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PROCHORDATA. 



The diagrams here reproduced are founded primarily upon conditions prevailing in 

 the larva of Amphioxus, which are of the utmost complication as soon as one approaches 



Fig. 17. Diagrams to elucidate some suggested comparisons between an Enteropneust (A), an Ascidian 

 larva (B), and a Craniate embryo (C). 



A. p. proboscis ; p.p.c. proboscis pore-canal opening close to the anterior neuropore which leads into 

 c.v. the collar nerve-tube; e.p. epiphysial roots; s. stomochord; m. mouth; c. collar-region; g. gut; t. trunk- 

 region; d.n.t. dorsal nerve-tract; a. anus; py. pygochord. 



B. f.o. organ of fixation; e. endostyle; m. mouth; a.n.c. anterior neurenteric canal which, together 

 with s.n.g., subneural gland, constitutes the neuro-hypophysis ; c.v. cerebral vesicle; sp. medullary tube 

 communicating with the gut by means of p.n.c. posterior neurenteric canal ; re. notochord. 



C. ep. pineal body or epiphysis cerebri; f.b. m.b. and h.b. fore-, mid-, and hind-brain; hy. hypophysis 

 cerebri ; th. thyroid gland ; other letters as above. 



them in a morphological spirit. In the Prochordata (including for the present occasion 

 the Enteropneusta, Tunicata, and Cephalochorda) the three principal canal systems of the 

 body, namely, coelomic, neural, and alimentary, open in various ways to the exterior in the 

 anterior region, where they form an intricate association of apertures which I have formally 

 designated as the anterior trematic complex. 



The following table will possibly be useful to future investigators who may venture 

 to wrestle with this problem. 



