ARTICULATA. 257 



THIRD DIVISION. 



AETICULATA, Citvier. DIPLO-GANGLIATA, Grant. 



The articulated animals afford a wide field for the investiga- 

 tions of the zoologist and comparative anatomist ; but their 

 palaeontological history is much behind that of the other 

 divisions of the animal kingdom. 



The Articulata have a body composed of a series of rings, 

 which give the animal an articulated or jointed appearance. 

 The skin is often indurated by a deposit of the carbonate and 

 phosphate of lime, as in the Crustacea, or by a peculiar horny 

 substance called chitine in insects. The rings of the body 

 support lateral 'appendages, which are metamorphosed into 

 various organs in different parts of the same : thus the 

 appendages of the rings of the head form the antennae, jaws, 

 and other parts of the mouth in insects, spiders, and crus- 

 tacea : those of the thorax form the feet in Crustacea and 

 spiders, and the wings and feet in insects. 



The nervous system is composed of a chain of ganglia dis- 

 posed in pairs, and united by nervous cords : hence the term 

 diplo-gangliata. The first pair, of ganglia forms the brain, 

 which reposes on the upper part of the oesophagus. A nervous 

 thread descends on each side of that tube, to unite the brain 

 with the second pair of ganglia which lie below it ; and thus 

 the oesophagus is surrounded by a nervous collar. In the 

 larval condition of insects the nervous system may be conve- 

 niently studied. Each ring of the body is seen to have its 

 own ganglionic centre, from whence nerves proceed to the 

 various organs contained therein. The nervous system 

 undergoes important changes during the development of the 

 animal. In the larva, it is worm-like, with ganglia corre- 

 sponding to each ring. In the perfect insect, several ganglia 

 coalesce to form two or more nervous centres, from whence 

 lashes of filaments are given off. As an equal distribution of 

 the ganglia is one of the characters of the larva state, a 

 union or fusion of the ganglia into masses indicates a higher 

 grade of organisation. In this division, all the Worm-like 



