130 



ZOOLOGY 



SECT. 



B 



Lamprey ; but it appears that no odontoblasts and no calcined 

 substance of any kind are formed in connection with them. The 

 nostril (na. ap.) is a large unpaired aperture situated in the 

 dorsal margin of the buccal funnel, and is continued into a 

 passage, the pituitary sac, which opens into the pharynx. Myxine 

 commonly lives nearly buried in mud, and the respiratory current 



passes through this pas- 

 sage to the gills. 



The only fin is a nar- 

 row caudal surrounding 

 the end of the tail. The 

 respiratory organs pre- 

 sent striking differences 

 in the two genera. In 

 Bdellostoma there are 

 six or seven very small 

 external branchial aper- 

 tures (br. cl. 1) on each 

 side. Each communi- 

 cates by a short tube 

 with one of the gill- 

 pouches, which is again 

 connected with the phar- 

 nyx by another tube. 

 Behind and close to the 

 gill-slit, on the left side, 

 is an aperture leading 

 into a tube, the ceso- 

 phageo - cutaneous duct 

 (ces. ct. d.), which opens 

 directly into the pharynx. 

 In Myxine (Fig. 762) 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 oesophageo-cutaneous duct (oes. ct. d.) opens. In both genera 

 the internal branchial apertures communicate directly with the 

 pharynx ; there is no respiratory tube. 



The neural canal is over-arched merely by fibrous tissue (Fig. 762, 

 nt.) ; there is no trace even of the rudimentary neural arches of 

 the Lamprey. Similarly the roof of the skull is entirely mem- 

 branous. The nasal passage (na. t.) is strengthened by 'rings of 

 cartilage, and the buccal tentacles are supported by rods of the 



br.cl.f 



br.ap 



oe.s.cl.d 



Fro. 761. Head of Myxine glutinosa (A) and of 

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

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

 'iidh. mouth ; na. ap. nasal aperture ; a-s. ct. d. ceso- 

 phageo-cutaneous duct. The smaller openings in A 

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



