so ZOOLOGICAL PHILOSOPHY 



If the arachnids are quite distinct from the crustaceans, through 

 not breathing by gills but by very limited air-carrying tracheae, 

 they are also to be distinguished from the insects. It would be quite 

 as improper to combine them with the insects of which they lack 

 the classic character and from which they differ even in internal 

 organisation, as it would be to confuse the crustaceans with the insects. 



The arachnids, indeed, although having strong affinities with the 

 insects are essentally distinct from them : 



1. In that they never undergo metamorphosis, that they have at 

 birth the shape and all the parts of an adult and that, consequently, 

 they have eyes in their head and jointed legs throughout their lives. 

 This is an order of things that follows from the nature of their internal 

 organisation and therein differs greatly from that of insects ; 



2. In that in the arachnids of the first order (pedipalp-arachnids) 

 we begin to see the outlines of a circulatory system ; ^ 



3. In that their respiratory system, although of the same order 

 a.s that of insects, is nevertheless very different ; since their tracheae 

 are limited to a small number of sacs, and do not constitute the very 

 numerous air-canals extending throughout the animal's body that are 

 witnessed in the tracheae of insects ; 



4. Lastly, in that the arachnids procreate several times in the course 

 of their life ; a faculty which the insects do not possess. 



These considerations suffice to show how faulty are those arrange- 

 ments in which the arachnids and insects are combined into one class, 

 through paying exclusive regard to the joints in these animals' legs, 

 and the more or less crustaceous skin which covers them. It is almost 

 as if we were to consider only the more or less scaly integuments of 

 reptiles and fishes, and thus to combine them into one class. 



The general degradation of organisation that we are seeking through- 

 out the entire animal scale is extremely obvious in the arachnids : these 

 animals indeed breathe by an organ inferior in organic perfection 

 to lungs and even to gills, and have only the rudiments of a circulation 

 apparently not yet finished off. They thus confirm in their turn the 

 continuous degradation in question. 



This degradation may even be observed in the series of species 

 belonging to that class ; for the arachnids with antennae, making up 

 the second order, are sharply distinguished from the others, are very 

 inferior to them in progress of organisation, and come close to the 

 insects ; they differ from the latter however in undergoing no meta- 



' "It is especially in the spiders that this heart may be easily observed : it may 

 be seen beating through the skin of the abdomen in species that are not hairy. On 

 removing this skin, a hollow oblong organ is seen, pointed at the two ends, with the 

 anterior end directed towards the thorax and from the sides of which there issue 

 visibly two or three pairs of vessels " (Cuvier, Anatom. conip. vol. iv. p. 419). 



