446 



PCEGILOPODA. 



of the chelae of the cephalothoracic appendages arise as buds. The abdo- 

 minal 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- 

 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 

 central lobe forms a well-marked keel, and at the line of insei'tion of the 

 rim-like edge of the lateral lobes are placed the two pairs of eyes (se and 

 ce). The abdominal region is also distinctly trilobed and divided into nine 



c.s 



FIG. 246. Two STAGES IN THE DEVELOPMENT OF LIMULUS 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., viii. First, seventh, arid eighth appendages. 



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



segments ; the last, which is merely formed of a median process, being the 

 rudiment of the caudal spine. The edges of the second to the seventh are 

 armed with a spine. The changes in the appendages are not very con- 

 siderable. The anterior pair nearly meet in the middle line in front of 

 the mouth ; and the latter structure is completely covered by an upper 

 lip. Each abdominal appendage of the second pair is provided with four 

 gill-lamellae, attached close to its base. 



