96 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
2. PROTECTIVE TISSUE. 
The protective tissue of the central. nervous system is of two kinds; 
an outer membrane, the neurilemma, and within this a spongy tissue, 
the neuroglia. The neurilemma is better developed in the anterior than 
in the posterior region of the animal, and better in the older epitokal 
individuals than in the atokal forms. In the cephalic segment it is very 
thick and forms a capsule around the brain (n’lem., Plate 2, Fig. 9), 
and it also envelops the nerves from the brain. A tissue similar in 
texture lines the dorsal wall of the head, there taking the place of a 
basement membrane (Plate 3, Fig. 20). The brain capsule is continuous 
with this lining of the wall of the head along the mid-dorsal line, and 
also around the nervous connections that unite the brain with the poste- 
rior dorsal surface of the head. It likewise serves as a place of attach- 
ment for some of the muscles of the head, as the neurilemma of the 
ventral cord does for some of the diagonal muscles ; but elsewhere the 
capsule is free from the wall of the head, being suspended in the cavity 
of the cephalic lobe. 
The neurilemma of the brain is continuous with that of the ventral 
cord along the circum-cesophageal commissures. Except where it is 
pierced by nerves, the neurilemma of the cord (Plate 2, Fig. 18) forms 
a closed tube, whose walls are thickest at the anterior end. Along its 
dorsal side the wall of the neurilemma tube is continuous with the 
tunica intima of the ventral longitudinal blood-vessel by means of a 
narrow membrane which connects the two tubes throughout their 
entire lengths (Fig. 18). 
In structure the neurilemma is uniformly fibrous ; it does not stain in 
iron-hematoxylin or osmic acid. On the outer surface of the brain cap- 
sule there is a layer of nuclei which may possibly belong to the neuri- 
lemma. I have not clearly seen such nuclei on the neurilemma of the 
ventral cord. The connective tissue of the muscles is continuous with 
the neurilemma in many places, and resembles it in every respect. 
The neuroglia of the brain is a network of delicate fibres with numer- 
ous small elongated nuclei located at the nodes of the network. This 
tissue lines the inner surface of the brain capsule, from which it pene- 
trates into all parts of the brain except the neuropil and the masses of 
small nuclei connected with it. The neuroglia of the ventral cord is like 
that of the brain in texture, and it surrounds and penetrates the nervous 
structures of the cord in the same way as in the brain. The portion 
immediately surrounding the cord, however, is somewhat differentiated 
ee ee oe 
