160 TEE BIOLOGY OF AN ANIMAL. 



into which each sends a single branch or nerve-fibre. They are 

 confined to the ventral and lateral parts of the ganglion. 



2. Fibrous Portion. This occupies the central part. It con- 

 sists of a close and complicated network of nerve-fibres inter- 

 mingled with connective-tissue. Some of these fibres communi- 

 cate with branches of the nerve-cells, as stated above ; others run 

 out into the lateral nerves, while still others run along the com- 

 missures to connect with fibres from other ganglia. 



A section through the brain or supra-oesophageal ganglia shows the 

 same essential structure, but the muscular layer of the sheath is very thin 

 and the nerve-cells are on the front and upper sides of the ganglia, in- 

 stead of on the lower. 



Sections through the ventral commissures are similar to those through 

 the ganglia, but the central portion (i.e., that within the sheath) is smaller, 

 is divided into two distinct parts, and the nerve-cells are less abundant. 



Sections through the nerves show them to consist only of parallel fibres, 

 surrounded by a sheath which gradually fades away as the nerves grow 

 smaller, and finally disappears, the muscular layer first disappearing, and 

 then the epithelial covering. 



Other Organs. The structure of the ovaries has already been de- 

 scribed. That of the spermaries varies according to the state of develop- 

 ment. When young they consist of closely-packed, rounded cells, each of 

 which gives rise to a number of spermatozoa, which are at first attached by 

 their heads in bundles, the tails being parallel. These bundles are easily 

 seen in sections. 



The Clitellum is produced by an enormous thickening of the hypoder- 

 mis, caused especially by a great development of the gland-cells. Three 

 forms of these may be distinguished, which probably produce different 

 secretions. The tissue is permeated by numerous minute blood-vessels 

 which ramify between the cells. 



With this brief sketch of the histological structure of the 

 earthworm we conclude our morphological study of the animal. 

 Those who desire fuller information on the histology will find an 

 exhaustive treatment of it in the work of Claparede, already cited 

 at p. 158. 



