GASTEROPODA. 265 



sequently capable of being withdrawn with it. In the adult animal, 

 the eye is found to be at the base of the tentacle ; and although it can 

 be protruded at pleasure for a short distance, it seems to be de- 

 pendent upon the tentacle only for an external coverlid as it inva- 

 riably draws it down over the eye whenever that organ needs protec- 

 tion. The eye itself is pyriform, somewhat resembling the round 

 figure of the human eye-ball, with its optic-nerve attached. In colour 

 it is very dark, with a single central pupillary-opening for the admis- 

 sion of light. The tentacle, which is rounded in the young animal, 

 becomes flat and triangular in shape in the adult. The young animal 

 is for some time without teeth ; consequently it does not very early 

 betake itself to a vegetable sustenance : in place of teeth it has two 

 rows of cilia, as before stated, which drop off when the teeth are fully 

 formed. The lingual band bearing the teeth, or, as it is termed, the 

 "tongue" of the mollusc, consists of several rows of cutting spines, 

 pointed with silica, which, as we have before stated, is a most interest- 

 ing object seen under the microscope. 



It is an interesting physiological fact, to find that if the young 

 animal be kept in fresh water alone, without vegetable matter of any 

 kind, it retains its cilia, and arrest of development follows ; it acquires 

 no gastric teeth, and never attains perfection in form or size. If, at 

 the same time, it be confined within a narrow cell, or space, it grows 

 only to such a size as will enable it to move about freely; thus 

 adapting itself to the necessities of its restricted state of existence. 

 Some young animals in a glass-cell were, at the end of six months, 

 alive and well, and the cilia retained around the tentacles in constant 

 activity ; whilst other animals of the same brood and age, placed in a 

 situation favourable to growth, attained their full size, and produced 

 young, which grew in three weeks to the size of their elder relations. 



Should any injury occur to the shell, or a portion of it become 

 broken off, the calcareous deposit is quickly resumed, in order to 

 replace the lost part ; the cells being only half the size of those first 

 deposited in the original formation. This may be cited in proof of 

 what was stated by Professor Paget, in his lectures delivered at the 

 Royal College of Surgeons, 1852, "that, as a rule, the reparative 

 power in each perfect species, whether it be higher or lower in the 

 scale, is in an inverse proportion to the amount of change through 

 which it has passed in its development from the embyronic to the per- 

 fect state. And the deduction to be made from them is, that the 

 powers for development from the embryo are identical with those 

 exercised for the restoration from injuries ; in other words, that the 



