BRANCHIOPODA. CLADOCERA. 379 



the middle line to form a large, continually trembling, frontal 

 eye, largely developed in the Polyphemidae. Beneath this the 

 unpaired simple eye, reduced in Daphnia to a streak of pigment in 

 connection with the brain, usually remains. A special sensory 

 apparatus, whose function is not quite clear, appears in the region 

 of the neck, in the form of an aggregation of ganglion cells. 



The heart has the form of an oval sac, with a pair of trans- 

 verse lateral venous ostia and an anterior arterial opening. Its 

 pulsations are rhythmic, and succeed one another quickly. In 

 spite of the want of arteries and veins, the blood, which contains 

 amoeboid cells, circulates along definite channels in the body. 

 The looped and coiled shell gland (Sd) is always present. The 

 cervical gland, which functions as an organ of attachment, is 

 less widely distributed. The sexual glands lie in the thorax 

 as paired tubes by the side of the alimentary canal. In the 

 ovaries groups of four cells are separated ; in the formation of 

 the summer-eggs one cell of each group, usually the third from 

 the front end, becomes an ovum, while the rest are employed 

 as nutritive cells for the nourishment of the ovum, which in- 

 creases in size and absorbs fat globules. In the formation of 

 the winter-eggs every other group of four cells, or a larger number, 

 likewise breaks up and subserves the nourishment of the cells 

 destined to become the eggs (Weismann). The ovary is directly 

 continuous with the oviduct, which opens dorsally beneath the 

 shell into the brood-pouch. The testes, like the ovaries, lie at 

 the sides of the intestine and are continuous with the vasa- 

 deferentia, which open to the exterior ventrally by a common 

 opening behind the last pair of appendages or at the extreme 

 end of the body, the openings being sometimes situated on small 

 slightly protrusible prominences. 



The smaller males usually appear in the autumn ; they may, 

 however, also be present at any other time of the year, and, as 

 recent investigations have proved in a tolerably satisfactory 

 manner, always when the conditions of life and nourishment 

 are unfavourable (vide p. 373). Before the appearance of the 

 males, hermaphrodite forms * are sometimes produced with an 

 organization which is partly male and partly female. 



At the season when males are not present, normally in the 



* Compare especially W. Kurz, Ueber androgyne Missbildung bei 

 Cladoceren, Sitzungsber der Akad. der Wissensch. Wien, 1874. 



