256 LECTURE XIX. 



Many of the lower parasitic species of arachnidans are blind : 

 not any of this class have compound eyes, although the stemmata 

 in some, as the Galeodes and the genus Pliolcus, have their ocelli 

 arranged in two lateral groups. The scorpions have eight ocelli, two 

 of which are situated near the middle line, and three on each side, 

 near the anterior angles of the cephalothorax. 



In the spiders the ocelli are generally arranged in a group, upon an 

 eminence at the middle of the anterior part of the cephalothorax ; 

 they are generally eight, never less than six in number. The posi- 

 tion of the four median ones is the most constant ; they generally 

 indicate a square or a trapezium, and may be compared with the 

 median ocelli in hexapod insects. The two, or the two pairs of 

 lateral ocelli may be compared with the compound eyes of insects ; 

 the anterior of these has usually a downward aspect, whilst the pos- 

 terior looks backwards ; the variety in the arrangement of the ocelli 

 of spiders always bears a constant relation to the general conformation 

 and habits of the species. Dujes has observed, that those spiders 

 which hide in tubes, or lurk in obscure retreats, either under-ground 

 or in the holes and fissures of walls and rocks, from which they only 

 emerge to seize a passing prey, have their eyes aggregated in a close 

 group in the middle of the forehead, as in the bird-spider {Jig. 109.), 

 the clothos, &c. The spiders w^hich inhabit short tabes, terminated 

 by a large web exposed to the open air, have the eyes separated, and 

 more spread upon the front of the cephalothorax. 



Those spiders which rest in the centre of a free web, and along 

 which they frequently traverse, have the eyes supported on slight 

 prominences which permit a greater divergence of their axes; this 

 structure is well marked in the genus Thomisa, the species of which 

 lie in ambuscade in flowers. Lastly, the spiders called Errantes, or 

 wanderers, have their eyes still more scattered, the lateral ones being 

 placed at the margins of the cephalothorax. The structure of these 

 simple eyes resembles that which has been so well described by 

 Miiller in the scorpion ; Lyonnet had recognised the crystalline lens. 

 The iris, or process of pigment which advances in front of the lens, is 

 green, red, or brown in the diurnal spiders, and black at the back 

 part of the eye. The nocturnal species, as Mygale and Tarantula^ 

 have a brilliant tapetum, but no dark pigment. 



In the scorpion the transparent prominence which indicates each 

 ocellus is a thick dermal cornea, not divided into facets ; it is deeply 

 excavated at the middle of its inner surface for the lodgment of the 

 spherical lens : the back part of this body rests upon, without sinking 

 into, the anterior surface of a hemispherical vitreous body. The 

 interspace between this body and the lens forms a circular channel 



