134 THE PINEAL ORGAN 



namely, the cephalothorax, covered by the dorsal shield or carapace ; 

 and the abdomen, also covered by a carapace. The body terminates in 

 a tapering sword-like tail, which has given origin to the name " Xiphosura " 

 applied to the Order to which these animals belong (Fig. 69). Beneath 

 the head-shield are placed two pairs of eyes which were especially investi- 

 gated by Lankester and Bourne ' in 1883, who pointed out the funda- 

 mental resemblance of the central and lateral eyes of Limulus to those of 

 the scorpion. 



Each lateral eye of Limulus is covered superficially by a continuous 

 transparent corneal stratum, the under-surface of which presents a series 

 of papilliform downgrowths which lie over a corresponding number of 

 barrel-shaped retinulae (Fig. 86, p. 124). The papilliform downgrowths are 

 believed to function as separate lenses for the separate retinulae which lie 

 beneath them. Each of the latter consists of a group of elongated sensory 

 cells, the inner ends of which are continuous with the constituent fibres 

 of the retinular nerve. The papilliform downgrowths forming the 

 separated lenses of the composite eye were believed by Lankester and 

 Bourne to correspond to the numerous two to seven small lenses belonging 

 to the groups of lateral eyes which are present on each side of the scorpion's 

 head. In addition to the receptive or sensory cells of the retinulae, there 

 are refractile elements or rhabdomes which on transverse section are seen 

 to be arranged in a radial manner and also " intrusive " connective tissue 

 cells containing pigment. 



Each central eye consists of a single cuticular lens which is almost 

 spherical in form and is continuous with the surrounding cuticle ; beneath 

 this is a vitreous layer which consists of refractile columnar cells which 

 are continuous circumferentially with the hypoderm cells beneath the 

 general cuticle. We thus have an immovable cuticular lens of the corneal 

 type, which is supplemented by a vitreous layer of hypoderm cells (Fig. 87). 

 Beneath this is the receptive portion of the eye, which is composed of 

 groups of large sensory cells, the inner ends of which are continuous with 

 fibres of the optic nerve ; outside there are from five to seven fluted 

 rhabdomes and between the sensory cells and around the retinulae a 

 considerable number of " intrusive " connective tissue pigment cells. 

 The retinulae differ from the retinulae of the lateral eyes in their more open 

 character, as contrasted with the compact barrel-shaped retinulae of the 

 lateral eyes. This loose character of the retinulae of the central eyes, 

 coupled with the large amount of intrusive connective tissue, was con- 

 sidered by Lankester and Bourne to be an indication of loss of function 

 and degeneration of the central eyes as compared with the lateral. 



1 Lankester and Bourne, " The Minute Structure of the Lateral and the 

 Central Eyes of Scorpio and Limulus" Q.J. Micro. Sc, 23, 1883. 



