I 876. J PROF. T. H. HUXLEY ON CERATODUS FORSTERT. 27 



Cerutodus, the Elasmobranchs mentioned are hare-lipped ; and as 

 regards the position of the external nostrils, Cestracion and Scyllium 

 are intermediate between Chimcera and Ceratodus. 



It may be. asked, what is the use of a nasal passage and of internal 

 nares in a purely branchiate animal ? Without actual experiment it 

 is hard to give a definite answer to this question ; but I will venture 

 upon two suggestions. In the first place, these communications be- 

 tween the cavity of the mouth and the exterior must permit slow 

 respiration to take place when the jaws are shut ; and it is easy to 

 imagine that this, under many circumstances, may be an advantage. 



In the second place, the large olfactory sacs of these animals sug- 

 gest that the sense of smell is of value to them ; and the communi- 

 cation of the nasal passages with the mouth must enable them to do 

 what they could not do otherwise — namely, accelerate the rapidity of 

 the contact of odoriferous particles with the Schneiderian membrane 

 at will. The fish with posterior nasal apertures, in fact, can "sniff" 

 effectually, while that operation could only be very imperfectly per- 

 formed by compression and dilatation of the walls of the olfactory 



Fig. 2. 



£;.a 



an 



F.SjA 



Ceratodus forsteri. Left lateral view of the brain in sitti. 

 The details of the structure of the dorsal region of the spinal column are omitted. 

 Ch, notochord; E.O, exoccipital ossification; P.Sph, parasphenoid ; V.t, 

 vomerine teeth ; an, pn, positions of the anterior and posterior nares ; Op, 

 operculum ; I. Op, interoperculum ; Spl, splenial, and D, dentary bones of 

 the mandible ; Mck, Meckel's cartilage ; M.B^, M.W, anterior and posterior 

 mesobranchials ; Br. 5, fifth branchial arch ; 6, nodule of cartilage, which 

 possibly represents a rudimentary sixth arch; Py, pituitary body. The 

 other letters have the same signification as in the preceding figure. The 

 suprascapular bone is shown in place ; and its contour is given as if the 

 anterior part of the vertebral column were transparent. 



saca in the absence of any second opening. Probably the second 

 opening so generally present in the olfactory sacs of the Teleostei, 

 and the naso-palatine canal of the Marsipobranchs, have a similar 



