402 



GASTEROPODA. 



a given space, they will give birth in a few 

 weeks to five hundred thousand young slugs, 

 which multiplying in their turn would pro- 

 duce at the second laying two hundred and 

 fifty millions of eggs. This fact is well worth 

 the notice of the farmer, who, instead of dri- 

 ving away with so much assiduity crows and 

 other birds which live upon these destructive, 

 though apparently insignificant, animals, would 

 do well occasionally to cherish them as fellow- 

 labourers in his grounds. The Terrestrial Mol- 

 lusca, helpless and incapable of defence, 

 afford food to numberless indefatigable assail- 

 ants, and their preservation is provided for, 

 not only by the number of their eggs, but by 

 a peculiar tenacity of vitality which these ex- 

 hibit under circumstances which would be 

 thought sufficient to destroy the young before 

 they were hatched. The skin of the eggs of 

 the slug is coriaceous and very elastic, so that 

 when compressed they soon resume their 

 shape : exposure to intense cold does not de- 

 stroy their fertility, and they have been known 

 to resist a temperature of 40 without ap- 

 parent injury. When dried by artificial heat, 

 they shrivel up to minute points only distin- 

 guishable by the microscope, yet in this state, 

 if they be put into water, they readily absorb it 

 and are restored to their former plumpness. 

 The same thing happens to those which are 

 dried by the action of the sun and apparently 

 destroyed; a shower of rain is sufficient to 

 supply them with the fluid which they had lost 

 and to restore their fertility. This drying ap- 

 pears not to injure them. M. Leuchs found 

 that after being eight times treated in this 

 manner, they were hatched on being placed in 

 favourable circumstances, and even eggs in 

 which the embryo was distinctly formed, sur- 

 vived such treatment without damage. 



Reproduction of lost parts. Not less won- 

 derful is the power which snails possess of repro- 

 ducing lost parts, after mutilation by accident 

 or design. The results of the experiments of 

 Spallanzani upon this subject are very curious ; 

 he found that if the large tentacle of a snail 

 were amputated, the extremity of the stump 

 heals, forming a small swelling of a lighter 

 colour than the rest of the horn ; in this swel- 

 ling a black point soon becomes visible, which 

 is a new eye, and the mutilated member, in- 

 creasing in length, shortly equals its original 

 size, although it is for some time of a lighter 

 colour than its uninjured fellow, which in other 

 respects it perfectly resembles. The process 

 sometimes varies a little; it frequently happens 

 that the end of the stump, instead of becom- 

 ing round, is elongated and tapers to a point, 

 from the apex of which the new eye is seen to 

 " squeeze out ;" the end of the tentacle then 

 assumes a globular shape, and the most accu- 

 rate dissection cannot distinguish the newly 

 formed eye from the original. If, instead of 

 the horn, the head is cut quite off, a new one 

 will succeed : the new head, however, does not 

 at first contain all the parts of the old one, 

 but they are gradually developed, piece by 

 piece, at different intervals, until at length a 



head differing little, if at all, from the original 

 pattern is completed. In some cases the ob- 

 ject is effected by a different proceeding, the 

 new part appearing like a round tubercle, con- 

 taining the rudiments of the lips and of the 

 smaller horns, which is united to the mouth 

 and the new-formed tooth, the other parts, 

 as the larger horns and the anterior part of the 

 foot, being totally deficient. In another snail 

 the larger tentacle on the right side first ap- 

 peared, not more than one-tenth of an inch 

 in length, but already provided with its eye, 

 and at a short distance beneath this the linea- 

 ments of the lips separately developed them- 

 selves. In a third snail a group of three horns 

 is seen, two of which will acquire their full 

 developement, while the third is just above the 

 level of the skin. These and many other 

 varieties have been observed; but in most 

 instances there is no perceptible difference 

 between the new head and the one cut off, 

 the exact line of separation being indicated 

 by an ash-coloured mark distinguishable two 

 years after the experiment. The same effects 

 follow, whether the head be removed above or 

 below the brain, and in the latter case a new 

 brain, with all its nerves, is speedily con- 

 structed. The collar and foot are also per- 

 fectly restored after their removal. 



Slugs reproduce their horns as well as snails, 

 but their power of manufacturing a new head 

 is much inferior. 



Muscular integument. None of the Gaste- 

 ropoda have any thing analogous to an endo- 

 skeleton, a circumstance which sufficiently ac- 

 counts for the varied forms which the same in- 

 dividual assumes under different circumstances, 

 for the body being unsupported by any re- 

 sisting framework, readily yields to the con- 

 tractions of the muscular integument with 

 which it is covered. It is from this circum- 

 stance that the zoologist finds the preservation 

 of the natural forms of the recent animals a 

 task of such extreme difficulty, owing to the 

 corrugation and distortion produced by the or- 

 dinary modes of preservation; it is scarcely 

 possible indeed, in many cases, to recognise 

 with tolerable accuracy the natural appearance 

 of these creatures in the shrunken specimens 

 generally preserved in our cabinets, and the 

 collector of these objects would do well never 

 to omit, when circumstances allow him the op- 

 portunity, to preserve some sketch of the living 

 forms of such exotic species as may come into 

 his possession. 



Body. In the naked Gasteropods the whole 

 body is found to be inclosed in a muscular in- 

 tegument, the basis of which is a cellular web 

 of extraordinarily extensible character, in which 

 the muscular fibres may be seen to cross each 

 other in various directions, some passing longi- 

 tudinally from one extremity of the animal to- 

 wards the opposite end, while others, assuming 

 different degrees of obliquity, are interwoven 

 with the rest, so as to occasion the elongation 

 or contraction of the body in every assignable 

 direction. Within this muscular bag the vis- 

 cera are contained, as well as the organs sub- 



