OF THE NERVOUS SYSTEM. 81 



In insects, annelids, and mollusca, the bulk of the buccal 

 mass, and other necessary modifications of the oral apparatus, 

 elevate the so-called brain, curving upwards the morpho- 

 logical axis of the body of the animal. 



By comparing the indications of segments in front of the 

 mouth, and their corresponding diverging appendages, with 

 the arrangement and distribution of the nerves given off from 

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

 the aggregate of the segmental nervous centres in front of the 

 mouth. 



In like manner indications afforded by the segments, and 

 their appendages immediately behind the mouth, enable us to 

 determine whether the so-called sub-cesophageal 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-oesophageal mass, of an cesophageal 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 haemal axis 

 applied against, the vitellary mass.* 



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

 appear to consider the dorsal heart, with its anterior and posterior arterial 

 trunks in the decapod crustacean, and consequently the dorsal vessel in the 

 insect, arachnidan, and annelid, as corresponding to the thoracic, abdominal, 

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

 can we understand his assertion, that when the so-called belly of the crus- 

 tacean is turned upwards, its alimentary canal is still interposed between the 

 aortic trunk and the neural canal. Embryology, comparative anatomy, and 

 physiology, appear to me, however, to afford ample proof that the cardiac - 

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

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



G 



