622 



ZOOLOGY 



SECT. 



FIG. 517. One of the book -gills of Limulus, with 

 the appendage to which it is attached. (After 

 Lankester.) 



the external characters of the groups. They are all (Fig. 518) of 

 the tvpe of the ocelli or simple eyes of Insects, except the central 



eyes of the Scorpions 

 (Fig. 519) and the com- 

 pound eyes of Limulus.. 

 The former are intermedi- 

 ate in character between 

 ocelli and facetted eyes,, 

 possessing the single cuti- 

 cular lens (lens) of the 

 ocellus, and resembling 

 the facetted eye in having 

 the retinal cells arranged 

 in groups corresponding 

 to ommatidia. Each re- 

 tinula, composed of five 

 cells, contains a thick axial 

 rod or rhabdome (rliabd.). 



In Limulus the com- 

 pound eye has a continu- 

 ous chitinous cornea-lens of the nature of a thickening of the 

 cuticle. This, though non-facetted, differs from the corresponding- 

 part in the compound eye of the Scorpion in being produced 

 internally into a number of conical papilla?, each of which lies 

 over one of the ommatidea and may be looked upon as its lens. 



A considerable variety is observable in the exact arrangement 

 of the parts of the re- 

 productive apparatus 

 in different groups of the 

 Arachnida. In general, 

 testes or ovaries are either 

 paired or (more rarely) 

 unpaired tubes, with 

 paired vasa deferential, or 

 < Tul nets, which unite in 

 a median duct opening 

 on the exterior by an 

 unpaired genital open- 

 ing. Viviparity is ex- 

 ceptional. In the Spiders 

 the ovaries (Fig. 113, ov.) 

 are two wide tubes, on 

 the surface of which 

 follicles project promi- 

 nently ; sometimes they 

 unite into a single circular ovary. Each ovary has a short oviduct, 

 or, when the ovary is single it has two, right and left : these unite 



Tiervf 



FIG. 518. Section of the lateral eye of Euscorpius 

 italicus. 'nit. intermediate cells; l:.ns, cuticular 

 lens; ncn: c. terminal nerve cells; nerv. /. nerve 

 fibres of optic nerve ; /7yW. rhabdomes. ' (After 

 L:inkestcr and Bourne.) 



