194 



MUSEUM OF ANIMATED NATURE. 



[Slugs. 



itpeciftcally, identical. We have already instanced 

 the co»vry as havinjc a porcellaneous shell. The 

 shell, at first, instead of having its lips rolled in- 

 wards and toothed, with the aperture narrow, much 

 resembles the olive shell ; the substance is thin, and 

 the colour dull and opaque, and the outer edge, 

 instead of being rolled inwards and thick, is sharp. 

 But a change occurs ; the mantle begins to be re- 

 fleeted on each side round the shell, covering it 

 externally. The lip curves inwards, the tet>th ap- 

 pear, and beautiful markings are beginning to show 

 themselves. At length the shell is thickened by 

 repeated layers of porcellaneous matter, the colours 

 are vivid, and the spire is completely hidden. If we 

 take the shell of a cowry, and look at it, aline down 

 the centre of its back will be seen marking the place 

 where the two reflected expansions of uie mantle 

 met each other. 



Among the most singular of the univalve shells 

 is that covering the Chiton, and which forms an 

 exception to the turbinated or whorled character 

 which most exhibit. On examining these slug-like 

 creatures, we find the back covered with a tough 

 leathery mantle, which extends considerably beyond 

 the body of the animal beneath. On the top of this 

 mantle are eight transverse plates of shell, usually 

 overlapping each other like the slates of a house, 

 but never reaching to the edges of the mantle : these 

 free edges have patterns of different insculpture 

 according to the species. 



In many instances, among aquatic species, the 

 «ntrance of the shell is closed when the animal is 

 withdrawn by means of a horny or calcareous plate, 

 termed the operculum, and which is attached to the 

 portion of the foot last withdrawn. It fits the en- 

 trance precisely, and shuts the raollusk in ; protecting 

 it from the assaults of enemies. The periwinkle is 

 a familiar example. 



The operculum is, however, sometimes a lid at- 

 tached by a hinge to the coliimellar portion of the 

 lip, and may be regarded as a distinct valve, or, as 

 it is called, a clausium, which may be defined as a 

 modified operculum, attached by an elastic hinge 

 to the columella; and which shuts spontaneously 

 when the mollusk retires within. The membrane, 

 with which the common garden snail and others 

 shut themselves in while hybernating in holes of trees 

 or walls, or under the roots of hedges, or even at- 

 tached to palings, &c., during winter, is termed the 

 epiphragraa. It appears to be indurated mucus, 

 secreted for the occasion. 



Gastropodous mollusks have the power of re- 

 pairing their shells ; if, for example, a portion of the 

 shell of a common snail be leraoved, without in- 

 juring the animal, which may be easily effected, in 

 twenty-four hours a pellicle will be found extended 

 across the vacant space, but beneath the level of 

 the edges of the fractured part ; this pellicle is 

 thickened by additional layei-s, till it has acquired 

 nearly the thickness of the original shell ; the whole 

 process occupies about a fortnight. The repaired 

 part is veiy distinct, as it is not by a growth of the 

 edges, but by the affixing a plate from within to 

 block up the aperture occasioned by the removal of 

 a portion, that the injury is repaired. 



Various are the systems of arrangement adopted 

 by naturalists, with respect to the present great 

 group of mollusks, but, as it is far from our design 

 to enter into the "deep things" of science, we shall 

 merely give a brief review of that of Cuvier. He 

 divides the Gastropods into nine ordere, according 

 to the characters presented by the respiratory system, 

 viz.— 1, Pulmonobranchiata; 2, Nudibranchiata ; 

 3, Inferobranchiata; 4, Tectibranchiata ; 5, Hetcro- 

 poda; 6, Pectinibranchiata; 7, Tubulibranchiata ; 

 8, Scutibranchiaia; 9, Cyclobranchiata. 



1. The Pulmonobranchiata, of which the slug, 

 the snail, the Limnaus, and Planorbis, &c., are 

 examples, are distinguished by respiring atmospheric 

 air, which is alternately drawn into and expelled 

 from a cavity lined with a most delicate vascular 

 network : the respiratory organ opens on the right side 

 of the body near the margin of the shell, below the 

 collar of the mantle ; and it may be further stated 

 that the muscular floor of this cavity performs 

 movements analogous to those of the diaphragm in 

 quadrupeds. Of these air-breathing mollusks, some 

 are terrestrial, others live in streams, or in sluggish 

 or stagnant waters ; and we may here add that, of 

 the aquatic forms, some at least, as Planorbis, are 

 organized to respire both air and water ; of this 

 section some are shelled, others naked. 



2. The Nudibranchiata are marine mollusks 

 destitute of a shell, as Doris, Tritonia, Glaucus, &c. 

 They have no pulmonary cavity ; and the branchi.-e 

 are exposed on some part of the back. In the 

 Doris, which swims reversed, the foot appearing 

 uppermost like a boat, the branchiae are flovver-like, 

 and radiate veiy beautifully at the end of the back. 



