//. ACEEATA: ARACHNIDA. 445 



which bears six pairs of appendages; the four posterior pairs, con- 

 sisting typically of seven joints, are locomotor, so that the posses- 

 sion of eight legs is as characteristic for 

 an arachnid as ten for a decapod or six 

 for a hexapod. The first pair of append- 

 ages, the chelicerce (fig. 459), are preoral, 

 the second, m pedipaipi, beside that open- 

 ing. The chelicerae are short and con- 

 sist of two or three joints, the terminal 

 joint either folding back upon the other 

 or, pincer-like, meeting an opposable 

 thumb. In the spiders the last ioint or 



n ... , . . . . * , FIG. 459. Mouth parts of Epeira. 



claw is forced into the prey, introducing i, cheiicera; *, pedipaipi; p 



, , , -, . . ! palpus; J, basal plate. 



poison irom a sac in the basal joint. 



The pedipaipi are elongate, leg-like, their basal joints often form- 

 ing a lip, the other joints forming the palpus, which may end with 

 a claw or a pincer. 



The question has often been discussed as to whether the chelicerae are 

 the homologues of the antennas of other arthropods. The embryological 

 evidence, which cannot be detailed here, is in favor of their equivalence to 

 the second antenna of the Crustacea, and to the mandibles of insects. 



Since the Arachnida usually suck their food, the oesophagus is 

 frequently widened to a sucking stomach, behind which comes the 

 true stomach, with which, as well as with the intestine, a number 

 of so-called liver tubes may arise (fig. 455, da, dt). These may 

 be restricted to the abdomen alone, as in the scorpions. The 

 hinder part of the intestine is often enlarged into a rectal vesicle 

 (stercoral pocket), just in front of which the excretory tubules (so- 

 called Malpighian tubules) empty. These resemble the true Mal- 

 pighian tubes of insects in function, but differ in being entodermal 

 in origin. Besides there also occur, coxal glands (modified ne- 

 phridia), of which only one pair comes to development, and this 

 may lose its external opening on the base of the appendage. 



The oesophagus is always closely surrounded by a nerve ring 

 composed of brain above and of part of the ventral chain on the 

 sides and below, the thoracic and more or fewer of the abdominal 

 ganglia entering into its composition (fig. 405, D). Of sense organs, 

 besides tactile hairs, only the eyes (fig. 406), 2-12 in number, are 

 well known. Hearing is well developed, but it is uncertain whether 

 certain hairs on the legs and palpi are the seats of the recognition 

 of sound. The function of the ' lyriform organs/ which occur in 

 the skin of body and legs in several groups, is unknown. 



