94 



THE BIOLOGY OF AN ANIMAL. 



great regularity (Fig. 39). The finer branches have no muscu- 

 lar layer, consisting of the epithelium alone. 



Dissepiments. These often appear in cross or longitudinal 

 sections. They consist chiefly of muscle-fibres irregularly dis- 

 posed, intermingled with connective-tissue cells and fibres, and 

 covered on both sides with the peritoneal epithelium. 



Nervous System. A cross-section of a ganglion (Fig. 41) 

 shows it to be composed of two distinct parts, viz. , (1) the gan- 



FIG. 41. Highly magnified cross-section of a ventral ganglion, g.f, giant-flbres; I.n, 

 lateral nerve; .c, nerve-cells; s, muscular sheath of the ganglion; s.v, sub-neu- 

 ral vessel ; s.n.r, supra-neural vessel. 



glion proper on the inside, and (2) a sheath which envelops it. 

 The sheath (.s, Fig. 41) consists of two layers, viz. : 



1. Peritoneal Epithelium. On the outside. 



2. Muscular Layer, or sheath, a thick layer of irregularly 

 arranged muscle-fibres intermingled with connective tissue. Im- 

 bedded in it are the sub-neural blood-vessel on the lower side 

 and the supra-neural blood-vessels on each side above. In the 

 middle line are three rounded spaces (g, f, Fig. 41), which are 

 the cross-sections of three hollow fibres running along the entire 

 length of the ventral nerve-chain. They are called "giant- 

 fibres, ' ' and possibly serve to support the soft parts of the nerve- 

 cord. 



The Ganglion proper is distinctly bilobed, and consists of 

 two portions, viz. : 



1. Nerve-cells (n.c). Numerous pear-shaped nerve-cells near 

 the surface, with their narrow ends turned towards the centre, 

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

 confined chiefly to the ventral and lateral parts of the ganglion. 



