The following works will be found useful for further study of this portion of the subject:-- =F. Bernard=, Recherches sur les organes palléaux des Gastéropodes prosobranches: Ann. Sc. Nat. Zool. (7) ix. (1890), pp. 89–404. =G. Cuvier=, Le Régne animal (ed. V. Masson); Mollusca, Text and Atlas. =C. Grobben=, Beiträge zur Kenntniss des Baues von Cuspidaria (Neaera) cuspidata Olivi, nebst Betrachtungen über das System der Lamellibranchiaten: Arb. Zool. Inst. Wien, x. (1893), pp. 101–146. =E. Ray Lankester=, Encyclopaedia Britannica, 9th ed., vol. xvi. (1883), Art. ‘Mollusca.’ =A. Ménégaux=, Recherches sur la circulation des Lamellibranches marins: Besançon, 1890. =K. Mitsukuri=, On the structure and significance of some aberrant forms of Lamellibranchiate gills: Q. Journ. Micr. Sc., N.S. xxi. (1881), pp. 595–608. =H. L. Osborn=, On the gill in some forms of Prosobranchiate Mollusca: Stud. Biol. Lab. Johns Hopk. Univ. iii. (1884), pp. 37–48. =R. Holman Peck=, The structure of the Lamellibranchiate gill: Q. Journ. Micr. Sc., N.S. xvii. (1877), pp. 43–66. =P. Pelseneer=, Contributions à l’étude des lamellibranches: Arch. Biol. xi. (1891), pp. 147–312. CHAPTER VII ORGANS OF SENSE: TOUCH, SIGHT, SMELL, HEARING--THE FOOT--THE NERVOUS SYSTEM _Organs of Sense_: I. _Touch_ Tactile organs, although occurring in some of the Mollusca, do not appear to attain special or marked development, except in a few cases. The whole surface of the skin, and particularly of the foot, is very sensitive to the slightest impression. Nearly all Gasteropoda are furnished with at least two cephalic tentacles, projecting like horns from each side of the fore part of the head. At or near the base of these are generally situated the eyes. In the Helicidae the eyes are situated, not at the base, but at the apex of the tentacles, and in that case--except in _Vertigo_--a second pair of shorter tentacles appears beneath the longer pair. It frequently happens that several senses are centred in a single organ; thus the upper tentacles of snails not only carry the eyes and serve to a certain extent as tactile organs, but they also carry the organs of smell. The edges of the mantle, which are sometimes specialised into lobes, appear to be keenly sensitive to touch in all Gasteropoda.