Digestive Organs.-The mouth is armed with an upper horny jaw, and adherent within the cavity is a horny muscular tongue, which is a mechanical organ for the attrition of the food. This lingual ribbon or tongue (fig. 6, c), as it is termed (often, but erroneously, pallet), is covered by more or less regular quadrangular plates, carrying erect amber-coloured and glossy teeth of extreme tenuity, which are directed backward. This tongue acts in concert with the horny jaw (a), the one holding and the other rasping the vegetable food into the mouth.

As the lingual ribbon is such a pretty and interesting object for examination with the microscope, and as it plays so important a part in the economy of all snails and slugs, land, fresh-water, and marine; and also because the teeth vary in number, in arrangement, and in ornamentation in the different genera and species, I will now give a method of preparation, and will also point out, in its proper place, the value in a systematic arrangement of the species of these objects.

The tongue forms the floor of the mouth, and

the front part, which is the only part in use, is frequently curved or bent quite over, and its teeth are often broken

Fig. 6.- Diagrammatic view of and blunted; the hinder

the dental apparatus.

portion descends obliquely behind the mouth, and its edges are united to form a tube (c), and enclosed in a membranous sheath, which opens gradually as the part is brought forward to replace the worn portion.

The most simple plan to prepare these as microscopic objects is to boil the head of the mollusk in a solution of potash in a test-tube, by which all the parts, with the exception of the tongue and jaw, are dissolved: care must be taken to thoroughly wash the tongue before mounting.

The most instructive method is doubtlessly that of dissection; but certainly, when we have some of our minute snails to deal with, that of maceration will be a great saving of time and patience. The head should be pinned down in a guttapercha trough containing water enough to cover the part; the floor of the mouth may be laid open by passing the lower point of a pair of scissors into the mouth, and cutting upwards; now pin back the severed portions, and by the aid of a lancet or needle, work out the lingual apparatus. The ribbon should be cleaned by washing with a camel's hair brush, or by soaking in potash-water; if the latter, wash the tongue well before mounting. The preparation may be mounted in glycerine, or if intended as an object for the polariscope, it should be mounted in Canada balsam.

E

The length of the lingual ribbon is short in Paludina, but is of varying length for different species; in the marine limpet it is longer than the whole animal. The teeth are distributed in straight longitudinal rows, and in transverse rows, which are variously curved, angular, or rarely straight. The number of the teeth in the transverse row is nearly constant for the same species, and the number of rows is exceedingly variable in different species; longitudinally the teeth are usually arranged in a triple series, and each transverse row is but a repetition of the rest. The central area is called the rachis, and the teeth form usually a single series; the lateral areas are called the pleura, the teeth on which are termed uncini, and usually are extremely numerous. The term laterals is employed in a restricted sense to designate a series of teeth intermediate between the rachidian and the uncini.

The lingual ribbon of Paludina vivipara con

(101)

a

Fig. 7.-Teeth of Paludina vivipara (Lovèn).

sists of a few transverse rows, each composed of

a central oval tooth (a), slightly hooked and denticulated; and three nearly similar uncini (b, c, d), which are oblong and toothed on the upper sides: the number of teeth in each transverse row is therefore seven.

The number and arrangement of the teeth are capable of easy representation by a numerical. formula. Thus, 3. 1. 3 represents the system in Paludina, signifying that each transverse row consists of one median or rachidian tooth, flanked on each side by three uncini.

100

In Vitrina the general formula is co. 1. ∞; where represents 37; and as there are 100 rows, the lingual teeth of Vitrina are 7,500; the formula will now stand 37.1.37 7,500. In the great slug, Limax maximus, there are 28,800 teeth, distributed in 160 rows of 180 teeth in each. The number of teeth has no relation to the size of the animal; thus, Helicella cellaria possesses 1,330, while H. nitidula, less in size, has nearly three times that number.

The above dental formula will be employed to indicate the number of teeth in each generic group or species, as the case may be.

The teeth of the fresh-water Gasteropods are characterized by their fewness, whilst those of air-breathing snails are remarkable for their extraordinary number.

The digestive apparatus further consists of a

long gullet (fig. 6, b), an intestinal canal, folded in the substance of the liver, bent to the right, and terminating close to the margin of the mantle in about the middle line of the body. The liver occupies a very large part of the abdomen.

The Circulatory System is somewhat more perfect than in the bivalves.

The Respiration is aquatic; the organ is a plume-like gill, formed of a series of triangular plates attached to the left side of the branchial cavity. The water passes into this chamber through a respiratory siphon formed by the folding of a small lappet of the neck.

The Nervous System is much the same as in the bivalves. The eyes are two in number, placed on short stalks attached to the conical tentacles. The organs of hearing are situated near the base of the tentacles, and are sacs containing an otolith suspended in a fluid. The sense of smell is possessed by Gasteropods, and that of touch in a high degree; the tentacles are endowed with great sensitiveness.

Reproduction. - The The sexes are distinct in Paludina; the eggs are retained within the oviduct of the parent until they are hatched, and the young are not excluded until they have attained a considerable growth. Paludina is therefore ovoviviparous. The young