CRUSTACEA. 343 



tomy in King's College, London, in which the new limb (one of the 

 cheliferous claws) has already attained the form of the old chela, but 

 still remains soft and uncovered by calcareous integument. The pro- 

 cess of reproduction is as follows : The broken extremity of the 

 second joint skins over, and presents a smooth vascular membrane, 

 at first flat, but soon becoming conical as the limb begins to grow. 

 As the growth advances, the shape of the new member becomes 

 apparent, and constrictions appear, indicating the position of the 

 articulation ; but the whole remains unprotected by any hard cover- 

 ing until the next change of shell, after which it appears in a pro- 

 per case, being, however, still considerably smaller than the cor- 

 responding claw on the opposite side of the body, although equally 

 perfect in all its parts. 



(378.) The observations made in a former chapter relative to the 

 organs by which the senses of touch, taste, and smell are exercised 

 in insects, are equally applicable to the animals composing the 

 class before us ; for in the Crustacea, although we are compelled to 

 admit the possession of the above faculties, we are utterly ignorant 

 of the mode in which they are exercised, and therefore it would be 

 only an unprofitable waste of time to enter at any length into a 

 discussion from which no satisfactory conclusions are, in the pre- 

 sent state of our knowledge, to be deduced. 



(379.) The eyes of Crustaceans are of three kinds, simple, ag- 

 glomerated, and compound. 



The simple eyes (ocelli, stemmata) resemble those of spiders, and, 

 like them, are said to consist of a cornea, a spherical lens, a gelatin- 

 ous vitreous humour, a retina and deeply-coloured choroid, all occu- 

 pying their usual relative positions. These eyes never exceed two 

 or three in number. 



In the agglomerated eyes, such as those of Daphnia (Jig. 155), 

 the organ seems to be composed of a number of simple eyes placed 

 behind one common cornea ; such eyes are moveable, and, in the 

 animal depicted in the figure, the muscles acting upon the visual 

 apparatus, which in this case is single, are arranged so as to form a 

 cone the base of which is formed by the eye and may be distinctly 

 seen under a good microscope. 



The compound eyes appear to be constructed upon the same prin- 

 ciples as those of insects. The cornese are extremely numerous 

 and generally hexagonal ; but sometimes, as in the lobster, they are 

 square. The vitreous humours equal the cornese in number, and 

 behind each of these a distinct retina would seem to be expanded. 



