Gasteropoda.] 



MUSEUM OF ANIMATED NATURE. 



191 



genus Clio, appear analogous to the Hyalaeao in the 

 simplicity of their wings and the absence of tentacles 

 between them; their conic or pyramidal shell, he 

 adds, is not slit on the sides; and he quotes M. 

 Rank's genera and subgenera, 



M^ Deshayes, in his edition of Lamarck, states 

 that the Cleodorae are much more allied to the 

 Hyalaeie than the Clios, appioaching the former not 

 only in having a shell, but also in the form of the 

 animal, which bears a great resemblance to that of 

 Hyalaea. It is not astonishing, proceeds M. Des- 

 hayes, that Lamaick, who had approximated the 

 Cleodorse to the Clios, should regard them as not very 

 closely related to the Hyalase ; for, when he wrote, 

 but a very small number of species were known, and 

 he could hardly foresee that the assiduous researches 

 of MM. Quoy and Gaimard, Rang and D'Orbigny, 

 should have contributed to throw so much light on 

 the Pteropods in general, and the Hyalsese and 

 Cleodor* in particular. If we have before us a 

 sufficient number of species belonging to the two 

 last-named genera, we shall see them blend into 

 each other so as to make it impossible to draw the 

 line between them. It is thus, continues M. Des- 

 hayes, that we proceed by insensible degrees from 

 the globular to the lanceolate species. 



The following is M. Rang's definition of Cleo- 

 dora : — 



Animal of an oblong or elongated form, furnished 

 with an intermediate demicircular lobe, but having 

 no lateral expansions ; mantle open in front; bran- 

 chiiB incompletely known. 



Shell fragile, vitreous, in form of a sheath or case 

 (gaine ou cornet), more or less pointed posteriorly ; 

 aperture very large, nearly always without a slit, and 

 without lateral appendages. 



The same zoologist having, as he states, ob- 

 tained many new species, and studied their organi- 

 zation, divides the genus into the following sub- 

 genera: — 



1. Cleodorae properly so called. 



Animal of an oblong form, having the mantle 

 very much dilated and advanced on each side. 



Shell pyramidal, angular, very much dilated an- 

 teriorly, with a very large aperture, canaliculated 

 on each side, and rarely slit. 



M. Rang makes this subgenus comprise (1829) 

 five species only, two of which he considers as very 

 doubtful. Type Cleodora lanceolata. 



Description. — Shell compressed, elongated, lan- 

 oeolate; aperture dilated. 



Locality, the seas of warm climates. 



M. Deshayes, in his edition of Lamarck, records 

 thirteen species, besides Hyalsea cuspidata. 



Referring to Fig. 2592, — a represents the animal 

 and shell; 6, the shell seen edgeways; c, the shell 

 seen from above. 



2. Cresis. (Rang.) 



Animal very slender ; the mantle not dilated on 

 its sides ; fins generally rather small. 



2593. — The Cresis. 



Shell very slender, fragile, and diaphanous, in the 

 form of a straight or curved case (cornet), with an 

 aperture almost always as large as the shell itself, 

 and generally without a canal ; no lateral appen- 

 dages. JVI. Rang, who gives this description, says, 

 that he formed this subgenus for some very small 

 new mollusks, which he frequently met with in the 

 middle of the ocean, and to which he unites, by 

 analogy, the genera Vaginella of Daudin, and the 

 Gadus of Montagu, known in the fossil state ; and 

 T.I. Rang reckons nine species. 



Referring to Fig. 2593, — a represents the fins; 

 h, the intermediate lobe ; c, the mouth ; i, the vis- 

 cera seen through the shell ; h, the shell. 



A closely allied form has received from M. Rang 

 the name of Cuvieria (see Fig. 2594), characterized 

 by the animal being elongated, furnished with two 

 rather large fins, and with an intermediate demi- 

 circular lobe : the exterior branchiae situated at the 

 base of the nitermediate lobe ; mouth furnished 

 with dentiform pieces adapted for maslication. 



Shell in the form of a cylindrical case, rather 

 flattened near its aperture, which is heart-shaped, 

 with sharp edges. The species on which M. Rang 

 has founded the genus Cuvieria is a curious little 

 moUusk, common in the Indian Seas, the ocean, and 

 the South Sea. 



From the perplexing forms which constitute the 

 Pteropodous Mollusks, and which, however curious 

 they may be from their structure, are interesting 

 rather to the naturalist than the general reader 

 (their habits and manners being indeed necessarily 

 but little "known), we shall pass to another great 

 division or class of the mollusks, termed by scien- 

 tific writers, generally. Gasteropoda, or Gastropodous 

 Mollusks, from the circumstance of the under sur- 

 face being modified into a fleshy disc for the 

 purposes of locomotion, as we see in the snail or 

 .••Jug. 



