96 



THE BIOLOGY OF AN ANIMAL, 



crossing within the cord to the opposite side of the body or making exit 

 on its own side. Some of the cells, however, are purely " commissural," 

 i.e., neither branch leaves the cord. 



The sensory fibres entering from the periphery terminate freely (not in 

 ■nerve-cells), breaking up into numerous fine branches on the same side of 

 the cord. (Fig. 43.) 



The nerves leaving the central system are mixed, i.e., they contain both 

 sensory and motor fibres. 



nc 



Fig. 43.— Transverse section of ventral part of the body, showing the nervous con- 

 nections, n.c, ventral ganglion, giving off a lateral nerve at l.n. ; p.e., peritoneal 

 epithelium; l.m., longitudinal muscles; ?)i/, hypodermis; s, seta. A single motor 

 nerve-cell (black) is shown sending a fibre into the nerve towards the left. In 

 the nerve to the right are sensory fibres proceeding inward from the sensory cells 

 (black) of the hypodermis, and terminating in branching extremities. (After 

 Lenhoss6k.) 



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. 



With this brief sketch of the histological structure of the 

 earthworm we conclude our morphological stu(ij of the animal. 

 Those who desire fuller information on the histology will find a 

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

 ■at p. 93. Many later works have been published on the de- 

 tailed histology. 



