COMPARISON OF ADULT ARTHROPODS AND VERTEBRATES. 



31 



atrial folds, together with the branchial and oral appendages, thus tend to enclose 

 the mouth in an ever deepening chamber. When this condition approaches its 

 extreme development — cirripeds, cladocera, etc. (Figs. 273-275) — the mouth be- 

 comes very inaccessible, and food can only reach it in a finely divided condition, 

 carried there by roundabout ways, in the currents of water produced by the swim- 

 ming, the oral, or the branchial appendages. Or the mouth may become com- 

 pletely closed, as in many dwarf, or parasitic cirripeds. (Figs. 280 and 281.) 

 Under these conditions the form and general appearance of a phyllopod-like 

 arthropod, with its large branchial, or atrial, chamber, and its oar-like cephalic 

 appendages, approaches that of some simple ostracoderms, like Cyathaspis, or 

 Pteraspis. (Comp. Figs. 176 and 244.) 



If we compare an adult Limulus viewed from the neural surface, with Cephal- 

 aspis seen from the same surface (Figs. 11 and 12), it will be seen that such an 

 arachnid could be made into an ostracoderm by the union and backward growth 



Fig, 22. — Young spider, showing the procephalon, transferred from the neural to the haemal surface, and the loca- 

 tion of the thoracic appendages, mouth, heart, and respiratory organs. Thoracic appendages removed. 



of the anterior margins of the cephalothorax, thus enclosing the mouth and 

 appendages in a large branchial chamber, like that in some phyllopods (Figs. 9- 

 10). The eyes and olfactory organs could remain in their original embryonic 

 position near the center of the head; the olfactory organs in front, the three 

 parietal ocelli in the center, and the lateral eyes on either side. (Fig. 12.) The 

 enlarged coxal joints of the anterior thoracic appendages, extending on to the 

 h^mal surface, would form the visceral arches about the mouth, the free append- 

 ages forming the external gills and the jointed, oar-like arms. A varying number 

 of infolded branchial appendages, similar to the lung books of arachnids, would 

 initiate the true gill pouches, and finally the elongated post-abdomen would form 

 the beginning of the flexible trunk, with its pleural or lateral folds, from which 

 the post-cephalic appendages later arise. 



From the cephalaspids we may easily derive the remaining ostracoderms. 

 In Bothriolepis, the old cephalo-thoracic portion remains comparatively small, 

 while the abdominal buckler has become greatly enlarged and closed on its neural 



