88 ZOOLOGICAL PHILOSOPHY 



CRUSTACEANS. 



Animals with a jointed body and limbs, crustaceoiis skin, a circulatory 

 system, and breathing by gills. 



We now enter upon the long series of animals, whose body and 

 limbs are jointed, and whose integuments are hard, crustaceous, 

 horny or coriaceous. 



The sohd or hard parts of these animals are all on the exterior. 

 Since nature created the muscular system very Httle in advance 

 of the earlier animals of this series, and since she had need of sohd 

 support to endow it with energy, she was obHged to establish the 

 method of articulation in order to secure the possibiUty of movement. 



All the animals that exhibit this method of articulation were held 

 by Linnaeus and subsequently as forming only a single class, to which 

 was given the name of insects ; but it was at length recognised that 

 this large series of animals has several important divisions which 

 muLst be distinguished. 



The class of crustaceans, which had thus been confused with that 

 of insects, although all the ancient naturalists had always kept it 

 apart, is a division indicated by nature and must be maintained. 

 It should follow immediately upon the annehds and occupy the 

 eighth rank in the general series of animals ; this is required by their 

 organisation and is not a matter of arbitrary opinion. 



The crustaceans indeed have a heart, arteries and veins ; a trans- 

 parent and almost colourless circulating fluid, and they all breathe 

 by true gills. This is unquestionable and will always constitute a 

 difliculty in the way of those who persist in placing them among the 

 insects on account of their having jointed legs. 



If the crustaceans are completely distinguished from the arachnids 

 and insects by their circulation and respiratory organ, and if their 

 rank is therefore obviously superior, they yet share one trait of 

 inferiority of organisation with the arachnids and insects as compared 

 with the annehds ; that, namely, of being a part of the series of animals 

 with jointed limbs : a series in the course of which the circulatory 

 system and consequently the heart, arteries and veins are seen to 

 diminish and disappear, and in which again the branchial system 

 of respiration is Hkewise lost. The crustaceans therefore again confirm 

 the continuous degradation of organisation in the direction in which 

 we are following the animal scale. The transparency and extreme 

 thinness of the fluid which circulates in their vessels, Hke that of 

 insects, is a further proof of their degradation. 



As to their nervous system, it consists of a very small brain and a 



