RELATION OF ARTHROPOD DEAD TO ANNELID I'ROSTOMIUM. 259 



has shifted behind it. The first pair of ganglia have attained 

 a great size, and differentiation in connection with their 

 anterior position and relation to the sense-organs. The pro- 

 storaiura is insignificant, and the archicerebruna no longer 

 clearly distinguishable. The antenna are not i)rostoraial ten- 

 tacles, but outgrowths of the first metamere. 



The Myriapoda. 



Unfortunately the development of the Myriapods is very 

 imperfectly known ; but, according to the account we liave of 

 Julus (Heathcote, 1), it resembles exactly that of Peripatus 

 as regards the segmentation of the head. In the adult there 

 are three pairs of cephalic appendages — the antennae, the 

 mandibles, and the labial plate (fused maxillae). The last two 

 pairs belong to undoubted metaraeres. The first pair, the 

 preoral antennae, are developed on the large procephalic lobes, 

 which give rise to the main segment of the brain (cerebral 

 grooves are formed here also). As in Peripatus, the antennary 

 segment contains the first pair of mesoblastic somites. 



The Hexapoda (figs. 7 and 8). 



Four pairs of appendages are borne on the adult insect's 

 head. A study of its development shows that, in reality, it 

 is composed of six regions. Of these the three posterior, 

 belonging to the labium, maxillae, and mandibles, are univer- 

 sally considered to represent true metameres. The next, 

 counting from behind forwards, the recently discovered pre- 

 mandibular segment (Wheeler, 18), although possessing in the 

 earlier stages a distinct pair of coelomic somites and cavities, 

 and in some cases rudimentary appendages, becomes reduced, 

 and disappears in the adult. The next anterior segment, 

 bearing the antennae, was for long considered to be not only 

 preoral in position, but prostomial in origin. Here, again, 

 embryology shows that, like the posterior segments, it has a 

 special pair of mesoblastic somites, with well-developed 

 coelomic cavities (as a rule).^ Moreover, since in the early 



' "The deutocerebrum [antennary segment] in all the Orthoptera which I 



