ISOPODA. 



485 



tirt 



into the brood pouch, and is introduced by a centrolecithal 

 segmentation, the central part of the egg (food yolk) remaining at 

 first unsegmented. The blastoderm soon consists of a periphe- 

 ral layer of nucleated cells and produces by a rapid growth of its 

 constituent cells the ventrally placed germinal bands, at the 

 anterior end of which the cephalic lobes are first marked off. 

 In Asellus two lobed prominences arise as paired appendages of 

 the dorsal part of the thorax and shortly disappear. These are 

 known as the " dorsal organ," but their nature is problematical. 

 Of the appendages the two 

 pairs of antennae and the 

 mandibles are first formed 

 in Asellus (Boas) and Ligia 

 (Nusbaum), and after these 

 have made their appearance a 

 new cuticle, corresponding to 

 that of the nauplius stage of 

 other Crustacea, is separated, 

 and envelops the embryo 

 until it is hatched. While 

 however the separation of 

 this nauplius skin is of fre- 

 quent occurrence among 

 Isopods, the formation of the 

 limbs in Cymoihoa and Onis- 

 cus is simultaneous, with the 

 exception of the last thoracic 

 legs. These do not appear in 

 the Isopoda until after the 

 embryo has been hatched for 

 some time. The caudal ex- 

 tremity is bent towards the dorsal surface during development. 



Metamorphosis. Besides the development of the last (eighth) 

 thoracic legs not inconsiderable changes occur in the form of the 

 appendages before the attainment of sexual maturity. Hence 

 the Isopoda may be said to undergo a metamorphosis. Among 

 some of the parasitic forms it is of a very marked character. 

 While the modification of structure in relation to the parasitic 

 habit is comparatively slight in the Cymobhoina and Aegina, 

 the Epicaridea present a series of modifications which culminate 



FIG. 299.Gyge branchialis. a female 

 seen from the ventral side ; 6 abdomen of 

 the same more highly magnified, and 

 showing the adherent male ; .BrZ oostegite ; 

 K branchial laminae (from Claus after 

 Cornalia and Panceri). 



