148 ZOOLOGICAL PHILOSOPHY 



FOURTH STAGE OF ORGANISATION. 



Nerves terminating in a ganglionic longitudinal cord, or in a brain 

 without a spinal cord ; respiration by gills ; arteries and veins for the 

 circulation. 



{Crustaceans, annelids, cirrhipedes and molluscs.) 



CRUSTACEANS. 



(Class VII. of the Animal Kingdom.) 



Oviparous animals with jointed body and limbs, a crustaceous skin, 

 several pairs of maxillae, eyes and antennae in the head. 

 Respiration by gills ; a heart and vessels for circulation. 



Observations. 



The great changes that we find in the organisation of the animals 

 of this class, indicate that in forming the crustaceans, nature has 

 succeeded in making great progress in animal organisation. 



In the first place, the method of respiration is altogether different 

 from that employed in the arachnids and insects ; and this method, 

 which is characterised by the organs called gills, continues as far as 

 the fishes. Tracheae will appear no more, and gills themselves dis- 

 appear as soon as nature can form a cellular lung. 



Then again the circulation, of which only rudiments are found in 

 the arachnids, is thoroughly estabhshed in the crustaceans ; for in 

 them we find a heart and arteries for the dispatch of blood to the various 

 parts of the body, and veins which bring back this fluid to the chief 

 organ which sets it in motion. 



We still find in the crustaceans the type of articulations, always 

 used by nature in the insects and arachnids, to facihtate muscular 

 movement by means of the induration of the skin ; but hereafter 

 nature abandons this type to estabUsh a system of organisation in 

 which it is no longer required. 



Most crustaceans live either in brackish or salt water. Some, 

 however, keep on land and breathe air with their gills : they all feed 

 on animal substances. 



