324 



LECTURE XV, 



always smaller than the female, but have longer and larger antennae. 

 Mijller deemed these antenna? to be themselves the male organs ; 

 they are, however, only accessories, like the claspers in the sharks, 

 being provided with a hooked claw for holding on. Strauss points 

 out a modification of the lateral borders of the shell, and suggests 

 that thereby the valves of the female's shell are divaricated, and the 

 sperm ejected into their inter- space. The ovaria are simple coecal 

 tubes, coiled subspirally, one on each side the hinder half of the 

 abdomen. Several groups of ova ai-e successively hatched within the 

 bivalve shell, and the young are excluded during the spring and 

 summer ; these young ones propagating in a similar way, without 

 fresh impregnation : the males do not appear until the autumn, and 

 the ova which are then impregnated are retained in pt^culiar recep- 

 tacles throughout the winter months, and ai'e hatched in the following 

 spring. 



In autumn an opake layer is developed on the inner surface of the 

 common incubating cavity, which hardens in tAvo pieces like a small 

 bivalve-shell, with their concave opposed inner surfaces forming a 

 special cavity when the valves are closed; this is called the "ephip- 

 pium," or saddle, being placed on the dorsal surface of the DapJuna, 

 but beneath the common shell. Between the valves of the ephip- 

 pium is formed a similar but smaller apparatus, attached only to the 

 dorsal symphysis or hinge of the carapace, which also affects the form 

 of a small bivalve shell: this is the "internal ephippium ;" it in- 

 cludes, also, two bivalve capsules, in each of which an egg is lodged, 

 which remains in the passive state through the winter. 



If the mother have strength to undergo a moult after th'' formation 

 of the winter nest, this is cast off with her outer skin, which remains 

 with the ephippial apparatus, protecting the eggs during the winter. 

 They are hatched by the early warmth of spring, and produce only 

 females, which, after moulting three times, exclude a batch of ova. 

 Now, if these be insulated, a successive production of fertile females 

 may be observed to the sixth generation. But the germ-cells 

 retained unchanged in the developing Daphnia acquire the superad- 

 ded yelk and chorion of an ovum, from which the young are ex- 

 cluded. The males are a later production, and the winter eggs are 

 always the products of impregnation. 



In the male Cyclops the right antenna is peculiarly thickened and 

 jointed a little beyond its middle, so as to permit it to be bent upon 

 itself. With this organ the male seizes the female, and then grasps 

 the base of her tail by the great hook of his hindmost pair of feet. 

 After a slight i-esistance, she is quiet, and both gradually sink to the 

 bottom. A cylindrical tube, filled with sperm, escapes from the 



