ARRANGEMENT OF THE TISSUES. 



159 



invested by chloragogue-cells, which radiate from them with 

 great regularity (Fig. 87). The finer branches have no muscular 

 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, 89) 

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



l.n 



FIG. 89. Highly magnified cross-section of a ventral ganglion; g.f, giant-fibres; l.n, lateral 

 nerve; n.c, nerve-cells; s, muscular sheath of the ganglion; s.v, sub-neural vessel; s.n.v, 

 supra-neural vessel. 



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

 The sheath (s, Fig. 89) consists of two layers, viz. :- 



1. Peritoneal Epithelium. On the outside. 



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

 ranged 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, /, Fig. 89), which are 

 the cross sections of three hollow fibres of unknown nature run- 

 ning 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, 



