THE ISOPODA 213 



of separate sexes. In certain Cymothoinae the external characters 

 of the male sex do not completely disappear when the individual 

 passes into the female phase, the copulatory appendage of tlio 

 second pleopods sometimes remaining of conspicuous size even in 

 specimens which have the marsupium filled with eggs (Fig. 127). 

 As in many other Crustacea, traces of hermaphroditism probably 

 exist normally in the young of many Isopoda. In Sj)haeroma, 

 vestiges corresponding to the three testicular follicles are found at 

 the anterior end of the ovary in young specimens, and what may 

 be a vestige of the oviduct is found in the male. 



Mention may be made here of the supposed occurrence of 

 " hypodermic impregnation " in the Asellotan Jaera. It is stated 

 that a spermatophore is inserted by the male between the thoracic 

 terga of the female, and that it penetrates the articular membrane 

 and passes into the body-cavity, discharging its contents into the 

 oviduct, while the empty cai)sule is expelled by the oviducal 

 aperture. The account of this extraordinary process cannot, 

 however, be accepted without further investigation. 



Development. 



The eggs are usually large, with superficial segmentation. In 

 Hemioniscus, however, which, from the peculiar formation of the 

 marsupium, is practically viviparous, the eggs are minute, without 

 yolk, and undergo complete and equal segmentation, giving rise to 

 a hollow blastosphere. It is characteristic of the Isopoda, as con- 

 trasted with the Amphipoda, that the developing embiyo is curved 

 towards the dorsal side. A "dorsal organ" is present in many 

 Isopod embryos, and assumes very diverse forms. In Cymothoidae 

 it arises as a thickened plate of cells which becomes invaginated, 

 forming a small cavity opening to the exterior by a narrow neck. 

 In Oniscus, on the other hand, the thickening is stated to become 

 constricted off from the dorsal surface and to form a saddle-shaped 

 plate partly enveloping the embryo and connected with it only by 

 a narrow stalk, but the accuracy of this account has lieen denied. 

 In Asellus a pair of trilobed hollow processes grow out from the 

 sides of the thoracic region. These have been regarded as repie- 

 senting the dorsal organ, but Claus has compared them with the 

 lateral wings of the carapace in the larva of Apseudes. A distinct 

 carapace-fold has been described in the embryo of Jaera, extending, 

 at first, over the region of the second thoracic somite, but afterwards 

 becoming reduced. Transitory rudiments of exopodites are stated 

 to be present on the thoracic limbs in early embryos of Ligia. 



In all Isopoda the young leave the brood-pouch with the last 

 pair of thoracic limbs undeveloped (as in Tanaidacea and Cumacea), 

 but otherwise in most free-living species the young are very similar 



