PCECILOPODA. 



445 



now distinctly divided into two regions the cephalothoracic in front, and 

 the abdominal behind, both divided into segments. 



VI 



FIG. 245. THREE STAGES IN THE DEVELOPMENT OF LIMULUS POLYPHEMUS. (Some- 

 what modified from Packard.) 



A. Embryo in which the thoracic limbs and month have become developed on the 

 ventral plate. The outer line represents what Packard believes to be the aninion. 



B. Later embryo from the ventral surface. 



C. Later embryo, just before the splitting of the chorion from the side. The full 

 number of segments of the abdomen, and three abdominal appendages, have become 

 established; m. rnouth ; i. ix. appendages. 



Round the edge of the ventral plate there is a distinct ridge the 

 rudiment of the cephalothoracic shield. 



With the further growth of the embryo the chorion becomes split 

 and cast off, the embryo being left enclosed within the inner membrane. 

 The embryo has a decided ventral flexure, and the abdominal region 

 grows greatly and forms a kind of cap at the hinder end, while its 

 vaulted dorsal side becomes divided into segments (fig. 245 C). Of these 

 there are according to Dohrn seven, but according to Packard nine, of 

 which the last forms the rudiment of the caudal spine. 



In the thoracic region the nervous system is by this stage formed as 

 a ganglionated cord (Dohrn), with no resemblance to the peculiar ceso- 

 phageal ring of the adult. The mouth is stated by Dohrn to lie between 

 the second pair of limbs, so that, if the descriptions we have are correct, it 

 must have by this stage changed its position with reference to the appen- 

 dages. Between the thorax and abdomen two papillae have arisen which 

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

 development they can hardly be regarded as segmental appendages. lu 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 



