DEGRADATION OF ORGANISATION 89 



ganglionic longitudinal cord. This is a sign of poverty of that system 

 observed among the animals of the two preceding and the two following 

 classes ; for the animals of these classes are the last in which a nervous 

 system is still to be seen. 



It is in the crustaceans that the last traces of an organ of hearing have 

 been identified ; after them it is no more found in any animal. 



Observations. 



Here ends the existence of a true circulatory system, that is to say, 

 of a system of arteries and veins, which is part of the organisation of the 

 most perfect animals and with which those of all the preceding classes 

 are provided. The organisation of the animals of which we shall now 

 speak is still more imperfect than that of the crustaceans, which are 

 the last in which a circulation is actually to be found. The degradation 

 of organisation is thus clearly in progress ; since according as we 

 advance along the series of animals, all features .of resemblance are 

 successively lost between the organisation of those we come to and 

 that of the most perfect animals. 



Whatever may be the nature of the movement of the fluids in 

 the animals of the classes that we are about to traverse, that move 

 ment is secured by less active methods, and consta!itly tends to 

 become slower. 



ARACHNIDS. 



Animals breathing by limited tracheae, undergoing no metamorphosis, 

 and having throughout their lives jointed legs and eyes in their 

 head. 



On continuing the order that we have hitherto followed, the ninth 

 rank in the animal kingdom necessarily belongs to the arachnids ; 

 they have so much affinity with the crustaceans that we shall always 

 have to bring them together, immediately following one another. 

 They are however entirely distinct ; for the arachnids furnish us 

 with the first example of a respiratory organ lower than gills, — one 

 never met with in animals which have a heart, arteries and veins. 



Arachnids in fact breathe only by stigmata and air-carrying tracheae, 

 which are respiratory organs analogous to those of insects. But 

 these tracheae, instead of extending throughout the body as in the 

 insects, are limited to a small number of sacs : this shows that nature 

 is bringing to an end in the arachnids the method of respiration 

 which she had to employ before the establishment of gills, just as she 

 brought to an end in the fishes or later reptiles that which she had to 

 make use of, before she could form a true lung. 



