them to be of much greater consequence in 

 the organization of these animals, and of more 

 avail in regulating their functions than the 

 anterior ones. 



In the Monomyaria the nervous system is in 

 general less perfectly developed than in the 

 Dimyaria. It is not quite symmetrical, and 

 the posterior are not larger than the anterior 

 ganglions. The nervous cords, too, are much 

 more slender, and not nearly so easy of de- 

 monstration as in the Dimyaria; it was not 

 without difficulty that we discovered them in 

 the common Oyster, the Pecten and the Spon- 

 dylus. Poli has said nothing upon the nervous 

 system of these genera. Our own researches 

 in quest of it were perfectly fruitless at first ; 

 but having bethought us that in the Dimyaria 

 the nervous cords" of the labial palps were 

 always to be discovered without difficulty, we 

 sought for the same filaments in the Mono- 

 myaria, and were lucky enough to find them ; 

 these led us by-and-bye to the anterior gan- 

 glions, and by degrees to the detection of the 

 entire nervous system. The anterior ganglions 

 in the Monomyaria are extremely small ; they 

 send a principal filament to each of the palps; 

 a cord proceeds from them to the anterior part 

 of the mantle which covers the mouth ; another 

 runs from the ganglion of one side to that of 

 the other, passing above the oesophagus ; and 

 from the posterior angle several branches are 

 detached to the liver, the stomach, and the 

 branchiae. Among these there is one, and 

 sometimes two, which, resting on the internal 

 aspect of the central muscle, bend obliquely 

 over its surface, and finally unite occasionally 

 to form a small posterior ganglion. This gan- 

 glion sends branches to the heart, to the ovary, 

 and to the posterior parts of the mantle. The 

 parallel cords traverse the thin part of the man- 

 tle, sometimes radiating in a slight degree, and 

 divide into numerous branches within its thick 

 margin and the tentacular ciliary processes that 

 fringe it. There is one among the monomyary 

 genera, the nervous system of which we have 

 not been able to study with due attention; this 

 is the genus Lima. From what we have seen 

 of it, however, it would appear that the ner- 

 vous system in this genus is every way as 

 perfectly symmetrical as in the Dimyaria. But 

 before admitting this as a fact definitively, it 

 were necessary to have verified its accuracy at 

 least several times, which we have as yet had 

 no opportunity of doing. 



When we consider the great simplicity of 

 the nervous system of the acephalous mol- 

 lusca, we can only conceive these animals 

 endowed with sensibilities extremely obscure, 

 and with instincts extremely limited. No 

 especial organ of sense can be detected among 



them, unless perhaps it be that of touch, which 

 appears to reside in every part of the body and 

 of the mantle, and probably also the sense of 

 taste, of which in all likelihood the maxillary 

 palps are the organ. The manner of existence 

 of these animals is in perfect accordance with 

 the great simplicity of their nervous system. 

 Many genera live attached to submarine ob- 

 jects, either by the shell immediately or by 

 means of a byssus, taking no pains to avoid or 

 to protect themselves from danger, and giving 

 no sign of existence but by opening and shut- 

 ting their shells : they shut them when any 

 foreign body comes in contact with their 

 mantle ; and they open them to admit the 

 water which brings suspended in it the nutri- 

 tious particles which they seize upon for their 

 subsistence, and which is in itself necessary 

 for the purposes of respiration. Among the 

 acephalous mollusca which are not fixed in the 

 manner of those now mentioned, those which 

 have no siphon, or which- have this part very 

 short, live at the bottom of the sea, in spots 

 covered with sand or mud, amidst which they 

 burrow by means of the foot, and support 

 themselves in an oblique position by resting 

 upon the half-open valves of their shell. The 

 acephalous mollusca again, which are furnished 

 with a siphon, almost all bury themselves more 

 or less deeply amid the sand or the mud of the 

 bottom, contenting themselves with an ascend- 

 ing or a descending motion, the latter sufficient 

 in the moment of danger to gain the limits of 

 their retreat, the former to enable them to 

 protrude the free extremity of their siphon 

 when they would establish the current of 

 water necessary to their nutrition and respi- 

 ration. It is easy to imagine that among ani- 

 mals whose functions of external relation are 

 so limited, the nervous system must continue 

 extremely simple, a fact which could in some 

 measure be predicated from observation of the 

 habits of the extensive class whose structure 

 and economy we are now engaged in consi- 

 dering. 



Of the skin and its appendages. The 

 mantle. The acephalous mollusca arc enve- 

 loped by two very thin fleshy laminae, which 

 are seen covering or closely applied to the 

 whole of the inner surface of the shell ; this is 

 the part to which the name of mantle has been 

 given (c, Jig. 359 ; a, a, fg. 360). This name 

 has been very appropriately given to this cuta- 

 neous envelope, for it appears to be applied 

 over the back of the animal, and to be extended 

 over the lateral parts, to meet by its edges 

 along the anterior middle aspect of the body. 

 The mantle is composed of two parts generally 

 equal, or nearly equal, each of which has been 

 designated one of its lobes. In the natural 

 position of the animal, one of these lobes is in 

 relation with its right side, the other in relation 

 with its left side ; they adhere intimately to the 

 superior and posterior part of the body ; they 

 become free at the origin of the branchiae, and 

 form around the whole inferior part of the 

 animal a cavity of various dimensions, within 

 which the abdominal mass, the foot, and the 