3. The Inferobranchiata, as Phyllidia, have no 

 shells, and their branchiae, like two long rows of 

 leaflets, are placed on each side of the body, under 

 a projecting edge formed by the mantle. 



4. Tectibranchiata, as Pleurobranchus, Aplysia, 

 Dulabella, &c., have the branchial fringe or leaflets 

 placed under the margin of the mantle on the right 

 side only. The mantle contains almost always in 

 its substance a little shell. 



5. The Heteropoda are so named because their 

 foot, instead of forming a horizontal disc, is com- 

 pressed into the form of a vertical muscular oar, 

 on the edge of which generally is a dilated part 

 hollowed out, representing the disc in other orders ; 

 the branchiae form tufts on the hinder part of the 

 back. These mollusks swim with the back down- 

 wards, and the vertical foot upwards, and are ena- 

 bled to distend themselves by filling the body with 

 water. 



(i. The Pectinibranchiata form beyond com- 

 parison the most numerous division of the Gastero- 

 poda. This order in tact includes all the inhabit- 

 ants of spiral or whorled univalve sea-shells : their 

 branchiae are comb or gill like, and are placed in 

 one, two, or three rows, suspended from the roof of 

 a branchial chamber, iff the body of the animal, con- 

 tained within the widest or last-formed whorl of the 

 shell. This chamber opens by a wide orifice or by 

 a siijhon which admits the sea-water. The sexes 

 are distinct. 



7. Tubulibranchiata. From the Pectinibranchiata 

 has been separated a group termed Tubulibranchiata 

 from having the branchiae in a tubular cavity. The 

 animals of this group, as Vermetus and Magilus, 

 inhabit long irregular shells spirally contorted, and 

 olten twisted in a serpentiforra manner with each 

 other, or with various objects. 



8. Scutibranchiaia. The Scutibranchiata, as 

 Haliotis, or sea ear, and Fissurella, &c., agree with 

 the Pectinibranchiata in the general characters of 

 the branchiae ; but they inhabit very open shells 

 which cannot be called turbinated ; and there are 

 other points in their economy which render it at 

 least convenient to separate them into an order by 

 themselves. 



9. The Cyclobranchiata, as the Patella, or Limpet, 

 and the Chiton, have the branchiae forming a fringe 

 around the body of the animal under the edges of 

 the mantle. 



We have already alluded to the senses with which 

 the gastropods are endowed ; and here we may add, 

 as respects the eyes, that they are sometimes seated 

 on the head, sometimes on the top of the tentacles 

 or horns, and sometimes at their base. In the 

 common snail there are two eyes seated each on 

 one of the larger horns or tentacles. In these 

 animals the horns are lour in number, and retractile. 

 In the marine gastropods the tentacles are mostly 

 two in number, and often not retractile, and the eyes 

 are usually seated at their base. In the fresh-water 

 pulmonobranchiata there are only two horns : they 

 are retractile, and the eyes are on their inner base, 

 as in Planorbis, Limnapus, Physa, &c. 



In the chiton, one of the cyclobranchiate marine 

 gastropods, there are neither eyes nor tentacles. 



The retractile horns of the snail are organs of 

 touch, especially the two lowermost or shortest, the 

 two uppeimost being, as we have said, furnished 

 with eyes. They are supplied with nerves from the 

 great supra- oesophageal ganglion ; are hollow or 

 tubular; and their retraction is eft'ected by a mus- 

 cular slip, which inverts them like the finger of a 

 glove, the tip being gradually drawn down from 

 within. When thus inverted they lodge in a cavity 

 for their reception. The protrusion of the horn is 

 effected by means of a system of circular muscular 

 fibres composing the walls of the cavity of lodgment 

 and also of each tubular tentacle, which by their con- 

 traction force out the inverted portion, and, having 

 accomplished this, in conjunction with longitudinal 

 fibres give it firmness and mobility. Now it would 

 appear that the nerves which run up the tentacle 

 must be stretched when the latter is extended, but 

 such is not the case ; nerves do not admit of such 

 rude treatment without pain and loss of functional 

 power; the provision is simple; the nerve (whether 

 of sight or feeling) is as long as the tentacle ex- 

 tended to its uttermost, and when the tentacle is 

 inverted it is thrown into series of coils lodged in 

 the cavity into which the tentacle is drawn. Very 

 variable, according to the nature of the food, is the 

 structure of the mouth in the gasteropoda. In the 

 snail and its allies the mouth is placed on the under 

 part of the head, and is furnished with an instru- 

 ment well adapted for cutting leaves and fruits. 

 The oral cavity, which is muscular, has affixed to its 

 upper part a horny plate, the lower edge of which 

 is free, extremely sh.irp, and dentated, and well 

 adapted for dividing the soft parts of vegetables, to 

 which it is applied ; the floor of the oral cavity is 

 provided with a small tongue of a cartilaginous 

 texture, with its surface transversely striated ; and 

 by its action it propels the food into the gullet. 

