OP THE INVERTEBRATE ANIMALS. 3 



•we are compelled to regard this connexion as essential; and we 

 might even now, in a modern garb, yield the first place in our 

 experimental sciences to the ingenious ideas with which Reil 

 formerly commenced his Archives. 



The greater the importance of an organ, the more does the 

 variety in the combination of its chemical elements disappear. 

 The nervous system, i. e. the primitive fibres and the ganglion- 

 cells, does not appear to present any chemical differences ; how- 

 ever, nothing certain can be based upon mere microscopic reac- 

 tions. The muscular system, i. e. the primitive bundle (both the 

 smooth and the transversely striated), exhibit the same composi- 

 tion. In the vascular system, i. e. the walls of the tubes, we 

 also find no difference — both belonging to the proteine com- 

 pounds, or being nearly related to them. The intestinal canal 

 with its appendages forms the transition to the cutaneous system; 

 the epithelia follow next; horny plates, and certain membranes 

 which are situated between the epithelia and the muscular 

 laminae, or rather which themselves perform the functions of 

 epithelia, exhibit the same composition ; whilst the appended 

 glands (the pancreas, liver and salivary glands), excluding their 

 separate secretions, consist of proteine compounds. The same 

 applies to the respiratory system. The external tunics of the 

 laminae of the gills, as also the trachea, correspond to the cuta- 

 neous system. Lastly, the latter, i. e. the teguments destined 

 as a protection from external influences, exliibit the utmost 

 variety in form and combination. In the highest grade of the 

 animal kingdom this system consists of proteine compounds ; it 

 is purely animal ; in the intermediate ones it is combitied with 

 the cutaneous system of jylants ; finally, in the lowest it is iden- 

 tical with the latter. Hence the Mollusca stand higher than the 

 Articulata ; the latter occupy the intermediate station ; the 

 Zoophytes, in the true sense of the word, are plant-animals. 



The transition stages are all extremely interesting, Tiius, in 

 the Cirripedia, from the cirri alone they should, in a histo- 

 logico-chemical point of view, be arranged among the Articulata 

 (Crustacea), whilst from the shells they should be placed with the 

 Bivalves. Again, the Ascidia, which form the transition from 

 the Mollusca to the Zoophytes, are arranged, from the delicate 

 structure and the chemical properties of their tunics, among 

 those animals which have a vegetable mantle. Lastly, also the 

 most simple forms of the animal world [Bacillarice) form ti-ans- 



b2 



