OF THE NKRVOUS SYSTEM. 81 



In insects, annelids, and niolhisca, the bnlk of the buccal 

 mass, and other necessary modifications of the oral apparatus, 

 elevate the so-called brain, curvinj:^ upwards the morplio- 

 logical axis of the body of the animal. 



By comparinf;; the indications of se^nents in front of the 

 moutli, and their corresponding diverging appendages, witli 

 the arrangement and distribution of the nerves given off from 

 tlie so-called brain, it appears very evident that this brain is 

 the aggi-egate of the segmental nei-vous centres in front of the 

 mouth. 



In like manner indications afforded by the segments, ami 

 their appendages immediately behind the moutli, enable us to 

 determine Avhether the so-called sub-wsophageal ganglionic 

 mass is a single segmental ganglion, or an aggregate of antero- 

 posteriorly united segmental ganglions. 



In this way I was enabled to perceive that the axis of the 

 nervous system of the annulose animal does not consist of a 

 supra-cesophageal mass, of an oesophageal collar, of a sub- 

 oesophageal mass, and a continuous sub-intestinal ganglionic 

 chain ; but of a continuous line of connected and serially 

 homologous ganglions situated in the mesial line of the 

 neural aspect of the body. 



The annulose, like the vertebrate animal, is developed 

 with its nervous axis turned away from, and its hremal axis 

 applied against, the vitellary mass.* 



* From the passage in his lectures already quoted, Profe,ssor Owen would 

 appear to consider the dorsal heart, witli its anterior and jwstcrior arterial 

 trunks in the decajxid crusticeau, and coiise<|uently the dorsal vessel in tlio 

 insect, arachnidan, and annelid, as correspoiulinj; to the thoraiie, alHloininul, 

 and caudal aortic trunk of the vertebrate animal. On this supposition only 

 can we niulerstand his lusscrtion, that when the so-calh-d belly of the crus- 

 tacean is turned ujiwards, its alimentary canal is still inteqwsetl Ix'tween the 

 aortic trunk ami the neural canal. Embryolog}', comparative anatomy, and 

 pliysiologj', appear to me, however, to affonl ample proof that the cardiac- 

 arterial doi-sid trunk of the annelid, crustacean, insect, or arachnidan, is ho- 

 mologous, not with the sub-spinal aorta of the vertebrate, but with the pri- 



r. 



