ANNELIDA. 449 



the important sense organs there located. The first swelling on the 

 ventral cord is called the sub-oesophageal ganglion, and the sub- 

 sequent swellings the ventral ganglia. The ganglionated condition 

 is not due to the exclusive presence of nerve cells at certain points, 

 for, as in the Vertebrata, nerve cells are found along the whole length 

 of the central organ; but is rather due to the fact that there are 

 more nerve cells and more nerve fibres in the ganglia than elsewhere, 

 in consequence of the fact that the nerve fibres do not as a rule 

 pass out all along the cords, but are gathered up into bundles, and 

 leave the ventral cords at any rate at segmental intervals. The 

 central nervous system is generally separated from the ectoderm 

 and placed within the muscular layer, but in the Archiannelida 

 and in one or two Chaetopoda it lies in and forms part of the 

 ectoderm. 



A vascular system is nearly always, probably always, present. It 

 has a canalicular character, and the principal vessels are the dorsal 

 and subintestinal longitudinal vessels. 



The perivisceral cavity is a portion of the coelom. It is derived 

 from the paired cavities of the mesoblastic somites of the embryo, 

 which swell up and surround the intestine. Primitively therefore 

 the body-cavity is divided into a series of paired cavities, the walls 

 of which are in contact with the walls of the somites anterior and 

 posterior to them in the series, and with the walls of their fellows of 

 the opposite side dorsal and ventral to the alimentary canal. Two 

 kinds of septa are thus formed, the one separating the cavities of 

 somites adjacent to each other in the series : these are the transverse 

 septa found running between the body-wall and the gut-wall in many 

 Chaetopoda ; and the other separating the cavities of the two somites 

 of a pair on the dorsal and ventral side of the alimentary canal : 

 these are the dorsal and ventral longitudinal mesenteries, which also 

 run between the body-wall and the gut-wall, but in a longitudinal 

 direction. These two kinds of coelomic septa are found coexisting 

 in the Archiannelida^ and possibly in one or two Chaetopods, but as 

 a general rule the dorsal and ventral mesenteries break down in the 

 adult, so that the two sides of the body-cavity become continuous 

 with each other on the dorsal and ventral sides of the alimentary 

 canal; while the transverse septa, though more often found in the 

 adult (Oligocliaeta and some Polychaeta), also generally break down 

 either partially or completely, so that the perivisceral cavity becomes 

 a continuous space from end to end of the animal (Echiuroidea, some 

 Polychaeta). In the Hirudinea the coelom generally (Acantliobdella 



2 G 



