654 



ZOOLOGY 



SECT. 



Acarida. Their number and arrangement have been given with 

 the external characters of the groups. They are all (Fig. 566) of 

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

 eyes of the Scorpions (Fig. 567) and the compound eyes of Limulus. 

 The former are intermediate in character between ocelli and faceted 



FIG. 562. Diagrammatic view of a median longitudinal section of Limulus. abd. app. ab- 

 dominal appendages ; an. anus ; brn. brain ; chil. chilaria ; hep. du. opening of one of the 

 hepatic ducts ; M. heart ; int. intestine ; 1. liv. " liver " ; mo. mouth ; ne. co. nerve-cord ; 

 oes. 03sophagus ; operc. operculum ; tels. telson ; ven. sinus, venous sinus ; 1 5, legs. (From 

 Leuckart, partly after Packard.) 



eyes, possessing the single cuticular 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, contains a thick axial rod or rhabdome (rhabd.). 



In Limulus the compound eye has a continuous 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 repro- 

 ductive apparatus in different groups of 

 the Arachnida. In general, testes or ovaries 

 are either paired or (more rarely) unpaired 

 tubes, with paired vasa deferentia or oviducts, 

 which unite in a median duct opening on the 



FIG. 563. Book-lung of 

 Spider (Zilla callo 

 phylla). 



. axis; b, 



(Fronfiiertwig.) stl exterior by an unpaired genital opening. Vivi- 

 parity is exceptional. In the Spiders the 

 ovaries (Fig. 561, ov.) are two wide tubes, on the surface 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 

 exterior by a median genital aperture at the base of the abdomen. 



