258 



ZOOLOGY 



are supplied by the first sub-oesophageal ganglia. 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 ganglia. 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 lie 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 Clans. 



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 



