445 



446 



"Certainly, if we consider the shells only, there is a very great 

 resemblance between a Physa and a Limncea, but all the Physic are 

 sinistral the Limncece are dextral ; the Physce have a polished and 

 shining shell, because the animal has its mantle lobated and turned 

 back upon the shell, which is not the case in Limncea ; the animal of 

 Physa carries on its head elongated and narrow tentacles, like those of 

 Planorbig, and not triangular and thick ones, like those of Limncea. 

 These characters seem sufficient to retain the two genera in the 

 system, and consequently to reject the opinion of Mr. Sowerby, who 

 unites them in his genera." 



Lamarck recorded four species of Physce (recent). M. Deshayes, 

 in the last edition of the ' Histoire,' increases the number to ten ; and 

 he regrets that M. Michaud has given no detail with regard to some 

 species indicated as found in France, but which do not appear to live 

 there. He observes that Lamarck has recorded two Physce (P. castcmea 

 and P. tubopaca), the first from the Garonne and the last from the 

 environs of Montpelier, which M. Michaud does not mention. M. Des- 

 hayes adds, that we must probably conclude, from the silence of 

 M. Michaud, that these species have not been found, and that 

 Lamarck, deceived by a false indication, has given them a habitat not 

 theirs. Conrad has described an additional species. 



P. foHtinalis, Drap. (Bitlla fontinalis, Linn.). Shell sinistral, oval, 

 diaphanous, smooth ; of a yellowish-horn colour ; spire very short 

 and rather pointed. 



Bhell of Phyia fontinatu. 



It is found in England, France, and North America (Claiborue, 

 Alabama). Conrad. 



Colonel Montagu (' Testacea Britannica ') notices the species as not 

 uncommon in stagnant pools, as well as running waters, in many 

 parts of the kingdom, and as most frequently found on the under part 

 of the leaves of aquatic plants. He gives a description of the animal, 

 and says that when in motion it covers a great part of the shell with 

 a thin pinnated membrane, thrown out on the right side, extending 

 quite behind and partly on the left side, covering the smaller volutions : 

 this membrane (mantle) is, he says, very deeply divided, or digitated, 

 the points of which meet and sometimes intersect on the back of the 

 ihell, and it is so transparent as scarcely to be distinguished but by 

 the assistance of a glass. The foot he describes as long and narrow, 

 and the foramen on the left side, " as must be the case with all the 

 animals of this kind inhabiting heterostrophe shells." Colonel 

 Montagu concludes his remarks on this species as follows : " It has 

 a very considerable locomotive power, and transports itself by 

 adhering to the surface of the water, with the shell downwards, 

 against which it crawls with as much apparent ease as on a solid 

 body; and will sometimes let itself down gradually by a thread 

 affixed to the surface of the water, in the manner of the Umax flans 

 (' Linn. Trans," iv. 85, t. 8) from the branch of a tree. The property 

 of crawling under water against its surface is not wholly confined to 

 this species ; but we know of no other testaceous animal capable of 

 suspending itself tinder water in the same way except a species of 

 Litiopa. [LiTIOPA.] It has the power of throwing its shell about 

 in an extraordinary manner, either in defence or to remove obstruc- 

 tions, continuing at the same time fixed by its foot. Probably this 

 singular motion is sometimes occasioned by a minute species of 

 Mirvdo (Gcrrdiut inquilinus, Mull., ' Verm.') which infests this and 

 many other fresh-water testaceous animals; twenty or more may 

 be seen adhering to its sides like slender white filaments." 



Limncea (Limneus, Limnreiu, Lymncea). Animal of oval form, more 

 or less spiral ; head furnished with two flattened triangular tentacles, 

 carrying the eyes at their base, on the internal side ; mouth furnished 

 with an upper piece for mastication, surmounted by a sort of very 

 short veil ; foot oval, bilobated anteriorly, narrowed posteriorly ; orifice 

 of the pulmonary cavity on the right side, on the collar, in form of a 

 furrow, and capable of being covered by a fleshy appendage which 

 borders it below; anus on the side; organs of generation distant, 

 the orifice of the male intromissive organ being under the right 

 tentacle, and that of the vagina at the entry of the pulmonary 

 cavity. 



