MALACOLOGY. 



81 



occurs that the skin of those animals is larger than 

 necessary merely to surround their bodies, or the 

 visceral mass exactly, and that its plaits appear to 

 envelop it, the name generally adopted for this portion 

 is pallium, or the mantle, to designate the skin of 

 molluscs, though in fact this does not always exist so 

 disposed ; so various is the general position of that 

 mantle, that we shall not attempt a description of the 

 differences, as it would lead us beyond our limits. The 

 edges of the mantle vary exceedingly in form, some 

 of them being lobated, or more or less digitated, the 

 protection of which characterises the form of the Ptero- 

 ceras, and other molluscs, familiarly said to have claws ; 

 but the most singular property of the mantle is that 

 of secreting, in addition to the colouring, a cretaceous 

 matter, whose accumulation and desication forms a 

 protecting cover, or shell properly so called. 



In the article CONCHOLOGY we have explained, to a 

 certain extent, nature's operation in this respect, illus- 

 trating it by the easily observed process of the snail, 

 in the formation of its dwelling ; our present object is 

 therefore confined to the anatomical organisation of the 

 animals constructing shells, without reference to the 

 shells themselves in a conchological point of view. The 

 apparatus and sense of touch appears to be seated in 

 the tentacula, or tentacular cirrhi, with which the 

 edges of the mantle are generally furnished ; and it 

 is probable, from a parity of reasoning, that certain 

 tentacular appendages, occasionally in the form of a 

 fringed membrane, as in the Janthina, may be consi- 

 dered as organs of touch, as well as serving for organs 

 of locomotion. In some molluscs the organ of taste 

 is not at all defined, as is the case with all the Ace- 

 phalophora, in others it is only a lingual mass, ill 

 defined ; but in some species that mass is provided 

 with small hooks and spines on its surface, closely re- 

 sembling the tongue of some of the feline mammals, and 

 leaving no doubt of the purpose of that organ ; this 

 is visible in the snail. The organ of smell has not 

 yet been positively determined, and a great diversity 

 of opinion exists upon the subject; some have thought, 

 from the texture of the skin of molluscs, they possessed 

 the faculty of smell dispersed over its whole surface ; 

 but as the sense of smelling can only be accomplished 

 by the odour of the particle of matter smelt being free 

 to escape through the medium of air, this sense is 

 only given, in all probability, to such of these animals 

 as breathe air, and is seated at the extremity of the 

 true tentacula, where the skin is invariably softer, more 

 smooth, and in every respect of a more delicate tex- 

 ture than any other portion of the animal's body ; 

 the nerves are also more numerously congregated at 

 that part. The organ of vision does not admit of so 

 much doubt as many other of the senses, because the 

 connexion, cause, and effect, is more easily remarked 

 and determined ; the headless molluscs do not pos- 

 sess eyes, neither do that class called articulated 

 molluscs ; and here, as in every other instance upon 

 which an opportunity of reasoning is afforded us, the 

 wisdom of Providence is manifested most strikingly. 



If the organs of locomotion are always distinctly 

 defined by a true muscular fibre, visible on the inter- 

 nal surface of the skin, which is there disposed in a 

 tendinous bundle, of a determined form and disposi- 

 tion of parts, and even sometimes is attached to the 

 most solid portion of the animal's body, the num- 

 berless other contractile fibres with which molluscs 

 are provided assist also, in all probability, to their 

 locomotion ; but as it is so difficult to see these crea- 



NAT. HIST. VOL. III. 



tures in their habits, considerable uncertainty exists 

 as to the precise part of their body which serves as 

 legs in all the species. In the Ccphaluphora, the name 

 of foot has been given to a muscular disk, or round 

 . substance of greater or smaller thickness, situated at 

 the extremity of the animal's bodv, and produced by 

 a prolongation of the muscular fibres. In some 

 molluscs, the so-called foot extends the whole length 

 of the body, forming a kind of sole ; by means of 

 which the animal is enabled to climb, which, occupy- 

 ing the surface of the viscera, has occasioned them 

 to be called Gasferopods,or Reptantia, meaning animals 

 with feet in their belly ; but in such species as from 

 their form are specially rolled within their shell, and 

 their congeners, the foot or sole does not cover the 

 viscera, but merely extends from that part which se- 

 parates the head, and may therefore be called the 

 neck, which has occasioned them the distinctive name 

 of Trachelipods, or having legs in the neck ; the propor- 

 tionate size, and even the form of this foot, varies very 

 considerably, even in examples of the corresponding 

 genera : thus, in the Patella it is nearly round ; oval 

 and very thick in the Haliotis ; rounded in front, and 

 gradually becoming thin on either side, in the Murex ; 

 ear-shaped in some of the Terebra, and a great num- 

 ber of the order Cyclostoma and in the Conus it is 

 divided by a transverse groove at its anterior edge. 

 All the species of paracephalons molluscs, without 

 shells, are eminently gasteropod, but that is not the 

 case with those possessing shells ; they are not, 

 nevertheless, trachelipods necessarily, although that 

 most usually is the case ; but a fact common to 

 both is, that these muscular fibres are always so placed 

 with regard to the shell, that they can be contracted 

 to a greater or lesser depth within it, as well as the 

 head itself, by means of a muscular bundle, denomi- 

 nated the muscle of the columella because in spiral 

 shells it is always attached to that part. The form of 

 this muscle and its disposition differ considerably 

 according to the form of the foot, and more particu- 

 larly with respect to the configuration of the shells. 

 To this columella muscle also, belongs the power of 

 opening or closing the aperture of the shell, by means 

 of its operculum, when that appendage exists. In 

 the Cepkalopods and Paraccphalophora, which are 

 sometimes provided on each side with numerous loco- 

 motive appendages, as in the Sepia, Calmar, and their 

 congeners, and generally indeed in the Pteropods, or 

 winged molluscs, these parts possess depressing or 

 elevating muscles attached at their root to the back 

 or the belly ; but when these do not serve to perform 

 locomption, they are merely formed of a contractile 

 skin, in which no muscles are distinguishable. In the 

 acephalous or headless molluscs, in the centre of the 

 abdominal part is situated a muscular mass, more or 

 less thick, which, besides its external, possesses inter- 

 nal contractile muscles, and this mass is in them called 

 the foot, similar to that occupying the lower portion 

 of Gasteropods; the shape and size of this varies very 

 considerably, till in some species no trace whatever 

 of it exists, as in the oyster. This foot is attached 

 more or less in front of the body, which depends upon 

 the habitual position of the animal, but it can also be 

 used in different directions, by means of true muscles 

 divided into a greater or smaller number of bundles, 

 directed towards different points of the shell, particu- 

 larly backward, forward, and sometimes towards the 

 intermediate space, as is the case with the Onodon 

 and its congeners. This extensible foot sometimes as- 



