cxvi Introduction. 



and corresponds precisely with the genital segment of the Scorpion (see 

 p. 1 1 6, infra), and with that of the Poecilopodous Crustacea. The seventh 

 post-cephalic segment ordinarily carries a double penis, which may how- 

 ever, as in Glomeris, be removed to the posterior extremity of the body, 

 indicating hereby an approximation to the Chilopoda. The larvae of 

 Chilognatha, Avhen they leave the egg, have ordinarily only three pairs 

 of appendages, which are carried upon three of the four first post-cephalic 

 segments ; and they contrast still further with the Chihpoda, by at- 

 taining their additional segments at each moult by intercalation only in 

 that portion of germinal membrane which is interposed between the 

 penultimate and ante-penultimate segments. 



Pcmropus appears to resemble the Chilognatha in having the generative 

 orifices situated anteriorly instead of posteriorly in the body, and its 

 larvae are hexapodal ; but it differs from them in many points of its 

 external anatomy. The shape of its body as a whole, its dorsal plates, 

 and elongated posterior legs, give it a resemblance to some Chiloj^oda. 

 As tracheae may be absent in the early developmental stages even of an 

 Insect, Chloeon Dimidiatum, too much weight must not be laid upon 

 their absence in Pauroims ; and it may be safely said that for our present 

 purpose, that, namely, of showing the relationships which subsist between 

 the various classes of Arthropoda, the most important morphological 

 point in this genus (or order '?) is its possession of bifid antennae, carrying 

 multi-articulate flagella, by which peculiarity, as also by the others 

 specified above, a very distinct affinity is shown to exist between Myrio- 

 poda and Crustacea. 



For the structure and affinities of the Myriopoda, see Sir John 

 Lubbock, on Pauropus, a new type of Centipede, Linn. Soc. 

 Trans., xxvi., 1867, ihkiue c'ltata ; and also Mr. Newport's 

 Papers on the Myriopoda, in the Transactions of the Royal 

 and Linnaean Societies. 



Class, Araehnida. 



Air-breathing Arthropoda, with well-marked heteronomy be- 

 tween the several divisions of the body, which are usually only 

 two, a cephalo-thorax to which the limbs are limited, and an 

 abdomen, usually marked off from it by a constriction, a post- 

 abdomen being developed only in the Scorpions. Their antennae 

 are modified so as to serve in the prehension of food ; and they 



