LESSON 90.] THE EYE IN THE ANNELLATA, ETC. 289 



LESSON XC. 



THE EYE IN THE ANNELLATA, CRUSTACEA, MYRIAPODS, AND INJECTS. 



1297. In the Annellata (red-blooded worms), eyes are constantly 

 found, and in variable number ; thus in the several species of Pla- 

 naria, they vary from one to three pairs, and in one species (Prosto- 

 ma armatum) the head is literally covered with them. 



1298. In the medicinal Leech (Fig. 362) there are ten simple 

 eyes ; in Nereis nuntia there are four large eyes on the upper part 

 of the head, and nearly one hundred smaller visual organs disposed 

 in rows and groups on all the prominent lobes about the mouth. 



1299. But in the higher forms of this class the eyes are reduced 

 to two. In all these animals the eyes consist of a transparent cornea, 

 a minute crystalline lens; a pigment, not always, but generally black, 

 and an optic nerve ; whether other organs are superadded is not 

 known, the extreme minuteness of the eyes forbidding further inves- 

 tigation. 



1300. In the higher forms of Crustacea (Lobsters and Crabs), 

 the eyes are pedunculated and movable, by means of muscles inserted 

 within their sclerotic covering ; in the inferior Crustacea, they are 

 sessile (sitting close upon the body, without support) and immovable, 

 and in the lower Crustacea the two sessile eyes are frequently united 

 on the median plane, and appear to form but one organ (Monoculus, 

 Daphniaj &c.). 



1301. The eyes themselves are constructed on the same plan as 

 those of Insects, hereafter to be described, with one remarkable ex- 

 ception : the facets in the transparent cornea of Insects are hexago- 

 nal (see Fig. 420), but in the Lobster they are perfectly square (Fig. 

 410), and resemble a ruled glass micrometer ; pio 410 

 moreover, it consists of a series of laminae 



(plates). It will be shown hereafter that in 

 the Insect eye a great tendency to the square 

 form of the facet prevails, notwithstanding it 

 is always found associated with the pure hex- 

 agonal figure. 



1302. In the Myriapods (Centipedes, lu- 

 lidse) the eyes of some species resemble those 

 of the Annellides, whilst others approximate 



to the Insects; most of them present numerous separate simple 

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