CLASS GASTEROPODA 



(Gastropodous Mollusks, as Slugs, Snails, 

 Whelks, Cowries, &c.). The Gastropods, or Uni- 

 valve Mollusks, present us with a tolerably high 

 grade of organic structure ; far higher than is 

 found in the subsequent class of Bivalve Mol- 

 lusks, viz. Oysters, Mussels, &c. The nervous 

 system, for example, is more concentrated, the 

 viscera are more elaborate ; the head is distinct, 

 and contains a large nervous ganglion ; it is fur- 

 nished also with retractile organs, viz., tentacula, 

 or horns, which are not only instruments of touch, 

 but which often bear upon them the eyes. There 

 is a mouth furnished with teeth, and also with 

 a tongue ; and the sense of taste is very acute. It 

 would appear, moreover, that though no apparatus 

 has been detected, the sense of smell is not alto- 

 gether wanting. 



The Gasteropoda, as they are endowed with 

 senses which open to them much of the world 

 around them, so are they gifted with according 

 powers of locomotion. As we have said, the title 

 Gastropod is given in allusion to the character of 

 their locomotive organ ; for the under surface of 

 the body is converted into a sort of foot, with a fleshy 

 disc beneath, on which they creep along by means 

 of the expansive and contractile movements of 

 which it is composed. 



The common garden snail is an instance in point ; 

 if we watch a snail crawling upon a window-pane, 

 on the opposite side of the glass, the muscular 

 working of the disc may be easily seen. 



Unlike the bivalve-shelled mollusks, the Gastro- 

 pods comprise both aquatic and terrestrial groups ; 

 some are formed for the respiration of air, others 

 of water ; and it is curious to find that some species 

 which tenant the water, as Limnaeus, Planorbis, &c., 

 breathe air, and come to the surface to respire. 



All the bivalve-shelled mollusks are housed ; not 

 so the Gastropods. The slugs have no shell, or at 

 most a minute plate imbedded in the short con- 

 tracted mantle covering the anterior part of the 

 body above. 



In Testacella, the mantle, which is minute and at 

 the posterior part of the body below, is protected by 

 a little oval shield. 



In many marine species, as Doris, and some allied, 

 and the Phyllidia, there is no shell at all. Others, 

 however, have an ample shell, into which the 

 animal can withdraw itself at pleasure; as the 

 Helix, the Limpet (Patella), the Conch, &c. 



The shell, secreted by an ample membrane, 

 varies infinitely in shape, in compactness, and other 

 particulars. 



A general view of the mode of formation of the 

 shells of these animals may not, perhaps, prove 

 uninteresting, the more so as it is a subject not com- 

 monly understood. 



It may be here, then, observed that shells differ 

 in their composition (and the same observation 

 applies to those of the bivalved or acephalous, 

 that is headless, mollusks also). In some, as the 

 conch and others (and the oyster among the bi- 

 valves), the carbonate of lime, with a superabund- 

 ance of animal gelatine, is deposited in layers, a 

 fine membrane interposing between each layer. 

 Hence shells of this composition are termed mem- 

 branous, and the solution of the lime by means of 

 acids will leave the membranes perfect, but soft, 

 having lost the earthy matter which gave them 

 hardness. Moreover, on exposing these mem- 

 branous shells to a red heat, they emit a strong 

 fetid odour of burning animal matter, and fall to 

 pieces. 



Other shells, as the cowry, present us with a 

 much more uniform composition and compact 

 texture. In these shells, which are termed por- 

 cellaneous, the animal gelatine is in little quantity; 

 and the particles of lime are more equally blended 

 with it, not in layers, but assuming a more or less 

 crystalline arrangement ; the minute crystals pre- 

 senting the form either of rhombs or prisms. Por- 

 cellaneous shells when exposed to the action of 

 fire lose their colour, and give out but little smoke 

 or odour, while they retain their form. On the 

 contrary, they are almost entirely dissolved by the 

 action of acids. These shells are far more brittle 

 than the membranous shells, and also more trans- 

 parent ; and when this brittleness and transparency 

 are very great, they have been termed vitreous, from 

 their resemblance to glass. 



The inner lining of most membranous shells, of a 

 white, roseate, or other tint, and with a smooth 

 polished surface, is called the nacre, or mother of 

 pearl, and it is from the arrangement of the nacre 

 in regular layers, effecting a series of exquisitely 

 minute ridges or grooves, not unlike, when viewed 

 through a microscope, the striae on the cuticle at 

 the pulpy tips of the fingers, that in so many mem- 

 branous shells we see the nacre iridescent with rain- 

 bow tints, the light being reflected at different 

 angles. In the lining of the fresh-water mussels, 



and especially in Haliotis, the nacre is very re- 

 splendent. 



