925 



GASTEROPODA. 



GASTEROPODA. 



626 



where they grow a long time, necessarily produce an oblique spire. 

 If the reader will imagine an oblique cone in which other cones are 

 euccessively placed, always larger in a certain direction than in the 

 others, it will follow that the whole rolls itself upon the side which ia 

 least. The part on which the cone is rolled is called the Columella, 

 or Pillar : this is sometimes solid, and sometimes hollow. When it is 

 hollow, the open end of it is named the Umbilicus. The whorls of 

 the shell may remain nearly on the same plane, or may extend 

 towards the base of the columella. In the last case, the preceding 

 whorls are raised one above the other, and form what is called the 

 Spire, which is pointed in proportion to the more rapid descent and 

 small enlargement of the whorls. Those shells with an elongated or 

 projecting spire are termed Turbinated Shells. When, on the con- 

 trary, the whorls remain nearly on the same plane, and are not 

 enveloped one within another, the spire is flat or even concave. 

 These are called Discoid Shells. When the upper part of each whorl 

 envelops the preceding ones, the spire is said to be concealed. That 

 part of the shell from which the animal comes forth is termed the 

 Aperture. When the whorls remain nearly on the same plane, the 

 animal, when it creeps, carries its shell disposed vertically, the 

 columella lying across the posterior part of the back ; and its head 

 passes under the border of the aperture opposed to the columella. 

 When the spire is elongated it is directed obliquely to the right in 

 almost all the species : a small number only have it directed to the 

 left when they creep ; these shells are called Reversed or Left-Handed 

 Shells. The heart is always on the side opposite to that where the 

 spire is directed. It is therefore ordinarily on the left side ; in the 

 reversed or left-handed shells it is on the right. The contrary of this 

 disposition holds good with regard to the organs of generation. 



The organs of respiration, which are always situated in the last 

 whorl of the shell, receive the ambient element under its edge, some- 

 times by means of the mantle being entirely detached from the body 

 along the whole length of this edge, sometimes in consequence of its 

 being merely pierced by a hole. The border of the mantle is some- 

 times prolonged into a canal, so that the animal can advance to seek 

 the surrounding fluid without exposing either its head or foot beyond 

 the shell. For this purpose the shell, in such cases, has also on its 

 edge, near to that end of the columella (the base) which is opposed to 

 that whereto the spire tends (the apex), a notch or a canal for the 

 lodgment of that of the mantle. The canal is consequently on the 

 left in the ordinary species, and on the right in the reversed shells. 

 The animal being very flexible is able to vary the direction of its 

 shell, and most frequently when there is a notch or a canal it is 

 directed forwards ; the spire is thus behind, the columella on the left, 

 and the opposite border, or external lip, as it is termed by some cou- 

 chologists, on the right. A directly contrary disposition is manifested 

 in the Reversed Shells, and these, in consequence of this contrary 

 disposition, turn towards the left instead of turning towards the right, 

 as in the normal structure. It follows as a consequence that the 

 aperture of the shell, which is formed principally by the last whorl, is 

 more or less large in proportion to the other whorls, accordingly as 

 the head or foot of the animal, which is to be constantly protruded 

 therefrom and retracted thereunto, is more or less voluminous com 

 pared with the mass of the viscera which remain fixed within the 

 shell The aperture is moreover wider or narrower in proportion as 

 the same parts are more or less thick. There are shells whose aper- 

 ture is narrow and long ; the foot, in such cases, is delicate, and 

 doubles together for the purpose of re-admission. The greater 

 number of aquatic Gasteropoda with a spiral shell have an Operculum, 

 or separate piece, which is sometimes horny, sometimes calcareous, 

 attached on the posterior part of the foot, and which shuts the shell 

 when the animal has re-entered it and is entirely retracted within. 



Cuvier, in continuation, remarks that there are Gasteropods with the 

 sexes separate, and others which are hermaphrodites : of these lasl 

 some have the power of reproduction without the aid of a seconc 

 iudividual, while the others require a reciprocal copulation for the 

 continuation of the species. He adds that the organs of digestion 

 present as many differences as those of respiration, and he divides the 

 class into the following orders : 



1. Les Pulmoues, Pulmonifera (Pulmobranchiuta of De Blainville). 



This order is distinguished from the Mollusks inasmuch as they 

 re^i'ire the elastic atmospheric air by means of a hole opened under 

 the border of their mantle, and which they dilate or contract at their 

 pleasure. They have consequently no branchiae, or gills, but only a 

 net-work of pulmonary vessels, which creep around the walls am 

 principally upon the plafond of their respiratory cavity. Some are 

 terrestrial, others aquatic ; but these last are obliged to come to the 

 surface of the water from time to time, in order to open the orifice o, 

 tlioir pectoral cavity for the purpose of respiration. 



The Terrestrial Pulmouiferous Mollusks have all four tentacles 

 two or three only, of very small dimensions, have not permitted the 

 observer to see the lower pair. They are divided into those which 

 are naked, and those which are protected by a shell. They are al" 

 hermaphrodites. 



Tlirme which have no apparent shell formed the great genus Limax 

 of Linnaeus ; and of these every one may find examples in the common 



Slugs. [I.IMAX.j 



ParmaceUa and Testacella lead the way to those which have a com- 

 _>lete and apparent shell, the borders of whose aperture, in the 

 majority of instances, are reflected into a little roll (bourrelet) when 

 he animal is adult. These were placed by Linnaeus under his great 

 genus Helix. The shell varies much in form ; being for instance sub- 

 ;lobular or subdiscoid, as in many of the shell-snails ; or elongated 

 and pyramidal, as in liulinia, &c. [HELICID.E.] 



