DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



347 



semicircular ridge 



mcu 



the incurved segments in front of it (Fig. 159 A). At the base of 

 this segment lies the anal aperture. The limbs now grow more and 

 more like those of the adult, but the teeth are still wanting on the 

 coxal joints, which are modified into masticatory ridges. The two 

 anterior pairs of abdominal appendages, behind which a third pair 

 has become recognisable, have attained the typical biramose form by 

 the development of a small inner lobe (endopodite Fig. 158, a v 2 , 

 Fig. 159 13). 



The stage at which the embryo emerges from the blastodermic 

 cuticle is known as the Trilobite stage (Figs. 158 A, 159). The 

 cephalo-thoracic shield is now flatter and wider, and has lost all traces 

 of segmentation. It is distinctly divided by two longitudinal 

 furrows into a central region with a well-marked keel and two 

 lateral regions. The large eyes are situated in the furrows separa- 

 ting the median from the lateral lobes, and 

 connects the median with the 

 lateral eyes. The abdominal 

 region still appears divided up 

 into segments. Six movable 

 thorns can be recognised on 

 its lateral margins (Fig. 160) 

 from the second segment to 

 the seventh. The rudiment 

 of a fourth pair of abdominal 

 limbs now appears. The first 

 pair is changed into the oper- 

 culum and a fusion of the 

 inner margins of the two exo- 

 podites takes place, accom- 

 panied by a degeneration of 

 the endopodite. In the second FlG _ 160 ._ Larva of Li muius, at the Trilobite 



and following abdominal limbs 8te S e ( after WATASE). do, dorsal organ ; I, 



hepatic diverticula; la, lateral eye ; via, median 

 the rudiments of the branchial ey es ; *, rudiment of the caudal spine. 



lamellae now appear, only four 



being at first apparent on each limb. Their number is increased later 

 by the budding of new lamellae at the bases of the limbs. The 

 endopodites of these limbs become divided up into three segments. 



The young which hatch at the Trilobite stage are endowed with 

 great activity, and already burrow in the sand like the adults. By 

 means of their abdominal limbs they can swim about freely, and are 

 consequently occasionally taken in tow-nets, A. AGASSIZ having found 



