Characteristics of the Arthropoda. cvii 



The most essential difference between the various Classes of the Sub- 

 kingdom Arthropoda, is that which tracheal and branchial breathing re- 

 spectively correspond to, and with this principle of classification in view, 

 we place the Crustacea, the earliest representatives of the Arthropodal 

 type in geological times, apart from the other three Classes. 



Within the limits of any one of these four Classes, the greater or lesser 

 heteronomy of the several regions of the body, the greater or lesser 

 extent, that is, to which the specialization of segments, and, even more, 

 of appendages, has been carried out, constitutes the most impoi-taut dif- 

 ference between one order and another, next to that which the actual 

 abortion of segments or appendages entails. 



The two Classes, Crustacea and Arachnida, differ from the Myriopoda 

 and Insecta in comprehending much more varied forms ; the Myriopoda 

 contrast with the other three Classes by their low degree, whilst the 

 Insecta are distinguished by their high degree of heteronomy. 



The Crustacea and Arachnida are very closely approximated by such 

 forms as the Cyamidae and the Pycnogonidae. This latter family is re- 

 ferred to the Class Arachnida mainly on account of the lateral diverticula 

 which the digestive tract is furnished with, but there is reason to believe 

 that some of its species possess free and functional antennae, and it should 

 be considered therefore as Crustacean. If the parasitic Hedrioiihthalmata, 

 such as the Cyamidae, connect the Crustacea with the Arachnida on 

 the one side, they connect them also with the Insecta through Pediculus 

 on the other. A more striking, though perhaps not more real, link be- 

 tween the Crustacea and the Insecta and Myriopoda, is presented by the 

 air-breathing Isopoda, such as Oniscus, on the one side, and the apterous 

 Orthoptera and Glonieris on the other. The Arachnida approximate to 

 the Insecta very obviously by such forms as Galeodes ; and though their 

 marked heteronomy and the definite number of their body segments 

 cause them to differ very widely in external appearance from the Myrio- 

 poda, the peculiarities of their reproductive and respiratory systems 

 appear to speak to the existence of a real affinity between them and the 

 Chilognathous division of that class. 



The Arthropoda have frequently been classed together with more or 

 fewer of the Vermes in one Sub-kingdom, that of the 'Annulosa;' and 

 whilst by such highly-organized forms as the Marine PolycJiaeta an 

 approximation appears to be made to certain of the less specialized of the 

 Crustacea ; or even of the Myriopoda, or the lai'vae of Insects, amongst 

 the air-breathing Arthropoda : the microscopic Rotifera connect the Vermes, 

 to which Sub-kingdom they are to be referred, very closely to the Crus- 

 tacea. The possession at one period, or, as usual throughout life, of 



