418 PHYSIOLOGY. 



Ehrenb. ) and the crustaceous Arthrozoa (Crustacea, 

 formerly called Malacostraca by me, not the Malacostraca 

 of Leach). 



3. The EARTH-ARTHROZOA. - Here are arranged the Myriapoda 



and the Araclmodea (or Arachnides). 



4. The AIR-ARTHROZOA. Which comprise the hexapod insects 



(Insecta). 



Each of these groups has a peculiar organ whereby it is characterised, 

 and as the general character of the Arthrozoa is expressed in the pre- 

 sence of organs of motion, we shall necessarily have to seek for the 

 characters of the subordinate groups among those organs. The character 

 of the WORMS or water-Arlhrozoa is, that in them we first observe the 

 presence of distinct organs of motion, but which yet are of no deter- 

 minate type, and which, therefore, sometimes present themselves as 

 sucking cups upon the head (Cestodes), or upon the head and belly 

 (Trematodes), or upon the head and contiguous to the arms (Hirudinei), 

 then as setae (Naidei, Lumbricini sive Chcetopodes'), and, lastly, as 

 short pedal warts with booklets (Annelides antennati, Lam.). In the 

 following class they transform themselves partly to swimming organs 

 (the rowing organs) and partly to jointed swimming and coursing feet, 

 both of which forms are simultaneously common to the majority of 

 Crustacea. In the earth- Arthrozoa the limbs are conformably shaped, 

 feet adapted only to running ; in the air-Arthrosoa, or INSECTS, we 

 first find wings as the organs of motion for this element, they possess also 

 legs for running and exercising other functions like the earlier ones. 



249. 



Is the law indicated by the earlier physiologists, and applied by 

 Oken, especially, to the natural system, correct, that the higher groups 

 are repetitions of the lower ones in their development; or must we 

 rather, with Von Bar *, thus explain it, that the development of every 

 class of animals admits of recognising the progressive perfection of the 

 animal body as well by morphological as histological separation, as 

 also by the progressive construction of a particular form from one 

 more general ? In either case it will necessarily be applicable to the 

 development of insects. It is evident that both propositions tend to 



* C. v. Biir iiber Entwickelungsgeschiclite der Thiere. Konigsb. 1828. 4to. vol. i. 

 p. 231. 



