84 



MALACOLOGY. 



The Chiton moves in a crawling position by means 

 of its abdominal foot, much in tiie same way as the 

 Patella. 



The Lepas and its congeners, as we have already 

 stated, cannot quit the spot upon which they were 

 born. 



The apparatus for nutrition is much less known 

 than that of locomotion ; but it appears complete in 

 all the molluscs, consisting of the organs of digestion, 

 respiration, and circulation ; a very small number of 

 molluscs are able to seize their prey before introducing 

 it into their mouths these are the Brachioccphala, or 

 such as have those singular appendages like arms, or 

 tentacula, fixed at the top of the head ; with these they 

 lay hold of or entwine the living animal they are 

 about to swallow. In molluscs, whose buccal orifice is 

 furnished with teeth, they appear to possess the power 

 of masticating or crushing their food as well as seizing it 

 with them. When one tooth only exists in the upper 

 part, it serves as a fastening to the object upon which 

 the tongue acts ; this is clearly exhibited in the com- 

 mon snail and other species of Helices. But little is 

 yet known of the trunk or proboscis with which some 

 molluscs are provided ; the teeth they sometimes 

 possess at their extremity, may possibly serve to drill 

 holes in the shells, which contain their sustenance, 

 and through them they suck the juices by means of 

 their trunk. The use made of the spiral tongue in the 

 Chiton and certain Putcllce is quite unknown, at least, 

 with any degree of certainty ; but little more is 

 known of the manner the acephala derive their nou- 

 rishment ; it would appear that it is most generally in 

 a molecular form, suspended in the water, which is 

 conveyed to the mouth by means of the appendages 

 surrounding that part ; for they indicate no appear- 

 ance of salivary or masticatory organs. 



The Palliobranchia, assisted by their long labial 

 appendages, must more readily seize their food, since 

 they are capable of extending them beyond the shell, 

 and moving them in every direction. Deglutition 

 must go on as it is effected in the higher orders of 

 animals, at least we know it does so in the cepha- 

 lopods. With regard to the digestive organs of mol- 

 luscs, that process is probably carried on very slowly ; 

 nevertheless, the slug and snail, which are those we 

 can more readily observe, eat ravenously during an 

 abundance of their favourite food, which would imply 

 a very active digestion, as few creatures eat more 

 than will suffice for immediate want. Digestion is, 

 doubtless, in these as in other animals greatly assisted 

 by the action of bile, which generally is abundantly 

 supplied, to judge from the size of the liver, the 

 number of vessels receiving it, and the size of the 

 hepatic canals ; in fact, the bile is often poured into 

 the stomach itself, or at the pyloric orifice. If, as it 

 appears certainly to be the fact, acephalous molluscs 

 receive their aliment in a molecular or microscopic 

 size, the action of bile must be very secondary, as 

 digestion cannot require its assistance, and in this 

 group of animals the liver is very inconsiderable. 

 We know nothing certain respecting the chyle ; but 

 it is, doubtless, absorbed in the intestinal canals. 

 The respiration of molluscs is carried on under similar 

 circumstances with that of animals in a higher grade; 

 we know that oxygen is absorbed by them from the 

 air they live in ; but whether solely by the organs of 

 respiration is not so certain ; indeed, it is extremely 

 unlikely, the general envelope of their bodies being so 

 absorbent ; but as the organ of respiration contains 



a far greater number of vessels than any other part, 

 the breathing of air must at that point be chiefly con- 

 ducted. We know also by experiment, that those 

 species furnished with a pulmonary cavity soon die 

 when immersed to a certain depth in water, without it 

 be possible for them to gain the surface ; and, on the 

 contrary, the species with branchiae cannot live long- 

 in the open air, particularly when the branchiae are 

 exposed ; but when these are internal, the animal can 

 exist for a considerable period, by means of the mois- 

 ture humecting them, and by its slow evaporation. 



The mechanism by which the ambient fluid is 

 brought in contact with the fluid medium, or blood of 

 molluscs, is simple and well understood. In those 

 species whose branchias are external (as the Tritons, 

 &c.) the animal has only to ssvim in order to breathe ; 

 those, on the contrary, such as the Pulmobranchiata^ 

 where the respiratory organs are contained in a 

 cavity like,also, nearly all the paracephalous molluscs 

 the air or water is introduced or driven in by the 

 dilatation or contraction of the cavity, and of its plain 

 or tubular orifice, and these two effects are facilitated 

 in all the species, particularly those furnished with 

 a shell, by the extension or the contraction given to 

 the anterior part of the body at which the apparatus 

 is placed, and by its advanced position into the larger 

 part of the shell, but in no one instance is there any 

 regularity of inspiration or expiration. It does not 

 even exist in the Brachiocephala, where the water, 

 introduced into the cavity of the mantle in which the 

 branchiae are, assists, at the same time, for locomotion. 

 The headless molluscs, which are all of them aquatic, 

 present nearly the same mode of respiration, the labial 

 appendages, by their continual movement, determin- 

 ing a sort of current in the water in which the animal 

 is plunged. It may easily be distinguished in such 

 species, the extremity of whose mantles is prolonged 

 into two tubes varying in length, the water entering by 

 the one beneath, and passing out by the other above ; 

 thus respiration takes place when the fluid traverses 

 the branchial cavity, it is suspected that the effects 

 thus produced on the blood which fills the pulmonary 

 or branchial arteries are analogous to those of animals 

 more elevated, but that cannot positively be ascer- 

 tained, because there is no physical distinction be- 

 tween the arterial and venous blood of molluscs. In 

 the circulation of blood, the progress of blood through 

 the veins seems nearly as slow as that of the arteries ; 

 neither have they any true pulsations, although the 

 heart exhibits regular movements of the systole and 

 diastole ; they are, however, very slow, but can easily 

 be seen in the acephalous and cephalous molluscs. 



The mode in which nutrition is carried on in mol- 

 luscs, by the aid of external and internal absorption, is 

 not more known than in the other classes of animals : 

 that which appears certain is, that these animals can 

 support a long fast, particularly when their bodies 

 are completely sheltered by means of a shell, of which 

 the snail is an example and proof; neither does this 

 animal, like many others which hybernate, possess 

 any internal supply of tat, that substance not existing 

 in any mollusc ; what is so called in oysters being 

 nothing more than a peculiar condition of the ovariurn. 



Molluscs possess the extraordinary power of 

 reproducing in a very short time, under favourable 

 circumstances, certain external portions of their 

 body, destroyed by accident or other causes, which 

 may easily be conceived with respect to the mantle 

 and other parts ; it becomes, however, less easy for 