Shell delicate, fragile, of an oval oblong, with a spire more or less 

 sharp and elongated, and an aperture longer than it is wide, oval, 

 sometimes very large, with a sharp edge, not continuous, on account 

 of the convexity of the preceding whorl ; on the coluuiella an oblique 

 plait. (Rang.) 



M. Deshayes observes (last edition of Lamarck), that the animal of 

 Limnrea presents peculiar characters. On the head are two triangular 

 tentacle* very much enlarged at the base, and having the eyes rather 

 projecting on the upper and internal part of that base. The head is 

 large and flattened, separated from the foot by a shallow furrow. The 

 foot inclines to oval, terminated in a point posteriorly, and delicate 

 and flattened on the sides. The mantle, closed anteriorly and narrow, 

 forms a nort of collar, as in the Helicei. There is a great cavity 

 behind its border. The upper wall of this cavity, delicate and trans- 



parent, is covered on its internal surface by a very well-developed 

 vascular net-work, destined for respiration : it is near the aperture 

 of the mantle, and a little below it, that the orifice of the anus 

 is seen. 



The Limncece appear to occur in almost all parts of the world, but 

 the form is most seen in the temperate and northern regions. 



l.ininira stagnalis, 



Fresh waters, especially those which are stagnant, are the resort of 

 the Limncece ; in such situations they abound, feeding on the aquatic 

 plants on whose stems they creep, and coming to the surface to respire 

 the air. Here they may often be seen in a reversed position, and 

 probably maintained in it by the air in the branchial cavity. Like 

 the Physce they have the power of locomotion when so situated, and 

 may be observed moving their ventral disc, as if they were employing 

 it against a solid surface, whereas the animal only touches an extremely 

 thin lamina (so to speak) of water, which offers sufficient resistance 

 for its progression. In the reproduction of the species the animals 

 are employed somewhat differently from the Helicidcu and Limacidce, 

 though, like them, each individual is furnished with both male and 

 female organs of generation ; for the same Limncea is capable of 

 serving at the same time as a male for a second and as a female for 

 a third, and by this connection of one individual with two others a 

 continuous chain of some length is not unfrequently produced. 

 No. 2313 of the fifth or allotriandrous series of preparations illus- 

 trating the principles of generation, in the museum of the Royal 

 College of Surgeons in London (' Catalogue,' vol. iv., ' Physiological 

 Series'), exhibits the soft parts of the generative, anal, and respiratory 

 orifices of Limncea stagnalis, and shows how this Gastropod differs 

 from the Limaciclce and Helicidce in the separation of the above- 

 mentioned orifices from one another. The number of eggs is very 

 great, and they are deposited on stones, stems of vegetables, &c., in 

 elongated masses enveloped iu a glairy substance, which is said to 

 increase iu proportion to the development of the embryoes. For 

 details on the reproduction and embryogeuy of these Molluscs 

 we refer the reader to the works of M. Pfeiffer and of M. Dumor- 

 tier ; also a paper by Mr. Hoff in the ' Transactions of Microscopical 

 Society,' 1854. 



The recent species are numerous : Lamarck recorded twelve, includ- 

 ing L. columnar!*, which is considered to be an A chatina ; Deshayes, 

 iu the last edition of Lamarck's ' Histoire,' has added eight more ; 

 Bean and Troschel have each added one. 



L. stagnalis. This is Helix stagnalis, Linn. ; Buccinwm stagnate, 

 Miill. ; and Bulimus stagnalis, Brug. Shell ovate-acute, ventricose, 

 thin, pellucid, substriated longitudinally, of a horny colour ; the last 

 whorl subangulated above ; the spire conico-subulate ; the aperture 

 large. 



Limnrra itat/nalii. 

 a, the animal in the shell ; , mass of eggi, magnified. 



Montagu observes that it is frequently covered with a green epi- 

 dermis, and sometimes a concreted stony matter that almost obliterates 

 the upper volutions ; he adds that some authors have made this shell 

 into two or three species, apparently from size only. 



