ANATOMY AND PHYSIOLOGY OF A SNAIL. 21 



tentacles only are present they may be short and triangular, as in 

 Limncza, very long and cylindrical, as in Planorbis and Physa. 

 The eyes may be situated on the apices of the dorsal tentacles, as 

 in Hdix, or situated at their base, as in Limncea, Ancylus, and 

 Physa, or sessile, as in Planorbis, or placed upon tubercles, as in 

 Hydrobia, or placed upon pedicels, as in Paludina. 



The reproductive orifice is generally a single opening just 

 behind and slightly below the right tentacle, but in some genera, 

 as Limncea, Planorbis, and Ancylus, the generative openings are 

 separated from one another, the male orifice (penial orifice) being 

 situated a little anterior to the female orifice (ps vagina), a con- 

 dition of things which seems to be associated with the situation of 

 the eyes at the base of the tentacles. To those genera in which 

 these two last-mentioned features obtain Professor Ray Lankester 

 has given the name of BASOMMATOPHORA, as distinguished from 

 the STYLOMMATOPHORA, in which there is only one generative 

 aperture, and the eyes are elevated on tentacles which can be 

 invaginated into the body-cavity, as the genera Helix, Pupa, 

 Clausilia, Testacella, Limax, Amalia, Arion, and Onchidium. 



The visceral mass may be large and coiled spirally in the shell, 

 as in the Helicidce, Limnceidce, Cydostomatidce, Paludinidcz, and 

 Carychiidce, or it may be co-extensive with the foot, as in the 

 Limaddce. The sole of the foot is ciliated in the majority of 

 terrestrial forms, and this ciliation extends all over the surface of 

 the body in the aquatic species. In Arion the lateral margins of 

 the foot are also ciliated, and this feature also obtains around the 

 margin of the supra-p^dal gland and the pulmonary orifice in 

 Helix nemoralis and in Amalia marginata. In microscopical 

 structure the body-wall consists of an epidermis of columnar cells, 

 and internally to this a thickened layer of muscle-fibres and 

 connective tissue. The muscle-fibres are not so differentiated as 

 those in the muscle-tissue of vertebrates, but consist of cells which 

 generally are non-striated, although sometimes they are found to 

 present an appearance of fine striation. The connective tissue is 

 described as made up of " plasma-cells, a matrix, and fibrils.' 1 

 The plasma- cells may be irregular in shape, containing refractile 

 granules, or oval, nucleated, with a clear protoplasm, or they may 

 contain granules of carbonate of lime. In the oval-shaped cells 



