258 MANUAL OF ZOOLOGY. 



larvae have only six legs, and do not attain the proper four 

 pairs of legs, until after some moults. 



The Arachnida may be divided into two great sections or 

 sub-classes viz., the Trachearia, in which respiration is effected 

 by the general surface of the body, or by tracheae, and there 

 are never more than four ocelli ; and the Pulmonaria, in which 

 respiration is effected by pulmonary sacs, either alone or com- 

 bined with tracheae, and there are six or more eyes. 



CHAPTER XXXVII. 

 DIVISIONS OF THE ARACHNIDA. 



DIVISION A. TRACHEARIA. Respiration cutaneous, or by trachea. 

 Eyes never more than four in number. 



The Trachearia comprise three orders viz., the Podosomata. 

 the Acarina or Monomerosomata, and the Adelarthrosomata. 



ORDER I. PODOSOMATA (Pantopoda}. The members of this 

 order, sometimes called " Sea-spiders/' have been placed alter- 

 nately amongst the Arachnida and the Crustacea, their true 

 position being rendered doubtful by the fact that, though 

 marine in their habits, they possess no differentiated respira- 

 tory organs. They possess, however, no more than four pairs 

 of legs, and would therefore appear to be properly referable 

 to the Arachnida. The commoner forms of the Podosomata 

 (such as Nymphon and Pycnogonuni) may be found on the sea- 

 coast at low water, crawling about amongst marine plants or 

 hiding beneath stones. Some species of the latter genus are 

 asserted to be parasitic upon fishes and other marine animals, 

 but the common British species (P. littorale} is free when 

 adult, and does not appear to be parasitic at any stage of its 

 existence (fig. 99, a). The legs consist of four pairs, some- 

 times greatly exceeding the body in length, and sometimes 

 containing caecal prolongations of the digestive cavity for a 

 portion of their length. The mouth is provided with a pair of 

 " chelicerae," or chelate mandibles, and with two well-developed 

 maxillary palpi, behind which in the female are a pair of false 

 legs which carry the ova. The abdomen is rudimentary. 

 Though there are no respiratory organs, there is a distinct 

 heart. The sexes are in different individuals. 



ORDER II. ACARINA or MONOMEROSOMATA. The members 

 of this order possess an unsegmented abdomen which is fused 



