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Show 1!10 ON THE STHnTuT CTUUE OF THE • r OJ_JL, b bones but no ca?"tilaqe bones. D. The superadded mem. rane ' . osseous craniun~. . 1 11 t · tl l The three first-mentiOned kinds of stu are 111 WI ~ o~ y fi hes · A n~p~1~ ~' b ~· a ' Re:mr tilia' Aves, and Ma1nmaha ln-among s ' . . 1 . er or small r numb .r of cranial bones variably possessing a aig developed in cartilage. A. The membranous c1·anium. . The only a.ni.n la1 , a t 11r ' "\nt known ' wln h co1ncs umler th .s cateo-ory I.S tb a t si'ngnl·c1 r fi h ' the low st of all Vertebrata, ' I . 0 Amphwxus l anceo l a(;. ~.us (Pieor · 74). Th not chord (Oh), snr- ,...,. - ~-- ... ,: Fig. 74. ..._ --7- . !I .f -- -- Q f: ) Posit ion (\r olfactory (?) Fig 74 -Skull of Amphioxus lancaolatus (after uatre age .-a, . t- tiYes of neural . · · b optic nerves· c fifth (?) pair; d, spinal nern~s ; a, repl ef'en j1 sspa~m ' es ~, f , g , oral sk~leton ·' Ch • notochord; My, spinal chord, or mye on. rounded by a merely membranous sheath, extends very nearl~ to the anterior p01. nted extremi. ty of tl1 e b o dY · The Inyel.o n., or spinal chord (My), occupies the ordi·n ary pos1· t '1 011 a1 J 0 ve· It m a h tl canal formed by upward rroc s s of the memb~·anou~~;a:~ and gives off the spinal nerv s, d d, on each side. nt masses of somewhat den r tissu , e e, seem f am· t] Y to rcpreseth neural pineR. Just au ve th anterior boundary of the mou . ' THE SKULLS OF FISHES. ]91 bnt fnr behind the anterior end of the notochord, the myelon, dilating very slightly, suddenly terminates, and with it, the neural canal. The lateral muscles are divided into segments corresponding with the pairs of spinal nerves, and tho most anterior of these segments is situated just behind tho slightly dilated chamber of tho neural canal, which contains the correspondingly enlarged end of the nervous axis. The latter is all that represents the brain, and the chamber is the sl\.ull. A ciliated sac placed at b, in connection with the upper surface of the brain, has been considered to be the olfaetory organ of this fish, bnt it is possible the sac may simply represent tho pineal body; optic nerves (b) are given off to the rudimentary eyes, and the branches (c) appear to be analogous in function to the fifth pair. But no pituitary body has been recognised, and, what is still 1nore singular, there is no trace of auditory sacs. A cartilaginous ring, provided with tentacular prolongations (f, g), surrounds the mouth, and there is a singular branchial skeleton more like that of an Ascidian than any ordinary vertebrate structure ; but neither of these structures probably have anything to do with the true cranial or facial skeleton. It will be observed that this very remarkable skull, if it can be properly so called, is not strictly comparable to an arrest of develop1nent of a higher verteLrate skull; the notochord extending far beyond the end of the craniu1n, whic:h it never does in any embryonic condition of a higher Vertebrate. B. The Cartilaginous Cranium. Of this there are three forms : in the first (a) there is no mandible ; in the second (b) the mandible is present., and the suspensory apparatus by which it is counected with the skull forms one mass with the latter; in the third (c) the mandil>l is also present, but the suspensory apparatus by which it is connected with the skull is freely moveable. a. The cartilaginous cranium without a mand-ible. This ~ind of c~rtilaginous cranium is found only among the Ma.rs~pobranch~~, or Lampreys and Hags, and a d~scrip- |
