APPENDAGES OE AETHEOPODA. 



237 



Peripatus lias a simple form of body very similar to tliat of the 

 Aunulata. 



Among tlie Traclieata tlie most indifferent condition is found in 

 tlic Myriapodaj where the metameres are similar and separate. The 

 body of the Arachnida 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 Coleoptei'a. 



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 ventral appendages. 

 These structures are foreshadowed 

 by the parapodia which are found 

 in the higher Annulata. In the 

 Arthro|)oda these processes are more 

 highly differentiated, for they be- Fig. 120. Transverse section of a 

 come iointed (Fig. 120, «), and differ Wood-louse, p A pair of feet, p ' Ab- 

 ,1 . p ^ °. 'J. " dominal appeadages, wmch form a 



greatly m form, m correspondence tlioracic cavity (after Lereboullet). 

 with their different functions ; t h e i r 



lower stage of similarity to one another is only to be 

 seen in their earliest rudiments. 



