STKES : VARIATION IN KECENT MOLLUSC A. 255 



Abundant instances of discontinuous variation are found in studying 

 the animal. For example, double and even quadruple monstrosities 

 are found in the embryo in Limax and Plamrhis; double also in 

 Philine} 



Deformation and variation of the eyes and tentacles are by no means 

 uncommon. The tentacle and eye can be renewed, but this can only 

 be done if the supra-pharyugeal ganglion is uninjured.^ Patella vulgata 

 has been seen with two tentacles and two eyes on the left side, the 

 right being normal ; Nassa with two eyes on the right-hand tentacle ; 

 Littorina and Clausilia hidens with a similar character ; while in Peden 

 one eye has been observed imperfectly divided into two, the division 

 being at right angles to the mantle-edge. Duplication of the ex- 

 tremity of the tentacle has been observed in Helix lapicida, the nerve 

 being equally divided, and bifid tentacles have also been observed in 

 JSacella (enea, Littorina, Limncea, Anci/lns, Physa, and Helix. Triopa 

 claviger, again, has been examined with the upper tentacles, or rhiuo- 

 phores, abnormal owing to the right-hand one being divided into three 

 branches. A specimen of Subemarg inula was found bearing upon 



Cijclophorus, with bifurcate tentacle. (Sketch by Capt. A. J. Peile.) 



each of the eye-projections two eyes in place of one, the tentacles 

 being dwarfed. Adherent tentacles, attached throughout except for 

 a slight cleft at the end, are noted in Helix hispida, and a somewhat 

 similar case has been observed in Limax agrestis. Limax Icevis has 

 been observed without any tentacles at all. Perhaps, however, the 

 most interesting case is that recorded by Moquin-Tandon, namely, 

 a double-headed Limniea aiirimlaria, one head being a third larger 

 than the other. In the larger head the tentacles, eyes, mouth, etc., 

 were normal, but in the smaller the left tentacle was dwarfed and had 

 no eye. 



Like the other organs, the radula is liable to vary.^ It has been 

 shown that there is often an increase in the number and size of teeth 

 and some variation in their form as the animal becomes older [Hyalinia 

 and Limax). A,ge, however, affects many anatomical features as 

 well as conchological ; for example, the number of plicae in the gills 

 of Pelecypods is greater in the adult than in the young. Deformation 

 of the teeth of the middle row of the radula has been noticed in Helix 



1 Lacaze Duthiers : Arch. Zool. Exper., vol. iv, p. 482. 



2 Cf. Fischer: Journ. Conchyl., 1864, p. 89 ; 1856, p. 230; 1888, p. 131. Forbes & 



Hanley : Brit. Moll., vol. iv, p. 288. Jeffreys : Brit. Conch., vol. iv, p. 350. 

 Van den Broeck : Ann. Soc. Mai. Belg., vol. v, p. 29. Eoberts : Sci. Gossip, 

 vol. x.\ii, p. 259. Pelseneer : Res. Voy. Belgica, Mollusca. 

 ' See Binney : Bull. Mus. Comp. Zool., vol. v, p. 337. Jickeli : Nachrbl. Deutsch. 

 Malak. Ges., i873,p. 68. Sterki: torn, cit., p. 172. Esmark: Mag. NatiU'w., 

 vol. xxvii, p. 92. 



