TYPE ARACHN1DA. 441 



will be to the effect that the eyes of the Arachnids have been derived from 

 compound eyes similar to those of Limulus, and that in the median eyes of 

 the Scorpions, and the posterior dorsal and lateral eyes of the Spiders the 

 eutii'e optic area has been invaginated, making them comparable to the 

 median eyes of Limulus, while the lateral eyes of the Scorpions and the 

 anterior dorsal eyes of the Spiders have not undergone invagination and 

 hence are comparable to the lateral eyes of Limulus. Whether the com- 

 parability indicates also the homology from a phylogenetic standpoint of 

 eye to eye must remain at present uncertain (see p. 457). 



Iii addition to the Malpfiighian tubules already mentioned // 

 as excretory organs occurring in connection with the digestive 

 tract of the Spiders, there exist in many forms additional 

 glands which probably are also excretory in function or sig- 

 nificance. These are the coxal glands, so called on account of 

 their openings when present being on the basal joints (coxae) 

 of one of the pairs of legs. In the Scorpions and Spiders 

 the ducts of the glands open on the third pair of legs (i.e., the 

 fifth pair of appendages) in the embryo, but are usually 

 wanting in the adults. In the Solifugse and Harvest-spiders 

 coxal glands also occur in connection with the fourth pair 

 of legs, and similar glands have also been observed in several 

 genera of Mites, opening, however, at varying points. 



Glands are also of frequent occurrence in connection with 

 the pedipalps, having apparently varying functions in differ- 

 ent genera. They do not, however, seem to belong to the same 

 category as the coxal glands and are in no case excretory. 



The Arachnida are bisexual throughout. The ovaries not 

 infrequently fuse to form a single mass or a circular band, 

 and in connection with the oviducts, which are in direct com- 

 munication with them, there is usually developed a receptac- 

 uluin seminis, and in the Harvest-spiders, an elongated ovi- 

 positor. The testes are also frequently fused, and the vasa 

 defereutia are provided with vesiculse seminales and usually 

 terminate in a copulatory organ. The majority of forms are 

 oviparous, exceptions to the rule being found, however, as in 

 the genus Phrynus and in the Scorpions, which are viviparous. 



1. Order Scorpionida. 



In the Scorpions (Fig. 201) the body is composed of an 

 unsegmented cephalothorax and an elongated segmented 



