STRUCTURE AND CLASSIFICATION OP THE AETHROPODA. 5*71 



bears a pair of gnatbobasic hemignaths without rami or palps, 

 and is followed by two jaw-bearing* somites (maxillary and 

 labial). This enumeration would give six somites in all to 

 the head, three prosthomeres and three opisthomeres. Recent 

 investigations (Folsom, 4) show the existence in the embryo 

 of a prsemaxillary or supra-lingual somite which is sup- 

 pressed during development. This gives seven somites to 

 the Hexapod's head, the tergites of which are fused to form 

 a cephahc carapace or box. The number is significant, since 

 it agrees with that found in Edriophthalmous Crustacea, and 

 assigns the labium of the Hexapod to the same somite 

 numerically as that which carries the labium-like maxilli- 

 pedes of those Crustacea. 



The somites following the head are strictly nomomeristic 

 and nomotagmic. The first three form the thorax, the 

 appendages of which are the walking legs, tipped with paii'ed 

 claws or ungues. (Compare the homoplastic claws of Scorpio 

 and Peripatus.) Eleven somites follow these, forming the 

 abdominal " tagma," giving thus twenty-one somites in all 

 (as in the higher Crustacea). The somites of the abdomen 

 all may carry rudimentary appendages in the embryo, and 

 some of the hinder somites may retain their appendages in a 

 modified form in adult life. Terminal telescoping of the 

 abdominal somites and excalation may occur in the adult, 

 reducing the obvious abdominal somites to as few as eight. 

 The genital apertures are median, and placed far back in the 

 series of somites, viz. the female on the seventh abdominal 

 (seventeenth of the whole series) and the male on the ninth 

 or antepenultimate abdominal (nineteenth of the whole series). 

 The appendages of the eighth and tenth abdominal somites 

 are modified as gonapophyses. The eleventh abdominal 

 segment is the telson, usually small and soft ; it carries the 

 anus. 



The Hexapod a are not only all confined to a very definite 

 disposition of the somites, appendages, and apertures as 

 thus indicated, but in other characters also they present the 

 specialisation of a narrowly limited, highly developed order 



