cviii Introduction. 



hollow articulated and segmented motor organs, into the interior of which 

 transversely striated muscles are prolonged, and the absence at all periods 

 of cilia, are points which distinguish all Arthropoda from all Vermes; 

 and a third point of nearly equal generality is furnished by the early 

 appearance of a ' primitive streak ' and the partial segmentation of the 

 yolk in development. Fourthly, whilst in Vermes it is only rarely 

 possible to differentiate the postcephalic segments into several regions, 

 this is always possible in Arthropoda ; the history of the development 

 and that of the relation of the internal organs to the external skeleton 

 rendering this possible even in the externally nearly perfectly homo- 

 nomous Myriopoda, and in the most degraded representatives of the 

 Crustacean Class as well as in such homonomous forms as certain of the 

 Isopoda. Fifthly, true metagenesis is unknown in Arthropoda. The 

 various organs and systems of the Crustacean, as being a water-breathing 

 Class, appear to attain a lower degree of evolution than those of the other 

 Arthropoda, and the Vermes may be supposed to approximate to them 

 more closely than to any of the air-breathing classes ; but the points just 

 specified will always serve to differentiate the members of the two Sub- 

 kingdoms, howsoever closely they may at first sight resemble each other. 

 Still it must be said, that the two Sub-kingdoms have their boundaries 

 approximated at many points, if not along great lengths, in space ; and 

 for a concrete illustration of this principle, the student is referred to the 

 description of Echmoderes Dujardinii, an animal which, though classed 

 as a Crustacean, combines with many of the characters of Arthropoda 

 many also of those of such Vermes as the Nematelminthes and the Oli- 

 gochaeta, and has been pointed out by Claparede, (Anatomic und Entwick- 

 elungsgeschichte Wirbelloser Thiere, i86, p. 92,) as constituting a link 

 between these two Sub-kingdoms. 



Class, Insecta. 



Air-breathing Arthropoda with well-marked heteronomous divi- 

 sion of the adult body into three distinct regions, the head, thorax, 

 and abdomen. The middle region, or thorax, is composed of three 

 segments, the prothorax, mesothorax, and metathorax, each of which 

 has a pair of jointed appendages, the legs, articulated to it ventrally, 

 whilst each of the two posteriorly-placed segments has also^ ordi- 

 narily, a pair of unsegmented appendages, the wings, or the wing- 

 covers, articulated to it dorsally. 



A post-abdomen is never very obviously marked off from the 



