244 COMPAKATIVE AXATOMY. 



All the Tracheata have a single pair of anteniiEe; in this point 

 the Pcecilopoda among the Branchiata resemble them. In the 

 Poecilopoda and in the Arachnida these antennte are converted into 

 mouth-organs; in the Scorpions and Spiders they are known as 

 chelicer^e. Notwithstanding these relations thes'e structures are 

 homologous with the antennae of the Myriapoda and Insecta, for, 

 like them, they are innervated from the supra-CBSophageal ganghon. 

 They are developed in very various ways among the Insecta, in 

 adaptation to the various functions of the sensory organs connected 

 with them. 



The ventral appendages are disposed symmetrically, and in 

 Peripatus all but two pau-s retain this relation ; in the Myriapoda 

 most of the appendages are similar in character, but in all the other 

 forms these appendages, so far as they persist, take on various forms 

 in accordance with their variations in function. The appendages 

 of the anterior metameres become mouth-organs; those of 

 the posterior metameres are converted into feet, and those on the last 

 are often completely atrophied, and often are not even formed in rudi- 

 ment. On the whole the number of appendages is much less than in 

 the Crustacea ; within the several classes they are always definite in 

 number, and the number of gnathites as well as of feet is constant. 

 In Peripatus the first two pairs form mouth-organs; the first of 

 these is surrounded by the sides of the wall of the mouth, and the 

 second pair only lie close to the mouth. In the Ai'achnida there is 

 only a single pair of these mouth-organs. In the Arauea it forms 

 the base of a many- jointed palp, which correspond to the chelte of 

 the Scorpionea, and to the powerful hooked chelae of the Phrynida. 

 In the Mites the pieces of either side are united into a grooved 

 lower lip, in which the stylet-shaped jaw-parts are placed. The 

 four remaining pairs of appendages persist in all Arachnida as feet ; 

 the first of these is flagellate in form in the Phrynida. 



There are thi'ee pairs of gnathites in the Myriapoda ; the first 

 pair is generally developed into a strong jaw ; in the Chilognatha 

 the second and third pairs are converted into a kind of lower lip, 

 and are said to be represented by a single pair in the embryo ; in 

 which case this order has only got two pairs of gnathites. In the 

 Chilopoda, however, the second and third pairs are much more inde- 

 pendent, and the first pair of feet is also associated with the mouth- 

 organs. The other appendages of the body have all pretty much the 

 same form ; in the Chilognatha there are two pairs on each meta- 

 mere. The last pair frequently loses its locomotor function, and 

 forms an appendage which we shall again meet with in a modified 

 form in the Insecta. 



The appendages are not therefore separated into mouth-organs 

 and locomotor appendages in the same way in all Tracheata ; they 

 vary, but not so much as they do in the Crustacea. The develop- 

 ment of the gnathites, or rather their differentiation from locomotor 

 appendages, is correlated with the development of the head, that is 

 to say, the head is formed as a result of this differentiation. 



