258 ZOOLOGY 
are supplied by the first sub-oesophageal gangha. The right 
and left strands of the ventral cord remain distinct and separate 
from one another throughout their course, being united by 
commissures at the various gangha. Of these one pair supplies 
the muscles which move the mandibles and maxillae, and there 
is a pair for each pair of legs. The last pair of ganglia give 
off posteriorly a fine nerve, which supplies the sensory spines 
of the abdomen. 
The ovaries le one on each side of the alimentary canal ; 
they are tubular, and reach as far forwards as the heart, and 
their walls are continuous with those of the oviducts which 
open into the brood-pouch between the abdomen and the shell. 
The ovary is broken up into a series of segments, each of which 
contains four cells. One of these becomes an ovum and in- 
creases in size by absorbing the other three. In the case of 
the large winter eggs, the contents of two or more segments are 
absorbed by the cell destined to become the ovum. 
The male is but little more than half the length of the 
female; the first pair of antennae, which are minute in the 
Fig. 154.—Side view of male Daphnia 
similis, magnified to the same extent as 
Fig. 153. After Claus. 
1. Antennules. 
2. Antennae. 
3. Testis. 
4. Ductus ejaculatorius. 
5. Rectum, 
8. Hepatic diverticulum. 
9. Heart. 
11. Shell gland. 
female, are here of considerable size, though not so large as the 
second. They are provided with numerous olfactory hairs. 
The first pair of swimming legs are provided with a claw and 
a spine which project between the two valves of the shell. 
The space between the abdomen and the shell is a narrow 
