518 FORMATION OF THE LAYERS. 



stage of Penaeus and Euphausia. At this stage a cuticle is shed 

 (Van Beneden) which remains as an envelope surrounding the 

 larva till the time of hatching. Similar cuticular envelopes are 

 formed in many Isopoda. Subsequently the appendages of the 

 thorax appear, and finally those of the abdomen. Later than 

 the appendages there arise behind the mouth two prominences 

 which resemble appendages, but give rise to a bilobed lower lip 

 (Dohrn). 



In Asellus and Oniscus the ventral plate moulds itself to the 

 shape of the egg, and covers the greater part of the dorsal as 

 well as of the ventral side (fig. 241 A). As a result of this the 

 ventral surface of the embryo is throughout convex ; and in 

 Asellus a deep fold appears on the back of the embryo, so that 

 the embryo appears coiled up within the egg with its ventral 

 side outwards and its head and tail in contact. In Oniscus the 

 ventral surface is convex, but the dorsal surface is never bent in 

 as in Asellus. In Cymothoa the egg is very big and the 

 ventral plate does not extend nearly so far round to the dorsal 

 side as in Asellus, in consequence of which the ventral surface 

 is not nearly so convex as in other Isopoda. At the same time 

 the telson is early formed, and is bent forwards so as to lie 

 on the under side of the part of the blastoderm in front. In 

 having this ventral curvature of the telson Cymothoa forms 

 an exception amongst Isopods ; and in this respect is interme- 

 diate between the embryos of Asellus and those of the 

 Amphipoda. 



Amphipoda. Amongst the Amphipoda the segmentation 

 is usually centrolecithal. In the case of Gammarus locusta 

 (Ed. van Beneden and Bessels, No. 503) it commences with 

 an unequal but total segmentation like that of the Frog (vide p. 

 97), and the separation of a central yolk mass is a late occur- 

 rence ; and it is noticeable that the part of the egg with the 

 small segments eventually becomes the ventral surface. In the 

 fresh-water species of Gammarus (G. pulex and fluviatilis) the 

 segmentation is more like that of Insects ; the blastoderm cells 

 being formed nearly simultaneously over a large part of the 

 surface of the egg. 



Both forms of segmentation give rise to a blastoderm cover- 

 ing the whole egg, which soon becomes thickened on the ventral 



