Class 4. Arachnida. 



279 



pedipalpi. The clielicerae, wMch. lie in front of the mouthy 

 consist of two or three joints, and are entirely different from the 

 mandibles of Insects and Crustacea j in many {e.g., in the Scorpions) 

 they are in the form of small ohelse. The pedipalpi are usually 

 leg-like, longer or shorter; the basal joint is often furnished with a 



Fig. 227. Diagram of the anatomy of a, Spider, a anus, h offiomn of mesenteron, 

 6' its anterior end, i" branches of the caecum extending into the legs which are here cut 

 away) ; c cerebral ganglion connected with the ventral ganglionic mass, d mesenteron, 

 g poison glands, S heart, h^ cheHcerse, fc pedipalpi, I hepatic duct, L lung-sac, Le Hver, 

 M Malpighian tubules, M.^ dilation of the rectum into which M open, o eyes, ov ovaries, 

 vS large silk glands, 8' smaller do., T opening of the tracheal system, Z spinnerettes, ? 

 female genital aperture. — Modified from Krieger. 



kind of grinding ridge, whilst the rest of the joints are either all 

 ' simple, and form a strong palp, or the two distal joints are modified 

 to form larger or smaller chelae. Behind the pedipalpi are four 

 pairs of legs (ambulatory appendages), which are usually all 

 similar, and generally consist each of seven joints. 



According to tHe usual interpretation, the cephalothorax of tte AracTmida 

 ■con-esponds to the head and thorax of Insecta, the chelicerse represent the 

 mandibles, the pedipalpi the first maxillae, whilst the first walking legs are 

 comparable with the second maxillae (the labitun), and the remaining legs 

 with those of Insects. Against this view, however, may be mentioned, among 

 other things, the stnict\n-e of the cheHcerse, which are totally unlike the mandibles 

 of Insects (consisting of several joints, etc.). Moreover, the Arachnida, on the 

 whole, differ so essentially from the Insecta that it is impossible to make a 

 special comparison of this natui-e. It would, therefore, appear very doubtful 

 whether the jaws of the Arachnida can be compared with those of other 

 Arthropods; more probably they are thoracic limbs, which in correlation 

 with the degeneration of the head, have taken on the function of jaws ; in. 

 this case the mouth would have moved back to lie between the thoracic 

 limbs.* 



The skin in most Arachnids is not so hard as in the Insects, 

 usually the cuticle is leathery, often setose. Among the glands 

 of the skin, the spinning glands, present in certain divisions 



* As in Limulus, with which the Spiders are in no way allied, a view that has 

 been incorrectly held by some authorities. 



