364 WILLIAM PATTEN. 



It follows from what has precededj that in Vertebrates and 

 Arachnids, those surfaces of the head bearing a median row of 

 paired plates and those bearing a median row of unpaired 

 plates are homologous. This conclusion is supported by the posi- 

 tion of the nervous system, and also by the relation these plates 

 bear to ganglionic sense-organs. For example, each of 

 the median cranial plates of Vertebrates contains a primary 

 group of sense-organs supplied by a ganglionated " ramus 

 dorsalis;^' they therefore agree perfectly in this respect with 

 the coxal plates, each of which also contains a group of coxal 

 sense-organs, supplied by a nerve which, on other grounds, we 

 concluded was homologous with a " ramus dorsalis." Thus, 

 since the coxal plates of Arachnids and the paired cranial plates 

 of Vertebrates are homologous and segmentally arranged, the old 

 Goethe-Oken theory of the bony cranium appears in a new light. 



I trust in all that has preceded I have succeeded in showing 

 that there are important resemblances between the cephalo- 

 thorax of Arachnids and the head of Vertebrates. There are 

 other resemblances of a more general character, a few of 

 which we shall merely mention here: — (1) The division into 

 cephalothorax, body, and tail. (2) The tadpole-like larva of 

 Vertebrates, with its enormous head, and small body and tail, 

 is explicable on the Arachnid theory. (3) The Trilobites 

 probably swam, if at all, on their backs; and it is still more 

 probable that the Merostomata, from their shape and the posi- 

 tion of their oar-like appendages, swam in the same way. 

 The larvse of Limulus, according to my own observations, always 

 swim on their backs. Thus the way is prepared for the manner 

 of locomotion in fishes. (4) The development of the Arthro- 

 pod heart from the concrescence of paired mesodermic folds 

 is like that in some Vertebrates; and the aortic, arch-like 

 blood-vessels in the appendages may be compared to the 

 branchial arteries (Fig. 14, 5.) (5) Moreover, the enor- 

 mous liver of Arachnids, (6) the blood, (7) the histological 

 structure of the nervous system, and the manner of nerve 

 terminations, (8) the minute structure of tiie muscles, and (9) 



