92 Steel, Tracks of Garden Snail. [v<X Ict xxxv 



a series of connections or bridges between successive dis- 

 charges. 



Snails crawling on linoleum leave a very clear intermittent 

 trail, the waxy surface of the linoleum not lending itself to the 

 spreading of the mucus as glass does, and thus tending to 

 preserve intact the separate deposits. When a snail is crawling 

 on glass the muscular motion of its foot or sole is very clearly 

 seen from beneath, passing forward in a series of simultaneous 

 waves like those seen in the legs of a crawling myriapod. The 

 tracks of slugs, Limax, &c, closely resemble those of snails, 

 but on a smaller scale, and the bridging from one deposit to 

 another, on rough surfaces, is usually more complete. 



Both snails and slugs secrete two kinds of mucus, apart from 

 that secreted by the genital mucus gland. That forming the 

 track is, as has been mentioned, the product of the large ventral 

 mucus gland. In both cases it is a clear, glairy, very tenacious 

 substance. When the animal is crawling quietly, without 

 being disturbed, the mucus is quite translucent, and under the 

 microscope shows only a few scattered rounded cells and some 

 nucleated cells, which I suppose to be epithelium cells. As a 

 distinctive term I would propose to call this secretion the 

 ambulacral mucus. On the other hand, the secretion coating 

 the dorsal surface, both of snails and slugs, and which may 

 perhaps best be distinguished by terming it dermal mucus, is 

 turbid, and when examined microscopically is seen to be densely 

 crowded with minute structureless granules or cells of varying 

 dimensions, mostly sausage-shaped, and reminding one of a 

 crowd of diatoms. When the animal is irritated by touching 

 or in other ways, the secretion is poured out abundantly. It 

 is the product of numerous epidermal glands resembling those 

 described by Dr. A. Dend} 7 in his description of the anatomy 

 of a land planarian, Trans. Royal Soc. Victoria, i88g. (See 

 also " Outlines of Zoology," by J. Arthur Thomson, 1892 edition, 

 pp. 319, 321.) 



Sometimes, particularly when the animal is disturbed when 

 crawling, the mucus of the track contains portions of the 

 dermal mucus trailed off as the snail moves along. The ambu- 

 lacral mucus is frequently stained with streaks of yellow or 

 green, through contamination with excrementous matter. 

 Snail mucus is not coagulated by boiling water, but is coagulated 

 through dehydration, by alcohol. 



I have used the term " mucus " in referring to these secretions 

 because I think it a better term than slime, which is sometimes 

 used, and besides, in the text-books, the large ventral gland 

 is usually known as the mucus gland. 



It may be here mentioned that in Proc. Linn. Soc. New South 

 Wales, 1915, p. 114, there is an illustrated paper by the present 

 writer describing the feeding tracks of Li max maximus. 



