XII] AEACHNIDA 259 



undergoes a very thorough reorganisation and gradually the mature 

 Insect is built up; after a certain time this emerges and occupies 

 its comparatively short life in the propagation of its species 

 (Fig. 104). The female usually deposits its eggs on or near the 

 plants which serve as food for its offspring. 



Class III. AEACHNIDA. 



The third large group of the Arfchropoda is a very varied one 

 and contains many animals which differ markedly in their structure 

 one from another. Perhaps the most distinctive features of the 

 External Arachnida (Gk. apcr^, a spider; etSos, shape) are 

 features. Q There are no true gnathites. No appendage loses 

 all other functions and becomes exclusively a jaw, although the 

 proximal joints of several are prolonged inwards towards the mouth 

 and help to take up food ; in a word some of the limbs have 

 developed gnathobases; (ii) The most anterior appendages are 

 never antennae but always a pair of nippers, termed chelicerae; 

 (iii) The active catching and walking legs of the fore part of the 

 body or prosoma are strongly contrasted with the plate-like modi- 

 fied limbs of the middle part of the body ormesosoma when the 

 latter exist, but in many cases these have disappeared and in others 

 have become so modified that they are no longer recognisable as 

 limbs. Nearly all Arachnids moreover agree in having the anterior 

 end of the body, the prosoma 1 as it is called, marked off from the 

 rest and covered by a single piece, the carapace. The rest of the 

 body or abdomen is in some forms differentiated into two regions, 

 the mesosoma and metasoma, but in other cases this distinction 

 does not exist ; it may be segmented or it may not. The prosoma 

 bears six pairs of appendages and of these the last four are usually 

 walking.legs. The appendages of the abdomen are connected with 

 the respiratory function and are much modified, often in the 

 terrestrial forms forming floors for the respiratory chambers. The 

 breathing apparatus in the most primitive marine forms consists of 

 gills. In the many land forms these gills are retained but are 



1 The name cephalothorax is often applied to this region, but the term is 

 too misleading to be used. The cephalothorax of Decapod Crustacea includes 

 the first thirteen segments of the body : the prosoma of Arachnida only includes 

 six, and therefore corresponds roughly to the "head" of the higher Crustacea. 

 Similar criticism might be launched against the use of the word' "abdomen," 

 but here the error is too deep-rooted for correction since the term is used in 

 describing both Crustacea and Insecta, and in each case in a different sense. 



172 



