LESSON 93.] THE EYE IN AEACHNIDA, MOLLUSCA, ETC. 299 



convex part of the conical vitreous humor, the space around the 

 point of contact being filled with pigmentary matter. 



1353. This is illustrated by Fig. 425. The convex portion of 

 the transparent cornea is shown at a ; the concave surface of it at 

 b ; the descending walls of the cornea at c ; the prismatic bodies, 

 that are included in its substance, at d / the posterior layer, which 

 encloses the prisms, at #/ the crystalline lens is shown at /, its 

 posterior portion, in apposition with the conical vitreous humor, at 

 g ; and the conical vitreous humor, h. 



LESSON XCIII. 



THE EYE IN AEACHNIDA, MOLLUSCA, AND FISHES. 



1354. In the Arachnida, the simple eyes are the largest and 

 most perfect forms of ocelli met with in the articulated classes. In 

 the Spiders they are generally eight in number, arranged symmetri- 

 cally in one or two transverse rows on the upper and forepart of the 

 cephalo-thorax. The largest forms of these organs are met with in 

 the Scorpions, in which they have been the most frequently ex- 

 amined. 



1355. Beneath the transparent cornea, in the eye of the Scorpion, 

 there is a spherical, firm, transparent lens ; beneath this, a vitreous 

 humor, which fills half of the eyeball, surrounded by the pigmentuin 

 and the choroid, excepting on the forepart, where it bounds the pupil 

 like an iris, and on the back part, where it is penetrated by the 

 optic nerve. The optic nerve expands into a well-formed retina, 

 investing all the convex posterior portion of the vitreous humor. 



1356. Organs of vision are not required, neither are they devel- 

 oped in the fixed or slow-moving Molluscous animals ; and in those 

 individuals of this class which possess them, they are not aggregated 

 together like those of the Worms, the Myriopods, or the Arachnida, 

 neither are they compound organs, like the eyes of the Crustacea and 

 Insects. 



1357. In the Gasteropods the eyes are always two in number, 

 movable, and generally pedunculated frequently found at the sum- 

 mit of one pair of tentacles, as in the Slug and the Snail. Some of 

 the naked Gasteropods, as the Eolis (Fig. 263), the Doris, and 



