140 



ZOOLOGY 



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 aesophageo-cutaneous 

 dud (ens. ct. d.\ which opens 

 directly into the pharynx. In 

 Myxine (Fig. 807) 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 (ces. 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. 807, 

 n.t.) ; there is no trace of neural 

 arches in the trunk, but in the 

 posterior part of the caudal region 

 both neural canal and notochord 

 are enclosed in a continuous car- 

 tilaginous plate. Similarly the 

 roof of the skull is entirely 

 membranous. The nasal passage 

 (na. t.) is strengthened by rings of 

 cartilage, and the buccal tentacles 

 are supported by rods of the 

 same tissue. Behind the styloid 

 cartilage or hyoid bar (st.p.) is 

 a rod connected below with the 

 subocular arch ; it probably re- 

 presents the first branchial bar. 

 The "tongue" is supported by 

 an immense cartilage (m. v. c.), 

 corresponding to the lingual 

 cartilage of the Lamprey. The 

 branchial basket is quite rudi- 

 mentary, being represented only 

 by certain small irregular car- 

 tilages, such as one in the walls 



