428 



FORMATION OF THE LAYERS. 



This lobe rapidly grows in size, and at its extremity is placed the 

 narrow anal opening. It soon forms a well-marked abdomen bent 

 forwards over the region in front (figs. 239 B, and 240 A and B). 

 Its early development as a distinct outgrowth causes it to be with- 

 out yolk ; and so to contrast very forcibly with the anterior thoracic 

 and cephalic regions of the body. In most cases this process corre- 

 sponds to the future abdomen, but in some cases (Loricata) it appears 

 to include part of the thorax. Before it has reached a considerable 

 development, three pairs of appendages spring from the region of 

 the head, viz. two pairs of antennae and the mandibles, and inaugu- 

 rate a so-called Nauplius stage (fig. 240 A). These three appendages 

 are formed nearly simultaneously, but the hindermost appears to 

 become visible slightly before the two others (Bobretzky). The 

 mouth lies slightly behind the anterior pair of antennae, but distinctly 

 in front of the posterior pair. The other appendages, the number of 

 which at the time of hatching varies greatly in the different 

 Decapods (vide section on larval development), sprout in succession 

 from before backwards (fig. 240 B). The food yolk in the head and 



ft id 



md 



FIG. 240. Two STAGES IN THE DEVELOPMENT OF PALJEMON. 



A. Nauplius stage. 



B. Stage with eight pairs of appendages, op. eyes; at 1 , and at 2 , first and second 

 antenna ; md. mandibles ; mx 1 , mx*. first and second maxillaB ; mxp s . third niaxillipeds ; 

 Ib. upper lip. 



thoracic region gradually becomes reduced in quantity with the 

 growth of the embryo, and by the time of hatching the disparity in 

 size between the thorax and abdomen has ceased to exist. 



Isopoda. The early embryonic phases of the Isopoda have been 

 studied by means of sections by Bobretzky (No. 498) and Bullar 

 (No. 499) and have been found to present considerable variations. 

 When laid the egg is enclosed in a chorion, but shortly after the 

 commencement of segmentation (Ed. van Beneden and Bullar) a 

 second membrane appears, which is probably of the nature of a 

 larval membrane. 



In all the forms the segmentation is followed by the formation of 

 a blastoderm, completely enclosing the yolk, and thickened along an 

 area which will become the ventral surface of the embryo. In this 



