320 PHYLUM ARTHROPODA. 



cal correspondence can be traced between Arthropoda and 

 Annelida. 



In the development of the head, as of the rest of the body, 

 evidence of the existence of a segment is afforded by the forma- 

 tion in development (1) of a pair of mesoblastic somites, which 

 may retain their coelomic cavities, (2) of a pair of ganglia (forming 

 a neuromere) corresponding in position with the somite, and (3) 

 of a pair of appendages. 



Among the higher Arthropoda whose development is known 

 to us, the Chilopod Scolopendra appears to present the simplest 

 and most primitive condition of the segments of the body. It 

 has been investigated by Heymons, and Figs. 229.4 and B are 

 taken from his monograph. In front of the antennal segment 

 and marked, like it and all the segments behind it, by a pair of 

 coelomic sacks as well as by a neuromere, is a preantennal segment 

 from which for a short time small tentacular preantennal appen- 

 dages (1) project. Between antennae (2) and mandibles (4) a 

 segment is formed, marked by neuromere and coelomic sacks but 

 without appendages the premandibular or as it has been called 

 intercalated * segment (3). Then come the segments of the 

 mandibles and of the two maxillae of Scolopendra. The twenty- 

 three post-cephalic segments follow, the appendages of the first 

 (7) being transformed into the poison claws. 



The preantennae are formed at the sides of or a little behind 

 the mouth, though the latter subsequently moves back and lies 

 between the mandibles. In front of the mouth is formed (at 

 a later stage than that figured) the labrum, projecting back- 

 wards from the clypeus ; behind it the bilobed hypopharynx 

 subsequently appears. 



An unpaired median thickening of epiblast is formed from 

 the clypeal region. It is called by Heymons the archicerebrum. 

 Two paired pitted thickenings of the epiblast on either side of 

 it are the medial and lateral brain rudiments of Heymons. 

 From these five centres, one median and two paired, the syncere- 

 brum of Heymons is formed. 



* If the name " intercalar-segment " were to be taken literally, as mean- 

 ing that a new segment existed in a given individual between, two segments, 

 which were adjacent in its parents, the designation of the segments by 

 number would be at once recognized as inapplicable. We have however 

 no very satisfactory grounds for asserting or denying the possibility of 

 such an intercalation. 



