CRUSTACEA. 



517 



studied, there appears before any other organ a provisional 

 structure formed from the epiblast and known as the dorsal 

 organ. An account of it is given in connection with the de- 

 velopment of the organs. The general external changes under- 

 gone by the larva in its development are as follows. The 

 ventral thickened area of the blastoderm (ventral plate) shapes 

 itself and girths nearly the whole circumference of the ovum in 

 Oniscus (fig. 241 A) but is relatively much shorter in Cymothoa. 

 Anteriorly it dilates into the two procephalic lobes. In 

 Cymothoa it next becomes segmented ; and the anterior seg- 

 ments are formed nearly simultaneously, and those of the 

 abdomen somewhat later. At the same time a median depres- 



A 



pr 



FlG. 241. TWO LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF ONISCUS 

 MURARIUS. (After Bobretzky.) 



st. stomodfeum ; pr. proctodreum ; hy. hypoblast formed of large nucleated cells 

 'mbedded in the yolk ; in. mesoblast ; vg. ventral nerve cord ; sg. supra- cesophageal 

 ganglion ; //. liver; do. dorsal organ ; zp. rudiment of masticatory apparatus ; ol. upper 

 lip. 



sion appears dividing the blastoderm longitudinally into two 

 halves. The appendages are formed later than their segments, 

 and the whole of them are formed nearly simultaneously, with 

 the exception of the last thoracic, which does not appear till 

 comparatively late after the hatching of the embryo. The late 

 development of the seventh thoracic segment and appendage is 

 a feature common to the majority of the Isopoda (Fritz Miiller). 

 In Oniscus the limbs are formed in nearly the same way as in 

 Cymothoa, but in Asellus they do not arise quite simultaneously. 

 First of all, the two antennae and mandibles (the future palp) 

 appear, inaugurating a stage often spoken of as the Nauplius 

 stage, which is supposed to correspond with the free Nauplius 