Most, if not all membranous shells are covered 

 externally with an outer skin, or membranous 

 epidermis, which most collectors are anxious to 

 remove ; in conch, which has a tough thick epi- 

 dermis, we generally find that it has been carefully 

 obliterated, to render the appearance more attrac- 

 tive. 



The shells of the mollusca are the production of 

 a portion of the animal termed the mantle (Pal- 

 lium), and hence, according to the extent and 

 figure of the mantle, and its temporary develop- 

 ments, will be the figure and general characters of 

 the shell. 



If we take a common snail, or helix, and remove 

 it from the shell (killing it previously by immersion 

 in boiling water), we shall find, on examining the 

 animal, that the whole of the spiral portion, which 

 was lodged in the upper whorls of the shell, is 

 invested with a thin membrane, viz., the mantle. 

 The anterior part of this mantle, on what may be 

 called the back of the mollusk, is considerably 

 thickened, swelling into a portion called the collar. 

 This collar is provided with glands regularly ar- 

 ranged for the deposition of colouring matter, 

 as well as others for the secretion of lime and 

 gelatine, in order to add to the shell as is needed. 

 It is the smooth and thin portion of the mantle 

 that secretes the nacre ; but, as we have said, the 

 collar of the mantle secretes the layers by which 

 the growth of the shell is effected. If, as in the 

 conical-shelled limpet, the collar of the mantle 

 regularly deposits fresh material around the edge 

 of the aperture in equal quantity layer after layer, 

 then the shell will grow in one direction only, the 

 diameter of the mouth enlarging, according to the 

 extension of the collar of the mantle. It mostly, 

 however, happens, that the mantle of these mollusks 

 does not proceed with its work equally on all sides ; 

 indeed, the mantle does not, as in the limpet, hang 

 over the animal in a tent-like manner, but has its 

 margin across the back ; hence it adds only to the 

 edge, and not to the convex side which forms the 

 posterior boundary of the opening. In this case, 

 as in planorbis, the increase of the anterior margin 

 tends to the formation of a spiral shell, the coils 

 being all on one plane. 



Most commonly there is a preponderance of de- 

 velopment (from the form of the collar), laterally 

 as well as forwards, and then, as in the helix, a 

 spiral or turbinated shell will be the result. If we 

 take a whorled shell in our hand, we shall find that • 

 the turns of the whorl are directed from left to 

 right ; and this results from the situation of the 

 heart and great blood-vessels relatively to the shell, 

 these being placed on the left side ; the left side 

 of the mantle therefore is more active than the 

 right, so that the vis a tergo is from the left, forcing 

 (if we may so speak), towards the right. Occa- 

 sionally we see a shell whorled in a contrary di- 

 rection, the situation of the organs and figure of the 

 body being reversed ; such shells are termed sinis- 

 tral ; they are not common. 



The axis of the gyration of the whorls of shells is 

 termed the columella, and on dividing a shell longi- 

 tudinally, as the whelk for example, the columella 

 appears like the pillar or modiolus of winding stairs 

 in a tower or steeple ; round which the whorls are 

 wound. The edge of the aperture to which the 

 columella advances is termed the columellar lip: 

 looking on the back of the shell it is on the left 

 hand posteriorly. 



Instances occur in which the mollusk, for the 

 sake of convenience, removes portions of the interior 

 whorls as it increases in growth ; the cone thins by 

 some process the inner whorls, and the auricula, 

 which inhabits marshy places, obliterates them in its 

 progress of growth. 



In many shells, as Pterocera, and various species 

 of Murex, as Murex regius and Murex tenuispina, 

 we observe spines, or rows of antler-like projections, 

 at regular distances apart; now these are formed 

 by the collar of the mantle, which, at definite in- 

 tervals, during the growth of the mollusk, becomes 

 temporarily developed in a peculiar manner, shoot- 

 ing out into processes secretmg shelly matter, which 

 encases or nearly encases them. When spine, 

 are produced, as in Murex tenuispina, the slen- 

 der processes of the mantle are encased in the 

 shelly matter ; to this a lining of nacre is added layer 

 after layer, the process of the mantle retiring more 

 and more, till in due time the cavity of the spine is 

 obliterated, and the process of the mantle retracted 

 or absorbed. Where branches like antlers are pro- 

 duced, these will be found to be more or less like 

 troughs or canals, hollowed beneath, the under 

 surface of the extensions of the mantle not secreting 

 shell. 



In the Cowry (Cypraea)the formation of the shell 

 is very curious ; and, indeed, so greatly does it dif- 

 fer in its young and perfect state, that no one would 

 at first suppose the two to be generically, much less 



