XI 



PHYLUM ARTHROPODA 



671 



FIG. 555. 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. 556) of 

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

 ej^es of the Scorpions 

 (Fig. 557) and the com- 

 pound eyes of Limulus. 

 The former are intermedi- 

 ate in character between 

 ocelli and faceted eyes, 

 possessing the single cuti- 

 cular lens (lens) of the 

 ocellus, and resembling the 

 faceted eye in having the 

 retinal cells arranged in 

 groups corresponding to 

 ommatidia. Each retinula, 

 composed of five cells, con- 

 tains a thick axial rod or 

 rhabdome (rhdbd.). 



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-faceted, differs from the corresponding 

 part in the compound eye of the Scorpion in being produced 

 internally into a number of conical papillae, each of which lies 

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



A considerable variety is observable in the exact arrangement 

 of the parts of the reproductive apparatus in different groups 



of the Arachnida. In 

 general, testes or ovaries 

 are either paired or 

 (more rarely) unpaired 

 tubes, with paired vasa 

 defer entia or oviducts, 

 which unite, in a median 

 duct opening on the ex- 

 terior by an unpaired 

 genital opening. Vivi- 

 parity is exceptional. In 

 the Spiders the ovaries 

 (Fig. 551, ov.) are two 

 wide tubes, on the sur- 

 face of which follicles 

 project prominently ; 

 sometimes they unite 

 into a single circular 

 ovary. There are two short oviducts even when the ovary is 

 single ; these unite in a median vagina, which opens on the 



Tiervf 



FIG. 556. Section of the lateral eye of; Euscorpius 

 italicus: int. intermediate 'cells ; \len., .cuticular 

 lens ; nen: c. terminal nerve-cells ; nen: f. nerve- 

 fibres of optic nerve ; rhald. rhabdomes. (After 

 Lankester and Bourne.) 



