380 



GASTEROPODA. 



scribing it we shall avoid obscurity by applying 

 to its different parts the names ordinarily made 

 use of by anatomists to distinguish the tissues 

 enumerated as composing the human integu- 

 ment. 



The derwis is an extremely lax and cellular 

 texture, eminently elastic, and so intimately 

 blended with the contractile layers beneath it, 

 that it is difficult to recognise it as a distinct 

 structure : its great peculiarity consists in the 

 power which it possesses of secreting calcareous 

 matter, which being deposited either in a cavity 

 within its substance, or as is more frequently 

 the case, upon its outer surface, forms a con- 

 cealed or external shell : from this circum- 

 stance, and from the abundant quantity of 

 mucus which it constantly furnishes, we may 

 infer its great vascularity, while the high degree 

 of sensibility which it evidently possesses une- 

 quivocally demonstrates that it is plentifully 

 supplied with nerves, although the existence of 

 a true papillary structure cannot be satisfac- 

 torily distinguished. The colouring pigment 

 likewise exists, as is evident from the brilliant 

 markings which are often met with in some of 

 the more highly coloured species ; but there is 

 a circumstance in connection with this rete 

 mucosum which requires particular mention, as 

 it will enable us afterwards more clearly to ex- 

 plain the formation of shells ; the pigment is 

 not merely a layer which serves to paint the 

 surface of the body generally, but appears ra- 

 ther to be an infiltration of the lax tissue of the 

 ciitis with coloured fluid, which is poured out 

 in great abundance at particular points, espe- 

 cially around the margin of the shell, and there 

 being mixed up with the calcareous matter se- 

 creted by the collar, its tints are transferred to 

 tlit; exterior of the shell itself, tinging it with 

 similar hues. The epidermis is evidently defi- 

 cient, its place being supplied by the viscid 

 matter with which the surface of the body is 

 continually lubricated. The muciparous crypts 

 destined to furnish the copious supply of glairy 

 fluid with which the skin is so largely moist- 

 ened, have not been detected, but the pores 

 through which it exudes are sufficiently distinct. 

 It is> in connexion with the needful diffusion of 

 this secretion over the entire animal, that the 

 skin of the terrestrial species, as the Slugs and 

 Snails, is observed to be deeply furrowed by 

 large anastomosing channels, formed by the 

 rugae of the surface, and serving as canals for 

 its conveyance by a species of irrigation to 

 every point. No pilous system, properly so 

 called, exists in any of the Gasteropods, the 

 hairy covering of many shells being, as we shall 

 presently see, of a widely different nature. 



From the modifications observable in the 

 structure of the integument, it is not to be won- 

 dered at that names have been applied to diffe- 

 rent portions, which it will be useful to notice, 

 especially as they are not unfrequently used in 

 a confused and unprecise manner. That por- 

 tion of the skin which is more immediately 

 connected with the secretion of the shell, in 

 such Gasteropoda as are provided with a de- 

 fence of that description, has been termed the 

 mantle, and in certain instances, from the mode 



in which it seems to form a special co\ ering to 

 a part of the body, it has some claim to the 

 name ; the mantle is, however, extremely varia 

 ble, both in position and arrangement. In the 

 Nudibranchiata, which have no shell, it cannot 

 be said to exist, as no fold of the integument or 

 defined margin indicating a portion deserving 

 of a distinct appellation can be detected. In 

 the Tectibrunchiata the mantle is a small trian- 

 gular fold of the integument on the right side of 

 the body, inclosing a rudimentary shell, and 

 serving as a covering to the subjacent branchiae. 

 In the Inferobranchiata it invests the whole of 

 the back, and forms a fold around the margins 

 of the body, beneath which the branchiae are 

 found ; whilst in all the conchiferous Gastero- 

 pods it lines the interior of the shell, whatever 

 its shape, forming a distinct fold or thickened 

 rim around its aperture, to which when much 

 developed, as in Helix, the name of collar is 

 not improperly applied. 



In the naked terrestrial species the mantle 

 consists of a thickened portion, occupying a 

 variable position on the back, and more or less 

 defined by a distinct margin ; it is here not un- 

 frequently termed the corselet, and generally 

 contains a calcareous plate. In Vaginula it 

 covers the whole of the back ; in Limax it occu- 

 pies only its anterior portion ; in Parmacella it 

 is found in the middle of the dorsal region, 

 whilst in Testacella it is placed quite poste- 

 riorly in the vicinity of the tail ; yet whatever its 

 situation, shape, or size, it is the immediate 

 agent in the formation of the shell, and as such 

 we have deemed it necessary to be thus precise 

 in describing the different aspects which it 

 assumes. 



Growth of shell. The varied and beautiful 

 shells that form so important a part of the inte- 

 gument of many individuals belonging to this 

 order, however they may differ in external form 

 and apparent complication, are essentially simi- 

 lar in composition and in the manner of their 

 growth. These calcareous defences, although 

 serving in many cases as a support to the ani- 

 mal, from which important muscles take their 

 origin, differ widely from the internal skeletons 

 of vertebrate animals, being mere excretions 

 from the surface of the body, absolutely extra- 

 vital and extra-vascular, their growth being en- 

 tirely carried on by the addition of calcareous 

 particles deposited in consecutive layers. The 

 derm is or vascular portion of the integument is 

 the secreting organ which furnishes the earthy 

 matter, pouring it out apparently from any part 

 of the surface of the body, although the thicker 

 portion, distinguished by the appellation of the 

 mantle, is more especially adapted to its pro- 

 duction. The calcareous matter is never depo- 

 sited in the areolae of the dermis itself, but ex- 

 udes from the surface, suspended in the mucus 

 which is so copiously poured out from the mu- 

 ciparous pores, and gradually hardening by ex- 

 posure; this calciferous fluid forms a layer of 

 shell, coating the inner surface of the pre-exist- 

 ent layers to increase the size of the original 

 shell, or else is furnished at particular points 

 for the reparation of injuries which accident 

 may have occasioned. It is to the investigae. 