The Aquatic Pulmoniferous Mollusks have only two tentacula, and 

 always come to the surface to breathe ; they do not therefore inhabit 

 deep waters, but live for the most part in the fresh waters or salt lakes, 

 or at least near the sides and mouths of rivers. 



Cuvier goes on to give Onchidium, Buchanan (Peronia of De Blaiu- 

 ille) [NCDIBRANCHIATA], as an example of the Aquatic Pulmouiferous 

 dollusks without shells. 



Those with shells, which are sometimes discoid, as iu Planorlis, or 

 elongated and pyramidal, aa in I*imncea, &c., he illustrates by the 

 genera Phyaa, Scarabceui, Auricula,, and Conovulus. 



2. Nudibranchiata (Polybranchiata Tritonia, &c., of De Blainville). 



The Mollusks composing this order have no shell nor any pulmonary 

 cavity ; but their branchiae are naked, and placed upon some part of 

 the back. They are all hermaphrodites and marine. They often 

 swim reversed, with the foot concave like a boat, at the surface, aiding 

 their progression with their mantle and tentacles as with oars. 



3. Inferobranchiata. 



This order presents nearly the same form and organisation as the 

 Doridei and Tritonia : but their branchiae, instead of being placed 

 upon their backs, are arranged in two loug rows of leaflet-like append- 

 ages on each side of the body under the projecting border of the 

 mantle. Phyllidia and Dlphyllidiu,, Cuvier, belong to the Infero- 

 branchiata, 



4. Tectibranchiata(MonopleurobranchiataolDe Blainville). 



This order has the branchiae attached either along the right side or 

 upon the back, in the form of leaflets, which are more or less divided, 

 but not symmetrical The mantle covers the branchifo more or less, 

 and almost always contains in its thickness a small shell. The Tecti- 

 branchiata approach the Pectinibranchiata in the form of the organs 

 of respiration, and live like them in the sea ; but the Tectibranchiata 

 are all hermaphrodites, like the Nudibranchiata and Pulmonifera. 

 The genera Plewobrtmchus, Cuv., Pleurobranchcea, Meckel, Pleuro- 

 branchidium, De Blainville ; Aplysia, Linn. ; Dolabdla, Lam. ; 

 Notarchue, Cuv. ; Burtatella, Blaiuv. ; Atera, Mull. ; Bullcea, Lam. ; 

 part of Sulla, Linn. ; Gasteroptera, Meek. ; Umbrella, Lam. ; Gastro- 

 plex, Blainv. ; belong to this order. [BUBSATELLA; BULLID^E; 

 TECTIBRANCHIATA ; PATELLID.S.] 



5. Heteropoda, Lam. (Nectopoda, Blainv.). 



Distinguished from all the others, inasmuch as their foot, instead 

 of forming a horizontal disc, is compressed into a muscular vertical 

 plate, which serves them as a fin, aud at the edge of which, in many 

 species, a dilatation, in form of a hollow cone, represents the disc of 

 the other orders. Their branchiae, formed of feathery lobes, are 

 situated on the posterior part and left side of the back, directed 

 forwards ; and immediately behind them are the heart and a liver of 

 no great size, with a part of the viscera and the internal organs of 

 generation. Their body, lined with a muscular coat, is elongated, 

 terminating most frequently by a compressed tail. Their mouth is 

 formed by a muscular mass, and is furnished with a tongue beset with 

 small hooks. The oesophagus is very long, the stomach delicate iu 

 texture, and two tubes at the right side of the packet of viscera give 

 exit to the excrements and to the ova or to the prolific fluid. They 

 generally swim with the back downwards and the foot above, and 

 they can swell out then- bodies by filling them with water by means 

 which are not as yet well understood. 



To this description Cuvier adds, that the method of swimming 

 above described having induced Pe'ron to believe that the natatory 

 plate was on the back, and the heart and branchiae under the belly, 

 has given rise to many errors as to the proper place of these animals. 

 Cuvier adds, that the examination of their nervous system led him to 

 the opinion expressed in his memoir on the Mollusca, that they were 

 analogous to the Gasteropods. A more complete dissection, he observes, 

 made since, and that given by Poli, in his third volume, have com- 

 pletely confirmed this conjecture, and he states that the fact is that 

 the Heteropoda differ but little from the Tectibranchiata. M. Laurillard 

 believes that the sexes are always separate. Cuvier also remarks, that 

 M. De Blainville makes of his (Cuvier s) Heteropoda a family which he 

 names Nectopoda, and unites them with another family which he 

 names Pteropoda, and which comprehends none of Cuvier's Pteropoda, 

 except Limacina. To this, Cuvier observes, M. de Blaiuville refers 

 Aryonauta, upon what conjecture Cuvier knows not. 



Forskal places all the Heteropoda of Cuvier under his genus 

 Pterotrachea. [CAHINAHIA; ATLANTIA; HETEROPODA.] 



6. Pectinibranchiata (Paracephalophora Dioica, Blainv.). 

 This order is by far the most numerous division of the Gasteropods, 

 for it embaces nearly all those which have spiral univalve shells, and 

 miiny of those whose shells are simply conical. Their branchiae, 



