428 GENERAL CONSIDERATIONS ON 



more ventral position of the limbs, some of which, as jaws, bite one 

 against the other. This last point is of importance, because the jaws 

 of the Annelida are mere cuticular secretions of the stomodaeum, 

 and not appendages. We should mention further the degeneration 

 of the coelomic sacs and of the nephridial system. The former 

 undergo disintegration through the development of a secondary 

 pseudocoele, and the latter, in the higher forms, through the ac- 

 quisition of a new excretory apparatus. In direct connection with 

 the condition of the body-cavity we have the absence of a closed 

 blood-vascular system, and the development of that type of heart 

 characteristic of the Arthropoda. 



Another peculiarity recurring throughout the whole series of the 

 Arthropoda is the enlargement of the primary cephalic region by 

 the addition of originally post-oral segments. It might be worth 

 while to attempt to attain a fixed point for the homologising of the 

 anterior pair of limbs in the various Arthropoda, but in the present 

 state of our knowledge such an attempt would have to be made with 

 the greatest care. We may perhaps conclude from the segmentation 

 of the brain, that the antennae of Peripatus, the Myriopoda, and 

 the Insecta are homologous with the first antennae of the Crustacea. 

 We should then perhaps be able to consider the jaws of Peripatus, 

 which are included in the mouth, and the ganglion of which is 

 approximated to the brain (in the same way as are the antennal 

 ganglia of the Crustacea), as the equivalents of the second antennae 

 of the Crustacea, which, indeed, in the Nauplius still function as 

 masticatory organs. We are perhaps justified in assuming that, in 

 the Myriopoda and Insecta, these extremities are completely lost, so 

 that the mandibles of the Insecta (the homologues of the oral 

 papillae of Perijxdus), would then have to be related to the 

 mandibles of the Crustacea. We might further assume that the 

 chelicerae of the Arachnida correspond to the second antennae of 

 the Crustacea, a view that is supported by the condition of their 

 ganglia which fuse with the brain. The homologue of the first 

 antennae would, in this case, be lost, but it is seen to reappear 

 ontogenetically as a transitory structure (p. 111). The pedipalps 

 would thus be the equivalent of the mandibles of the Crustacea and 

 Insecta, whereas the chelicerae have until now, as a rule, been 

 homologised with these latter organs. 



A consideration of the accompanying table shows that the different 

 regions of the body (the head, the thorax, and the abdomen) are 

 not, in the various divisions of the Arthropoda, precisely homo- 



