io6 COMPARATIVE ANATOMY 



of hermaphroditism in A. cancriformis, or in the closely allied 

 Lepidurus productus* In these genera males are of rare 

 occurrence, and whole broods have been taken from ponds 

 consisting entirely of females. Yet the ova of these virgin 

 females develop, so we have in Apus an instance of the 

 phenomenon of parthenogenesis. Occasionally, however, 

 males are found in abundance. Thus out of seventy-nine 

 specimens taken by Sir John Lubbock in pools near the 

 Somme, thirty-three were males ; and similar instances have 

 been recorded by other naturalists. 



The reproductive organs, whether male or female, lie in the 

 perivisceral blood-sinus, and have the form of a long 

 sacculated tube extending on either side of the body from 

 about the third to the twenty-second segment, counting from 

 the first pair of thoracic limbs. In Apus glacialis^ the 

 sacculations of the ovary correspond with the annulations of 

 the body, and the organ therefore shows traces of metameric 

 segmentation, but this is not so clear in A. cancriformis. In 

 the latter species the main ovarian tubes lie along the sides 

 of the perivisceral sinus, and are incompletely cut off from 

 the more central part of the sinus by a sort of lattice-work of 

 diagonal muscles which traverse the cavity of the sinus. These 

 ovarian tubes are generally filled with mature ova. They open 

 by a short and wide oviduct at the base of the eleventh thor- 

 acic limb on either side, and internally they give off a number 

 of tubular-branched diverticula which project through the 

 above-mentioned muscular lattice-work, and nearly fill up the 

 central part of the perivisceral sinus. The walls of the 

 diverticula are lined by a columnar germinal epithelium, from 

 which primitive ova are derived. The primitive ova (oogonia) 

 pass to the extremities of the branches, and there undergo 

 maturation, each dividing into four cells, of which one only 

 develops into an ovum. When ripe, the ova distend the 

 extremities of the diverticula so that the latter look like 

 bunches of grapes. The ova, when ripe, are abundantly 

 provided with food yolk, and have relatively thick chitinous 

 shells. They pass into the brood pouches formed in the 

 manner already described in the eleventh thoracic limbs 

 (oostegopods), and there undergo the first stages of develop- 



* The genus Lepidurus is distinguished from Apus by the presence of 

 anal scales between and above the caudal setae. 



