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Show 1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 27 Ceratodus, the Elasmobranchs mentioned are hare-lipped; and as regards the position of the external nostrils, Cestracion and Scyllium are intermediate between Chimeera and Ceratodus. It m a y 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 between 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, m a y be an advantage. In the second place, the large olfactory sacs of these animals suggest that the sense of smell is of value to them ; and the communication 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 performed by compression and dilatation of the walls of the olfactory Fig. 2. Ceratodus forsteri. Left lateral view of the brain in situ. 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.B1, M.B2, 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 suprascapidar bone is shown in place; and its contour is given LS if the anterior part of the vertebral column were transparent. sacs 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 |
