DEVELOPMENT OP THE EXTERNAL FORM OF THE BODY. 347 



ma 



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 — Fio\ 158, a a 

 Fig. 159 B). 



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 natter 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 a semicircular ridge 

 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 

 )air is changed into the oper- 

 culum and a fusion of the 

 inner margins of the two exo- 

 )odites takes place, accom- 

 mied by a degeneration of 

 the endopodite. In the second 

 and following abdominal limbs 

 the rudiments of the branchial 

 lamellae now appear, only four 



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

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

 mdopodites of these limbs become divided up into three segments. 

 The young which hatch at the Trilobite stage are endowed with 

 L'eat activity, and already burrow in the sand like the adults. By 

 leans of their abdominal limbs they can swim about freely, and an 

 msequently occasionally taken in tow-nets, A. Agassiz having found 



Fig. 160.— Larva of Limulus, at the Trilobite 

 stage (after Watase). do, dorsal organ ; I, 

 hepatic diverticula; la, lateral eye ; ma, median 

 eyes ; s, rudiment of the caudal spine. 



