ANNULOSA: ARACHNID A. 319 



" cerebral " ganglion, a large thoracic ganglion, and often small 

 abdominal ganglia. In some of the lower forms the articulate 

 type of nervous system is lost, and there is merely a ganglionic 

 mass situated in the abdomen. In none of the Arachnida are 

 compound eyes present, and in none are the eyes supported 

 upon foot-stalks. The organs of vision, when present, are in 

 the form of from two to eight or more simple eyes, or " ocelli." 

 In all the Arachnida, with the exception of the Tardigrada, 

 the sexes are distinct. The great majority of the Arachnida 

 are oviparous, and in most cases the larvae are like the adult 

 in all except in size. In some cases, however (Acarina), the 

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

 pairs of legs until after some moults. 



CHAPTER XXXVI. 

 DIVISIONS OF THE ARACHNIDA. 



THE class of the Arachnida may be divided into the following 

 orders : 



ORDER I. PODOSOMATA (Pantopoda]. Respiration effected 

 by the general surface of the body ; limbs four pairs in number, 

 elongated ; abdomen rudimentary, unsegmented ; sexes distinct. 



The members of this order, sometimes called " Sea-spiders," 

 have been placed alternately amongst the Arachnida and the 

 Crustacea, their true position being rendered doubtful by the 

 fact that, though marine in their habits, they possess no dif- 

 ferentiated respiratory organs. They possess, however, no 

 more than four pairs of legs, and would therefore appear to be 

 properly referable to the Arachnida. According to Dr Dohrn, 

 however, the embryo is naupliiform, and this would support a 

 reference of the order to the Crustacea. The commoner forms 

 of the Podosomata (such as Nymphon and Pycnogonmn) 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 parasitic upon fishes and other marine ani- 

 mals, 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. 164, a). The legs consist of four pairs, some- 

 times greatly exceeding the body in length, and containing 

 caecal prolongations of the digestive cavity for a portion of 

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



