ORDER GYMNOBRANCHI ATA VEL NUDIBRANCH I ATA. NAKED-GILLS. 



131 



mon Whelk, Buctinum undatum, is an example of the former, and the 

 Garden Snail, Helix hortensis, of the latter. 



ILLUSTRATIVE EXAMPLES. 

 PLATE 3. 



LIMAX. Body long, contractile ; above convex, beneath flat, with a long 

 foot the whole length of the body ; tentanula four, unequal, the two upper 

 long, with the eyes at the end. 



This genus of naked, land, molluscous animals, was established by 

 LinnfEus, but restricted by Lamarck. They differ from the other mollusca 

 Snails by breathing free air in a pulmonary cavity lined with minute, 

 pulmonary vessels. They are usually called Slugs ; they differ from the 

 Snail in having no shell. They have been divided into two genera, one 

 being provided with a small shelly plate in the shield (arion, Plate 3), 

 while others have only a few crustaceous grains in that part. 



L. maximus vel antiquarius (Plate 3) is the type of the genus. It is 

 common in cellars and damp places in England. 



Illustrations : Arion antiqwrum, Umax variegatus. 



TESTACELLA. Body lengthy, slug-like, with the foot not very distinct, 

 and furnished posteriorly with a very small, external, very flat, ear-shaped 

 shell, slightly spiral at its apex, and having a very large oval aperture, the 

 left edge of which is sharp and a little inclined inwards ; its four tentacules 

 intractile, and the posterior larger pair supporting eyes on their tips'; 

 pulmonary aperture round, and on the right side near the top of the shell, 

 near to it the vent ; generative aperture beneath the right large tentacule. 



This genus has in its form and size much resemblance to a small slug, 

 but is distinguished from it by the cloak, which is very extensible, being 

 placed far back on the body, and containing a very small shell, and by two 

 grooves which pass from the base of the tentacules to the shell ; its branchial 

 and anal apertures are also near the tail, instead of, as in the Slugs, being 

 near the fore part. They live constantly under ground, sometimes at a 

 depth of three feet, and are rapacious, feeding upon worms, which they 

 absorb head foremost and gradually draw in as digestion proceeds. Though 

 this mollusk was discovered by Dugue at Dieppe in 1740, yet it is to 

 M. Mauge, who brought home specimens from Teneriffe some years since, 

 the honour is due of having drawn the attention of naturalists generally to 

 this genus. There are three species (Roissy speaks of four), viz., T. Halio- 

 tideus, T. Scutulum, and T. Maugei. 



Illustration : Testacella Maugei. The shell which covers the posterior 

 part of the pulmonary cavity is external, solid, auriform, with a large and 

 oval aperture : native of Teneriffe, naturalized at Bristol. This, it is sup- 

 posed, is the only carnivorous terrestrial mollusc. 



HELIX. Shell globular or conical ; spire short, conical ; whorls rapidly 

 enlarging ; last generally keeled when young, and sometimes so when full 

 grown ; the mouth semilunar, the edge of the mouth reflexed and thickened 

 internally ; axis perforated, often covered when full grown. 



The species of this genus are very numerous ; and every traveller who 

 takes the trouble to save the kinds which fall in his way, is almost sure of 

 adding to their number. 



Several experiments have been made on the property which snails possess 

 of reproducing a part which may have been amputated. Spallanzani was 

 the first to observe, that when the head was cut off, it was, after a short 

 time, reproduced. Adanson, in a most positive manner, denied this fact, 

 after trying the experiment on fifteen hundred individuals ; but he admitted 

 that the wound would heal if the head was left attached by a portion of 

 the skin. 



Like most terrestrial shells, only a few species Tiave been found fossil. 

 Brongniart has described seven species, found in the neighbourhood of 

 Paris. 



Illustrations: Helix naticordes, H. japonica, H. algira, H. carocolla, 

 H. nux denticulate*,, H. albella, H. epistyllium. 



BULIMUS (or BULIXAS). Shell oval, oblong, or turreted; aperture 

 entire, longitudinal ; margins very unequal, disunited above ; columella 

 straight, smooth, entire, and simple at the base. 



