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Show 1H2 ON '!'III~ STRUCrruHE OF 1.'IJ11: RKULL. tion of its characters in the former will suffice to illustrate its nature. Fig. 75 represents a vertical and longitudinal , ection Fig. 75. I• Fi O'. 75.-Yertical and longituuinal section ot' the an terior part of the bouy of a Lamprey "' (Patrom.';zon marinus).-A , the cranium, with its contained brain; a, section or the eJge of the cartil:-~ge marked a in Fig. '16 ; Olf, the entr3n ·c to the olfactory chamurr, which is prolonged into the crecal po11ch, o · Ph, the phnrynx; B1·, the braJJch ial channel, with the inner apertures ot t he branch ial sa" ; 11!, t he cavity of the mouth, with its horny teeth; 2, the C<'trtilage which supports the tongue; 3, the oral r ing. of the antm·ior part of the body of the large 10a Lmnprey (Petro- 1nyzon ma'tinus), and gives a very good notion of tho excessively 1ninnte proportions of th proper skull (A) to tho rest of the body in this animal. A and B (Fig. 7G) are lateral and superior views of the skull with it acce ory cartilage , separated from the soft parts. The notochord (Oh) i , a in An~phiox~~s, exceedingly large, and is surrounded by a merely n1embranous sheath, from which prolongations are given off above to form the sides of the small neural canal. In the walls of this canal, cartilaginous rods, which represent neural arches, are developed, and it dilates more distinctly in the head than in Amphioxus, though the cranial cavity is still very minute. The myelon also undergoes a very distinct enlargement as it enters the cavity, and all the typical divisions of the vertebrate encephalon are recognisable in the brain thus formed. The notochord terminates in a point i1nmediately behind the pituitary body. As it approaches the cranium, a rod of cartilage (l, Fig. 76, A) is· developed on each side in the lateral parts of its sheath, and gives attachment to the branchial skeleton ( m); still more anteriorly two other cartilaginous fila- THE SKULLS OF FISHES. 193 ments (lc) appear, Ride by side, in the under region of the sheath. These pass into the hinder part of the proper cranium, which is a sort of cartil~ginous box, closed in front, and through the greater .part of Its roof, only by membrane, but complete behind, wh~r~ It arches ov~r the myelon, nnd is perforated by the ocmp1tal foramen. The postero-lateral parts of this cranium are dilated to give rise to the two oval auditory capsules ( o), and bo~eath these they are produced into two processes, h and j, whiCh have a comn1on base, but diverge from one another below. The. process h gives attachment to a cartilage which is eonnected w1th that supporting the tongue ( i). The process J, on the other hand, passing down wards and forwards, become~ Fig. 76. Fig. 7G.a-tAh,e thtel skul~ of a .I.amprey viewed from the side; B, from above (after Miiller) ' e 1mo-' omerme plate · b tl lf: t 1 • d, the neural arches of the . ' 1' {e 0 ~c ory capsu e; c, the. auditory capsule; hyornandibular and s ·m .svma . co umn' e, the palato-pterygOid portion ; /, the arch; h, st lo-h al •} r plec~IC.po.rtwn, and q, the qu~drate portion of the sub-ocular of the crani:l cm;ilao~ ~c~ss 2 z3, lmgual ~arltdba.gel ; k, .mferior, l, la teral prolongation <"' , , , , accesso1 y a 1a cartilages. contiuuo~s at g, with another bar of cartilage e, which is connected With the antero-lateral part of the skull beneath the 0 |