Spiders and Their Near Relatives 



their prey, but do not masticate it so as to swallow the solid 

 parts.* 



The Arachnida are air-breathing. Two forms of respira- 

 tory organs exist in this class: first, the book-lungs; and second, 

 the tracheae. Both of these are described in the chapter on the 

 internal anatomy of spiders. In the mode of respiration, the 

 Arachnida differ from the Palaeostracha, with which they agree 

 in the number of cephalothoracic appendages and in the absence 

 of true jaws. 



The reproductive organs open near the base of the abdomen, 

 on the ventral side. In this respect the Arachnida resemble 

 Limulus, the millipedes, and the Crustacea, and differ from the 

 centipedes and insects, in which the reproductive organs open 

 near the caudal end of the body. 



The eyes are simple, that is, each eye is covered with a 

 single cornea, not facetted like the compound eyes of insects. 

 The number varies from two to twelve, and some species are 

 eyeless. 



The eyes vary greatly in their position. In some forms 

 (Avicularioidea) they are borne on an ocular tubercle near the 

 middle of the head. In this case there are two large round 

 median eyes, one on each side of the median line; on each side of 

 the median eyes, on the base of the tubercle, are three lateral 

 eyes, which differ in size, shape, and appearance, both from the 

 median eyes and from one another. This is probably the primi- 

 tive arrangement of the eyes; but in most forms, the lateral eyes 

 have wandered off laterally on to the cephalic lobes, which con- 

 stitute the lateral portions of the head, and the optic tubercle 

 is obliterated (Bernard '96). f A similar migration of the paired 

 ocelli of insects has been pointed out by the writer (Comstock 

 and Kochi '02). In some of the more generalized insects, the 

 three ocelli are borne by the front; but in most insects they 

 have wandered off from this sclerite into the epicranial suture 

 or even on to the vertex, which is developed from the cephalic 

 lobes of the embryo. 



Of the six pairs of appendages borne by the cephalothorax, 

 in most forms, the first two pairs are used for seizing and crushing 



•For possible exceptions to this see accounts of the habits of the Solpugida and of the Phalan- 

 gida, in the next chapter. 



tThc complete titles of works referred to in parentheses in the text a ( e given in the bibliog- 

 raphy at the end of the volume. 



IO 



