XTTI 



PHYLUM CHORDATA 



43 



SECTION I. ACRANIA (CEPHALOCHORDA). 

 Including only the little fish-like Lancelels. 



SECTION II. CRANIATA (VERTEBRATA). 



Including Cyclostomes, Fishes, Amphibians, Reptiles, Birds, and 

 Mammals. 



SECTION I.-ACRANIA (CEPHALOCHORDA). 



The section Acrania includes only two families, the Branchio- 

 stomidcB containing the genera BrancMostoma (usually known by 

 the name of one of its sub-genera, Amphioxus), Epigonichthys, and 

 perhaps some others ; and the Amphioxididce containing the 

 pelagic genus Amphioxides, which, however, may not be an adult 

 form. The differences between the genera and species of 

 Branchiostomidse are comparatively insignificant, and the following 

 description will deal exclusively with the best known and most 



ml pi 



alrp -vcnlfr 



orhcL 



dors/f 



mj/om 



or.hd 



FIG. 750. Amphioxus lanceolatus. A, ventral, _B,-side view of the entire animal. 

 an. anus ; atrp. atriopore ; al.f. caudal fln ; dr. cirri ; dors.f. dorsal fln ; dars.f. r. dorsal fin- 

 rays ; fion. gonads ; mtpl. metapleure ; myom. myomeres ; nch. notochord ; or. fid. oral hood ; 

 vent. f. ventral fin ; vent.f. r. ventral fin-rays. (After Kirkaldy.) 



thoroughly investigated species, the Lancelet, Amphioxus lanceo- 

 latus, found in the English Channel, the North Sea, and the 

 Mediterranean. 



Amphioxus is a small transparent animal, occurring near the 

 shore and burrowing in sand ; its length does not exceed 5-8 cm., 

 or less than two inches. Its form will be obvious from Fig. 750 

 and from the transverse sections, Fig. 751, A and B. The body 

 is elongated, pointed at either end, and compressed. The 

 anterior two-thirds is roughly triangular in transverse section, 

 presenting right and left sides, inclined towards one another above, 

 and a convex ventral surface. The posterior third is nearly oval 

 in section, the right and left sides meeting above and below in 

 a somewhat sharp edge. 



Extending along the whole of the dorsal border is a median 

 longitudinal fold, the dorsal fin (dors, f.) : this is continued round 