 Snails and slugs are the pests of the garden ; but 

 slugs, as the ordinary slugs, are very partial to 

 animal food, and when the dog has left a half-picked 

 bone on the grass-plot, we have found it covered 



with slugs at work upon it ; we have seen them also 

 at work upon dead worms ; and they appear to be 

 guided in their search for food by the sense of 

 smell. 



In some, as Pleurobranchus (Tectibranchiata), 

 and in Pterotrachea (Heteropoda), the mouth is a 

 simple tube, destitute of teeth, but capable of seiz- 

 ing soft and minute substances. 



A third kind of mouth is exemplified in the 

 Tritonia (Nudibranchiata). It is of an oval form, 

 furnished witn large fleshy lips, and a tongue covered 

 with spines: wittun the lips are two lateral horny 

 jaws resembling two sharp-edged blades, opposed 

 to each other like those of a pair of shears, and in 

 the same manner working upon an elastic hinge. 

 These blades are acted upon by powerful muscles, 

 and cut hard substances with great facility. The 

 spines on the tongue are recurved, and assist mate- 

 rially in propelling the food into the gullet. 



A more complicated mouth is found in many of 

 the Pectinibranchiata, as the Whelk (Buccinum). 

 It may be described !\s a flexible proboscis, movable 

 in various directions, and capable of being retracted, 

 like the horns of the snail. To the end of this 

 tubular proboscis, the tongue and also the gullet 

 are both carried out ; the tongue is cartilaginous, 

 and moreover supported by two cartilaginous slips, 

 of which the extremities form a sort of double lip, 

 capable of being opened and closed, and the carti- 

 lages can be moved upon each other by the action of 

 muscles. Now the tongue is armed with sharp, 

 hard, hooked spines ; and when it is applied to any 

 shell which the animal desires to drill for the pur- 

 pose of sucking out the contents, the supporting 

 cartilages by their movements alternately elevate 

 and depress the spines, which rasp aw.xy on a small 

 surface, and soon pierce through the substance. 

 Perhaps in this operation the saliva, which is car- 

 ried by long ducts to the tongue, may assist by some 

 solvent quality it possesses; but although this is 

 probable, it is not positively ascertained. Certain 

 it is, however, that with this slender rasp-like tongue 

 the whelk will pierce shells of great solidity for the 

 sake of feasting on their unfortunate inmates. 



The digestive apparatus of the gastropods varies 

 in the different groups ; but into this and other 

 points of anatomical detail we forbear to enter. We 

 must attend to our pictorial specimens. 



ORDER PULMONOBRANCHIATA. 



Family LIMACID^ (SLUGS). 



These destructive pests in gardens and cultivated 

 fields are too well known to need a detailed account 

 of their external characters. Almost all have four 

 tentacles, but Cuvier states that in two or three 

 small species the lower pair are wanting ; the mantle, 

 which is generally seen on the anterior portion ot 

 the back, behind the head, often contains a thin 

 shell ; sometimes only calcareous grains. 



2595.— The Red Slug 



(Arion ru/us). Limax rufus, Linn. In this genus 

 the orifice of respiration is towards the anterior part 

 of the mantle, and in the substance of the latter 

 are small calcareous concretions ; a mucous pore at 

 the end of the tail. 



The red slug is very common, and abounds in 

 some gardens almost as much as the small grey 

 slug, which it greatly exceeds in size. Its general 

 colour is rufous, sometimes deepening almost to 

 black. It is this species of which Cuvier says that 

 in France a decoction (bouillon) is used in diseases 

 of the chest : in some parts of England we have known 

 the small grey garden slugs swallowed in numbers 

 by weak or consumptive persons. 



2596. — The Great Gkey Slug 



(Limax antiquorum, Feruss.). Limax maximus, 

 Linn. In the subgenus Limax, as established by 

 M. Fi^russac, the respiratory orifice is situated more 

 backwards than in Arion ; and the mantle is marked 

 with fine concentric striae, and contains a minute 

 shelly plate. At Fig. 2596, a represents the internal 

 shell of the great grey slug ; b, the same enlarged ; 

 c, the under view of the shell from another indivi- 

 dual. There is no terminal mucous pore. 



This is the largest British species ; it is rugose 

 above, of a greyish colour, with longitudinal dashes 

 and lines of black. It frequents damp places, and 

 often invades humid cellars or outhouses. Another 

 large species, the black slug (Limax ater), is well 

 known, and is abundant along the banks of 

 hedgerows, and amidst the grass of meadows, dur- 

 ing the summer. It feeds on the leaves and roots ol 

 vegetables. 



Certain slugs of the East and West Indies, consti- 

 tuting the genus Vaginulus of Ferussac, are remark- 

 able lor haying the mantle extended over the whole 

 of the upper surface of the body, or even extending 

 beyond it, and forming in front a sort of luod, 

 beneath which the head can be withdrawn; there 

 is no rudimentary shell, nor any calcareous coiicre- 



