PHYSIOLOGY OF THE INVERTEBRATA. 5 



tumical elements. The more simple is the organisation of an 

 animal, taken as a whole, the simpler is also the structure of 

 each of the orders of anatomical elements. For example, the 

 muscular fibres of the Radiata, Aniinlom, and Mulliisca are 

 simpler than the same elements in the crab (Robin). But in 

 the higher animals there is a complete differentiation of parts 

 into organs having special physiological functions and varied 

 degrees of structure. In fact, the important law of Von Baer 

 — " the law of progi'ess from the general to the special " — 

 reigns supreme in the organic world. 



In the higher Invertebrates the various organs are localised 

 in different parts of the body. One area is restricted to 

 digestion, another to circulation, a third to respiration, and 

 a fourth to reproduction. The more highly organised the 

 animal, the more divided is its body into separate and dis- 

 tinct organs, each endowed with its own special function. 



The main object of this volume will be to consider in 

 detail the physiological functions of the various organs in 

 the Invcrtchrata ; but as it is impossible to investigate func- 

 tions without a knowledge of the organs performing them, 

 we shall refer (when necessary to a proper understanding of 

 the mechanism described) to their anatomy. 



As we shall have to allude to numerous classes, &c., of 

 animals, a classification of the Invertcbrata will hardly be out 

 of place in concluding the present chapter. 



The following tables are founded on the classifications of 

 Professor Huxley : — 



PROTOZOA. 



Monera. 



Protoplasta. 



Gregarinida. 



Catallacta. 



Infusoria. 



Foraminlfeia. 



Radiolaria. 



(«) Heliozca. 



(6) Cytophora 