The animals of this genus are all inhabitants of the land, and vegetable 

 feeders. The species consist of many of the land Testacea, which Linnaeus 

 placed in the genera Bulla and Helix. The animal has four tentacula, the 

 two larger of which, as in the Helices, bear the eyes on their summits. It 

 has no operculum. Its habits are similar to those of the Helices. Some of 

 the sheik of this genus are amongst the most beautiful and the largest of 

 the land division. 



Illustration : Btdimus ovatus ; not less than four inches and a half in 

 length. There are several small species, natives of Great Britain ; as B. 

 acutus, montanus, obscurus, lubricus, fyc. 



PUPA. Shell cylindrical, ribbed, blunt ; spire long, obtuse, composed 

 of whorls which gradually increase ; mouth sinuous, aperture rounded an- 

 teriorly ; peristoma reflected. The genus Pupa is composed of land shells 

 with plaited columella ; they are of variable form, and differ from the species 

 of Bulina in their spiral whorls, and in the plica? on the columella. They 

 differ from Clausilia (next genus) in the want of a clausium. They are 

 found in Britain, South of Europe, Mexico, West Indies, and in other 

 places. 



CLAUSILIA. Shell fusiform, slender ; slightly obtuse at the apex ; 

 aperture irregular, ovate ; peristoma complete, free, reflexed. 



The most curious circumstance belonging to these little animals, and 

 which distinguishes them from those of every other genus, is the existence, 

 in the adult, of a small shelly plate, serving as an operculum to the shell, 

 but fixed to the shell itself, and having no attachment whatever to the 

 animal ; it is found in the neck as it were of the shell, fixed in a groove in 

 the columella by a little elastic thread-like process ; when the animal pro- 

 trudes itself from the shell, it pushes aside this little plate, which, on the 

 animal's retiring, closes the aperture by its own elasticity. They are found 

 amongst moss, dead leaves, and on the bark of trees. There are not less 

 than four or five British species. 



Illustration : Clausilia. 



ACHATINA. Shell elongate, thin, white, turreted ; aperture moderate, 

 pyriform, or ovate ; outer lip thin, sharp, without any internal rib ; colu- 

 mella smooth, tortuous ; also so truncated in front as to form a notch at its 

 union with the outer lip. 



The notch occasioned by the abruptly-truncated termination of the inner 

 lip, or pillar of the shell, is that by which we are enabled to distinguish the 

 species of this genus from those of Bulina, to which, in their general cha- 

 racters, they are allied. 



These land shells are pretty generally diffused, but the largest and most 

 beautiful are found in tropical climates. 



Illustration : Achatina Virginea. 



PHYSA. Shell convoluted, oblong, with a prominent spire ; aperture 

 longitudinal ; peristome wanting in the body whorl ; pillar twisted ; margin 

 of the mantle loose, divided into lobes, and capable of being reflected over 

 the surface of the shell, near the mouth ; operculum wanting. 



Type of the genus, Bulla fontindis, Linnaeus ; Lister. Three species, 

 found in lakes and slow-running streams ; one of them not uncommon in 

 England. 



Illustration : Physa rivalis. 



ORDER GYMNOBRANCHIATA vel NUDIBRANCHIATA. 

 NAKED-GILLS. 



SOME of the Gasteropods are entirely naked, such as the Doris, which 

 swims with its foot upwards, and is moved by the fringed overlapping edge 

 of its dorsal tegument, and by a pair of club-shaped tentacules on the back, 

 which serve as a pair of oars ; its branchial apparatus is situated around 

 the aperture of the vent, also on the back, and, being free, presents an 

 example of the Dorso-nudibranchiate Order ; whilst, on the contrary, the 

 naked branchial fringes which depend between the foot and overlapping 

 dorsal tegument of Phyllidia indicate the Infero, or Ventronudibranchiate 

 Order. 



