M L L U S C A. 



613 



olltuci. heres to objects when at rest, and by which it crawls 

 Y"""' from one place to another. It is likewise used as a fin 

 in swimming, being aided at the same time by the ser- 

 pentine motion of the whole body. The tunic arises 

 on each side of the foot, and forms an arch over it for 

 the reception of the viscera. Behind, the sac thus form. 

 ed is closed, whose front is occupied by the neck and 

 head. The tunic on the back is, in some cases, forti- 

 fied by one or more testaceous plates ; in others, it is 

 protected by a conical or spiral shell. 



The head is connected at the sides and on the back 

 wflh the cloak, and beneath with the foot. In general 

 it is furnished with tentacula as organs of touch. 



The organs of circulation are more simple than in 

 the cephalopoda. There is no pulmonic auricle nor 

 ventricle, the veins which correspond with the vena 

 cava in the higher classes proceeding directly to the 

 lungs or branchiae. There is, however, a systemic au- 

 ricle, which receives the aerated blood, and a systemic 

 ventricle, which transmits it to the different parts of 

 the body. The heart is usually situated on the left 

 tide; but in those genera furnished with a spiral shell, 

 it is always on the side next the pillar, or the one op- 

 posite to that to which the spire of the shell U di- 

 rected. 



The organs of respiration exhibit the two modifica- 

 tions of lungs and gills. The lungs, destined to respire 

 free air, are usually seated in a cavity on one side, 

 whose orifice is capable of being closed at the will of 

 the animal. The walls of this cavity are lined with a 

 delicate net-work of vessels, in which the blood is ex- 

 posed to the decarbonizing influence of the atmosphere. 

 In those animals whose blood is aerated in water, the 

 gills exhibit every variety of form. In some they oc- 

 cur in a cavity, or exposed in the form of pectinated 

 ridges, in others assuming the appearance of shrubs. 

 In all cases these organs are well protected, delicate in 

 structure, and exceedingly sensible to any external im- 

 pression. 



These two conditions in the organs of respiration, 

 point out the propriety of forming the gasteropoda into 

 two subdivisions, which may be termed, according as 

 they respire by means of lungs or gills, Pulmonifera 

 and Branchifera. Cuvier appears to have been in some 

 measure aware of the importance of the distinction when 

 he instituted his order Pulmoni-s, But he afterwards 

 suffered himself to be more influenced by the presence 

 of an operculum, the shape of the aperture of the shell, 

 and the supposed separation of the sexes, than by the 

 characters of the respiratory organs. Hence he insert- 

 ed the pulmoniferous genus Cyclostoma among those 

 which are furnished with gills. 



SUBDIVISION I. GASTEROPODA PULMONIFIRA. 



The distinguishing character of this group of gaste* 

 **>* ropodous mollusca, is to respire free air. This is per- 

 tarj>vda. formed in a pulmonary cavity situated at the side, the 

 entrance to which the animal is capable of closing at 

 pleasure. The blood-vessels are spread upon the walls 

 of this cavity, chiefly on the roof, in the form of a deli- 

 cate net work. The opening of the cavity is usually on 

 the right side, with the anus behind it, and the sexual 

 orifice is in the front near the head. In some of the 

 genera these openings are situated on the left side. The 

 shells of the former are denominated dextral of the 

 latter siniitraJ. This change in the position of the ex- 

 ternal openings, is accompanied by corresponding alter- 

 ation in the arrangement of the internal organs. The 

 heart, for example, is always placed on the side oppo- 



site the pulmonary cavity. In the dextral shells, there- Mo!luea. 

 fore, it is sinistral, while in the sinistral shells it is dex- > "~*"Y""' ^ 

 tral. In both kinds, however, all the organs preserve 

 the same relation to the back and belly, the head and 

 tail. It is impossible, therefore, to conceive a destral 

 animal changed into a sinistral, by any circumstance 

 which could take place at the period of hatching, as 

 M. Bosc was inclined to believe. This arrangement 

 of the organs must.have been not merely congenital, but 

 coeval with the formation of the embryo. In some spe- 

 cies all the individuals are sinistral, while in others the 

 occurrence is rarely met with in a solitary example. 

 The former are in their natural state, the latter ought 

 to be regarded as monsters. Where the character is 

 permanent, it should constitute a generical difference. 



The animals of this subdivision have, in general, the 

 sexual organs united in the same individual, requiring, 

 however, mutual union. They are all oviparous. 



SECTION I. 



This is intended to include all those pulmoniferous Terrestrial, 

 gasteropoda which are terrestrial. These reside constant- 

 ly on the land where they have been hatched, and where 

 they collect theiffood and propagate their kind. They 

 admit of division into two tribes, from the circumstance, 

 that in some, the shell, which is either flat, or open and 

 subspiral, is concealed in a thickened plate of the cloak, 

 termed the Shield, while in others the shell is spiral or 

 conical, external, and contains the body of the animal. 

 TUIDE I. The genera included in this tribe are 

 three in number. They all possess four tentacula, ca- 

 pable of being withdrawn into themselves like the fin- 

 gers of a glove. The eyes are black points, seated at 

 the tips of the superior tentacula. 



I. LIMAX. Slug. The body is lengthened, and some- Lima*. ". 

 what pointed behind. The cloak is corrugated, and 

 supports the shield on the forepart of the back towards 

 the head, containing a thin corneous or testaceous plate. 

 The pulmonary cavity opens on the right side, at the 

 inferior margin of the shield. Behind this opening is 

 the anus. The sexual opening is underneath the su- 

 perior tentaculum, on the right side. At the termina- 

 tion of the body, above, is an opening, out of which a 

 thick glutinous fluid oozes. The surface of the whole 

 body is liberally supplied with a glutinous secretion of 

 the same sort, but thinner in its consistence. This fluid 

 issues from the pores of the skin and the pulmonary cavi- 

 ty, and when the animal is placed before the fire it is gi- 

 ven out in considerable quantities. The respectable edi- 

 tors of the Journal cF Histoire Nalurelle, vol. i. p. 477, re- 

 commended a lute made of this fluid, reduced to a pro- 

 per consistence by quicklime, for the purpose of secu- 

 ring the mouths of the bottles containing anatomical pre- 

 parations, and preventing the evaporation of the alcohol. 

 With this cutaneous secretion some of the species are 

 capable of forming a thread, by which they are able to 

 suspend themselves from the branches of trees. This 

 mode of assisting locomotion is recorded by Lister, 

 (Animalium Anglia:, p. 130.) and subsequently noticed 

 by other observers. Linn. Trans, vol. i. p. 1 82 ; and 

 vol. iv. p. 85. 



The mouth consists of lips, which are capable of 

 small extension, and above, the entrance is armed with 

 a concave corneous jaw with a notch in the middle. 

 The tongue is merely armed with soft transverse ridges, 

 pointed before, and terminated by a short cartilaginous 

 cone. There is a sensible dilatation of the gullet, which 

 marks the place of the stomach, at the under extremity 

 of which is the rudiment of a caecum at the pyloric 



