APPENDAGES OF AETHEOPQDA. 



237 



Peripatus has a simple form of body very similar to that of the 

 Aimulata. 



Among the Tracheata the most indifferent condition is found in 

 the Myriapoda, where the metameres are similar and separate. The 

 body of the Araehnida is very variously differentiated. The 

 Galeodea possess the largest number of segments. A head is sepa- 

 rated by three thoracic metameres from the succeeding and separate 

 abdomen, which is made up of separate metameres. In the Scorpions, 

 however, the cephalic and thoracic metameres are united into one 

 portion, and a post-abdomen is differentiated from the abdomen. The 

 abdomen is more sharply marked off from the cephalothorax in the 

 Phrynida ; the same happens in the Aranea, but they differ in the 

 fact that the abdominal segments have undergone more complete 

 concrescence. In the Acarina the metameres have altogether lost 

 their independence. 



The more richly segmented body of the Insecta presents less 

 variety in the distribution of the metameres in different portions. 

 In addition to the head, which is formed of several (3) meta- 

 meres, there are ordinarily three thoracic segments (Pro- Meso- 

 and Meta-thorax), which are either indifferent, as in the Thysanura 

 and many Pseudoneuroptera, and are only distinguished by the 

 organs appended to them; or they all form a portion which is as 

 sharply marked off from the head as from the abdomen (Neuroptera, 

 Hymenoptera, Diptera, Lepidoptera) ; or the first thoracic segment 

 only is specially modified, while the second and third are closely 

 attached to the abdomen ; this arrangement is indicated in the 

 Orthoptera (Saltatoria), and well marked in the Coleoptera. 



The characters of the abdomen are affected by its relations to the 

 thorax already noted. Its segments are always independent, and 

 the terminal ones, several of which are converted into parts of the 

 generative apparatus, are often atrophied. 



Appendages. 



§ 184. 



The appendages of the Arthropoda are paired, jointed struc- 

 tures, which are attached to the 

 metameres, and can be separated 

 into dorsal and venti^al appendages. 

 These structures are foreshadowed 

 by the parapodia which are found 

 in the higher Aimulata. In the 

 Arthropoda these processes are more 

 highly differentiated, for they be- 

 come jointed (Fig. 120, p), and differ 

 greatly in form, in correspondence 

 with their different functions ; their 

 lower stage of similarity to one another is only to be 

 seen in their earliest rudiments. 



Fig. 120. Transverse section of a 

 Wood-louse, p A pair of feet, p ' Ab- 

 dominal appendages, which form a 

 thoracic cavity (after Lereboullet). 



