394 , ZOOLOG V 
corresponding with the second, third, fourth, and fifth prosomatic 
limbs. In the adult this structure, which has been termed the 
coxal gland, appears to have no duct, but in the young, both an 
internal and an external opening have been described. The 
former opens into a space in the connective tissue which lies 
between the gland and the ventral blood sinus; the external 
opening is situated on the dorsal surface of the coxa of the fifth 
appendage. The chitinous covering of the animal is continued 
a short distance into the duct. The gland thus appears to 
have the relationship of a nephridium, and it is worthy of note 
that it opens upon the same appendage in numerical sequence 
as does the shell-gland of the Entomostraca, the latter having 
its orifice on the second maxillae, or fifth appendage. 
The nervous system of Limulus consists of a supra-oeso- 
phageal nerve mass which gives off five nerves, supplying the 
ocelli, the compound eyes, and the integument in the region of 
the head (Fig. 223). From the sides of this mass pass a pair 
of oesophageal commissures, which come together some dis- 
tance behind the oesophagus; the commissures are, however, 
united together by a number of transverse connectives situated 
behind the oesophagus. This elongated oval collar supphes 
nerves to all the limb-like appendages, and to the operculum 
which bears the genital orifices. After the fusion of the two 
circum-oesophageal commissures, the nervous system is prolonged 
backward as a ventral nerve cord, which during the first half 
of its course gives off no nerves, but its posterior half supplies 
five pairs of nerves, which pass to the last five appendages and 
the surrounding parts. 
Both the oesophageal collar and the ventral nerve cord, and 
many of the more important nerves, are ensheathed in blood- 
vessels. 
Limulus is dioecious, and the male can be distinguished 
from the female by the thicker and non-chelate character of the 
second pair of appendages. Both the ovary and the testis are 
retiform, the network of tubules which compose these glands 
extending through the pro-, meso-,and meta-soma. The tubules 
of the testes are in communication with a number of spherical 
sperm vesicles, which contain immature spermatozoa ; the latter, 
when ripe, are provided with a motile tail. Both the ovaries 
