536 



PCECILOPODA. 



so-called lower lip of the adult, but from their position and late development 

 they can hardly be regarded as segmental appendages. In the course of 

 further changes all the parts become more distinct, while the membrane in 

 which the larva is placed becomes enormously distended (fig. 246 A). The 

 rudiments of the compound eyes are formed on the third (Packard) or fourth 

 (Dohrn) segment of the cephalothorax, and the simple eyes near the median 

 line in front. The rudiments of the inner process of the chelae of the cepha- 

 lothoracic appendages arise as buds. The abdominal appendages become 

 more plate-like, and the rudiments of a third pair appear behind the two 

 already present. The heart appears on the dorsal surface. 



An ecdysis now takes place, and in the stage following the limbs have 

 approached far more closely to their adult state (fig. 246 A). The 

 cephalothoracic appendages become fully jointed ; the two anterior ab- 

 dominal appendages (vn.) have approached, and begin to resemble the oper- 



FlG. 246. TWO STAGES IN THE DEVELOPMENT OF LlMULUS POLYPHEMUS. 



(After Dohrn.) 



A. An advanced embryo enveloped in the distended inner membrane shortly 

 before hatching ; from the ventral side. 



B. A later embryo at the Trilobite stage, from the dorsal side. 

 I., vii., vm. First, seventh, and eight appendages. 



cs. caudal spine ; se. simple eye ; ce. compound eye. 



culum of the adult, and on the second pair is formed a small inner ramus. 

 The segmentation of the now vaulted cephalothorax becomes less obvious, 

 though still indicated by the arrangement of the yolk masses which form 

 the future hepatic diverticula. 



Shortly after this stage the embryo is hatched, and at about the time of 

 hatching acquires a form (fig. 246 B) in which it bears, as pointed out by 

 Dohrn and Packard, the most striking resemblance to a Trilobite. 



Viewed from the dorsal surface (fig. 246 B) it is divided into two 

 distinct regions, the cephalothoracic in front and the abdominal behind. 

 The cephalothoracic has become much flatter and wider, has lost all trace 

 of its previous segmentation, and has become distinctly trilobed. The 



