XIII 



PHYLUM CHORDATA 



135 



There is no true buccal funnel : the space on which the mouth 

 opens is edged with tentacles (Fig. 816) supported by cartilages ; 

 there is a single median tooth above the oral aperture, and two rows 

 of smaller teeth on the tongue. The papillae beneath the cone-like 

 horny teeth bear a still closer superficial resemblance to rudiments 

 (or vestiges) of true calcified teeth than is the case in the Lamprey ; 

 but it appears that no odontoblasts and no calcified substance of 

 any kind are formed in connection with them. A velum separates 

 the buccal cavity from the 



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\W^ 



*f^ 



W^--^-" 



mlh 



mlh 



pharynx. The nostril (na. 

 ap.) is a large unpaired 

 aperture situated in the 

 dorsal margin of the buccal 

 space, and is continued 

 into a passage, the pitui- 

 tary sac, which opens into 

 the pharynx by an aper- 

 ture which appears in late 

 embryonic life. Myxine 

 commonly lives nearly 

 buried in mud, and the 

 respiratory current passes 

 through this passage to 

 the gills. 



The only fin is a narrow 

 caudal surrounding the end 

 of the tail. The respira- 

 tory organs present strik- 

 ing differences in the two 

 genera. In Bdellostoma 

 there are in different species 

 six to fourteen very small 

 external branchial aper- 

 tures (br. d. 1) on each side, 

 each of which communi- 

 cates by a short tube with 

 one of the gill-pouches, 

 which is again connected 

 with the pharnyx by 

 another tube. Behind and close to the last gill-slit, on the left 

 side, is an aperture leading into a tube, the cesophageo-cutaneous duct 

 (CBS. ct. d.), which opens directly into the pharynx. In Myxine 

 (Fig. 817) the tubes leading outwards from the gill-pouches all unite 

 together before opening on the exterior, so that there is only a 

 single external branchial aperture (br. ap.) on each side ; into the 

 left common tube (c. br. t.) the cesophageo-cutaneous duct (CBS. 

 ct. d.) opens. In both genera the internal branchial apertures 



br.cl.ii 



br.a-p 



oes.cl.d 



FIG 816. Head of Myxine glutinosa (A) and of 

 Bdellostoma forsteri (B), from beneath, br. ap. 

 branchial aperture ; br. d. 1, first branchial cleft ; 

 mth. mouth : na. ap. nasal aperture ; as. ct. d. ceso- 

 phageo-cutaneous duct. The smaller openings in A 

 are those of the mucus-glands. (After W. K. Parker.) 



