n ORGANS OF SENSE. 



recover this faculty, according to Keaumur, upon being again 

 moistened. 



Phillirhoe bucephala, a pelagic mollusk inhabiting the Medi- 

 terranean, exhibits this quality in a remarkable degree. It 

 suffices to plunge it into fresh water to excite its luminosity, 

 and the addition of a drop of ammonia will cause its entire 

 surface to become resplendent for some time with a lively, 

 bluish light. 



Panceri has shown that the phosphorescence of Phyllirhoe 

 proceeds from special cellules provided with a golden yellow 

 pigment, and analogous to the chromatophores of cephalopods. 

 These Cellules of Muller are in intimate relation witii small 

 ganglionic swellings of the peripheric nerves, and it has been 

 advanced as probable that the phosphorescent emission is excited 

 by the nervous system. 



ORGANS OF SENSE. 



Touch. Cephalopoda. This is, of course, the most widely 

 diffused of the senses in the mollusca, every portion of the body 

 being extremely sensible of contact with external objects, but 

 in the Cephalopoda the arms may be considered as specialized 

 tactile organs ; the Octopus, for instance, using its dorsal pair 

 of arms in like manner with the horns of the snail or insects in 

 exploring neighboring objects. 



Gast7-opoda. The principal tactile organs are the tentacles 

 (vi, 72), and edge of the mantle, which is frequently fringed 

 (iii, 45) ; but there are in addition, in certain prosobranchiates 

 lobular productions of the head near the tentacles, which appear 

 to have a similar function (plate 3). 



In the prosobranchiates, the tentacles, always two in number, 

 are solid structures, not invaginate and capable of retraction 

 within the head as are those of the pulmonates ; they arise from 

 the front dorsal part of the head and in the proboscidiferous 

 species are situated at the base of it. The tentacles usually 

 bear the eyes upon stalks which are connate with or branch out 

 from them. The position of the eyes varies in different genera ; 

 thus they are found near the bases of the tentacles in Littorina, 

 Dolium (Ixii, 21), or near the middle, as in Murex, Fusus, Cassis, 

 Mitra, etc., or even at the end, as in Terebra. In Strombus the 

 robust eye-stalk originates about the middle of the filiform 

 tentacle (lix, 56). In many holostomates, as Trochus, Nerita 

 (Ixxviii, 5Y), Ampullaria, Paludina, etc., the ommatophores are 

 entirely separate from the tentacles. 



The tentacles are sometimes delicately hairy, and these hairs 

 are evidently tactile also. In the same category of tactile organs 

 must be included the lobes, filiform processes, etc., of the mantle- 

 margin, as well as the processes which beset the mantle lobes of 



