190 



ONYCHOPHORA. 



until the embryo is mature, and are even found in the adult 

 (v. Kennel). As development proceeds, the ventral organs shift 

 together and finally unite in the middle line ; they become flattened 

 and a slight depression is seen on their outer surface. Whereas at 

 first they were very massive (Figs. 100 and 101 B), they now appear 

 much smaller as compared with the size of the embryo (Fig. 102, 

 p. 205). As the embryo develops, they become less and less con- 

 spicuous, and, in the adult, are represented merely by a small 

 unpaired follicular depression of the epidermis situated medianly 

 between the bases of the limbs on each segment (v. Kennel), 

 and until recently overlooked. 



Fig. 96.— Transverse sections through the head of an embryo of P. Edwardsii (after v. Kennel). 

 In A, only half the section is drawn, an, antennal nerve ; n, brain (consisting of cell- and 

 fibre-substance) ; us, primitive segment of the head ; vo, ventral organ. 



The ventral organs of the anterior segments differ from the rest. 

 Those belonging to the segment of the oral papillae, as well as those 

 of the maxillary segment, are drawn into the buccal cavity, and can 

 still for a time be recognised on its floor (Fig. 94 A, vo 1 and vo 2 ). 

 Of these, the two posterior unite to form the ventral wall of the 

 oesophagus, while the anterior organs remain distinct. Consequently 

 each of these latter develops further independently, and in both 

 the external depressions are more marked than the other trunk 

 ventral organs (thus retaining, according to v. Kennel, the more 

 primitive character). This is still more the case with those structures 

 which must no doubt be regarded as ventral organs of the cephalic 

 segment. These are two deep epidermal depressions lying near one 



