THE PTEROBRANCHIA. 



439 



The posterior end of the thoracic nerve cord is united by nerves of consider- 

 able size with the median haemal nerve (Fig 297, h.nc), the latter representing the 

 median cardiac nerve of the arachnids. (Compare Figs. 78, 115 and 117.) 



V. The Pterobranchia. (Figs. 299, 300.) 



The pterobranchia have been shown to have a structure so much like that of 

 the enteropneusta in respect to the location of the functional mouth, medullary 

 plate, gill pouches, cephalic ceecum, coelomic chambers, and excretory ducts, 

 that the fundamental features in the morphology of both groups must be, with- 

 out doubt, interpreted in the same manner. If this is done, then it is apparent 



Fig. 299. — Diagrams of Cephalodiscus. A, Neural surface; B, side view, in optical section. 



that the features which are more specially characteristic of the pterobranchia, 

 such as the six pairs of appendages, the short, branchial region, the U-shaped 

 intestine with the anal end bent toward the posterior neural surface, and with 

 the genital ducts opening in the neural surface of the branchial region, give 

 a decidedly arachnoid character to the adults, and strengthen and confirm 

 the interpretation we have given for the enteropneusta. 



Cephalodiscus may therefore be regarded as a naked arthropod-like animal 

 with a closed neurostoma, the location of which is indicated by the pit in the ante- 

 rior portion of the neural plate (Fig. 299, n.st.), and by the cephalic cascum, di, 

 that probably united the primitive stomodasum with the enteron. The cephalic 

 disc with its procephalon, thoracic appendages, and thoracic neuromeres, m.i., 

 represents the cephalothorax; the excretory ducts, opening to the exterior by the 

 proboscis pores and collar pores, p. p. arid c. p., represent respectively the cephalic 

 and thoracic ducts of arthropods; and the single pair of gill sacs, g.p., represent a 



