INVERTEBRATA. 443 



blastic origin, the endosternite, as in scorpions and some 

 spiders. 



In Scoiplonida the segments and appendages correspond 

 with thote of Limulus. The prosoma or cephalothorax is 

 much smaller and narrow ; the chelate pedipalpi are large 

 and strong ; the operculum is small and bears the genital 

 apertures ; the next pair of appendages are fringed and 

 called the pectines, whose function seems to be tactile ; 

 they are modified gill-books ; the remaining four pairs of 

 gill-books are sunk into pits in the body-wall and form 

 the lung-books ; the last segment of the mesosoma is 

 without appendages and is succeeded by a metasoma of 

 six narrow segments without appendages ; the anus is at 

 the posterior border of the fifth segment, and the sixth 

 forms the characteristic pointed sting containing poison 

 glands. 



The Phalangida differ from spiders in having no con- 

 striction between prosoma and metasoma, and in the fact 

 that the latter is distinctly segmented. The four pairs of 

 legs are very long; they have no lungs, but breathe 

 entirely by tracheae ; they spin no web, but pursue their 

 prey. 



In the true Spiders the metasoma is unsegmented and 

 separated from the prosoma by a very deep constriction. 

 The chelicerae are not chelate, but the terminal joint is 

 pointed and provided with poison glands ; the pedipalpi 

 are also non-chelate, and in the males modified to serve as 

 copulatory organs. The genital openings are at the an- 

 terior end of the abdomen and protected by an operculum ; 

 in some there are two pairs of lungs, like those of the 

 scorpion ; in others the second pair of lungs is replaced by 

 a pair of openings leading to tracheae. Near the posterior 

 end of the metasoma are three pairs, or in some cases only 

 two pairs, of spinnerets, or papillae, on which the ducts of 

 the silk-glands open ; these papillae are modified abdominal 

 appendages. The prosoma bears two or three pairs of 

 simple eyes. 



In the Acarida the body is entirely unsegmented and 

 shows no distinction into prosoma and metasoma. The 

 chelicarae are sometimes chelate, but in the majority are 



