308 J. W. KTRKALDY-. 



of the four characters indicated by the terms notochord, pha- 

 ryngeal gill-slits, tubular myelon, and myelonic eye. 



The notochord is large and unconstricted, is continued 

 through the region of the head, and projects beyond the first 

 myotome into the snout or rostrum. The longitudinal muscu- 

 lature of the body-wall is divided by membranous septa into a 

 series of well-marked myotomes. There is no marked enlarge- 

 ment of the anterior region of the myelon as brain, but a slight 

 dilatation of its cavity anteriorly. The myelon terminates 

 anteriorly short of the termination of the notochord ; there is 

 no special protective skeleton (cranium) for the anterior por- 

 tion of the myelon. The vascular system is devoid of a spe- 

 cialised " heart.^^ Minute tubular nephridia_, arranged serially, 

 are present. Existing forms exhibit a deep-seated asymmetry, 

 masked by a secondary superficial symmetry; they are of small 

 size, and probably are degenerate representatives of symmetrical 

 bilateral ancestors of more fully elaborated organisation. 



Family (unica) BRANCHiosTOMiDiE. 



AmphioxidjE, Gray. ' Synopsis Brit. Mus.,' 1842, p. 150. 

 Amphioxini, Miiller. 'Abliaudl. k. Akad.,' Berlin, 1844. 

 BkanchiostomidjE, Bonaparte. ' Cat. metodico Pesci Europ.,' 1846. 

 Amphioxoidei, Blacker. 'Enum. sp. Piscium Archip. Ind.,' 1859. 

 CiRROSTOMi, Giintber. 'Cat. Fishes Brit. Mus.,' vol. viii, p. 513, 

 1870. 



Cephalochorda of minute size, and laterally compressed 

 elongate form — acutely terminated, both anteriorly and pos- 

 teriorly. The integument is raised dorsally into a median 

 plate or ridge forming the dorsal fin, which is uninterrupted 

 throughout the length of the entire body. Two similar plates 

 or outstanding ridges — the metapleura — exist on either side of 

 the anterior two thirds of the body, which is bluntly triangular 

 in section : the superior angle supports the median fin ; tiie 

 infero-lateral angles support the two lateral plates or meta- 

 pleura. A median ventral fin is present in the hinder third 

 of the body ; in some species it is continuous with the right 

 metaplcur, which is also continued in front of the mouth to 



