STEUCTUEE OF THE EYE AND EAR. 409 



insects, are equally applicable to the animals composing the class before 

 us ; for in the Crustacea, although we are compelled to admit the pos- 

 session of the above faculties, we are utterly ignorant of the mode in 

 which they are exercised ; 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 present state of our knowledge, to be 

 deduced. 



(1043.) The eyes of Crustaceans are of three kinds simple, agglome- 

 rated, and compound. 



. (1044.) The simple eyes (ocelli, stemmata) resemble those of Spiders, 

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

 tinous vitreous humour, a retina, and deeply- coloured choroid, all 

 occupying their usual relative positions. These eyes never exceed two 

 or three in number. 



(1045.) In the agglomerated eyes, such as those of Daphnia (fig. 212), 

 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. 



(1046.) The compound eyes appear to be constructed upon the same 

 principles as those of insects. The cornese are extremely numerous, 

 and in general 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. The com- 

 pound eyes of Crustaceans have not, however, as yet been examined with 

 the same patient diligence as those of the Cockchafer ; so that, as relates 

 to their minute anatomy, much is still left to conjecture and uncertainty. 

 One peculiarity connected with these organs is, that in the two highest 

 orders of Crustacea, hence called Podophthalmia, the eyes are placed at 

 the extremity of moveable pedicles articulated with the first cephalic 

 ring of the external skeleton, and thus they may be turned in various 

 directions without moving the whole body at the same time. This pro- 

 vision was not required in insects, owing to the mobility of the head in 

 those animals, but is absolutely indispensable in the case before us, 

 where, the head and thorax being consolidated into one mass, the extent 

 of vision commanded by sessile eyes would have been exceedingly 

 limited, and inadequate to the security of creatures exposed to such in- 

 numerable enemies. 



(1047.) It is in the higher Crustacea that we, for the first time, in- 

 dubitably find a distinct auditory apparatus ; and, from the simplicity 

 which the organ of hearing presents in this its earliest appearance, an 

 inquiry concerning its structure becomes of great physiological interest. 

 In the Lobster the ears are situated upon the under surface of the basal 



