PISCES. 



971 



straight, parallel, and fleshy, passing from the 

 superior margin of the upper jaw to the infe- 

 rior margin of the lower. 



The third presents a very singular arrange- 

 ment, having its fibres interlaced in a very re- 

 markable manner. These, however, may be 

 divided into three principal masses, two of 

 which are anterior and one posterior.* 



One of these masses is situated in front of 

 and above the upper jaw near the commissure. 

 It is attached to its superior margin, and runs 

 obliquely to join the external edge of the second 

 mass. This latter occupies nearly the same 

 position relative to the lower jaw ; it passes be- 

 hind the other and is conjoined with it exter- 

 nally. The third or posterior mass is derived 

 from the end of the upper jaw, and joins the 

 hinder margin of the second. All these fibres 

 so singularly interlaced co-operate in holding 

 the mouth closely shut when the Skate has 

 seized its prey. 



Lastly, there are two very long muscles de- 

 rived from the spine, which pass between the 

 palate and the cranium to be inserted into the 

 upper jaw. These bring the mass of the mouth 

 forward again after it has been retracted by the 

 broad oblong azygos muscle above described, 

 which passes between the pectoral zone and the 

 inferior maxilla. 



Fig. 507. 



a- 



Head of Lamprey, after Carus, shewing muscles. 



a, b, c, cartilages of the mouth ; d, e, f, external 

 muscles inserted into ditto ; g, h, muscles de- 

 rived fromjthe hyoid apparatus. 



In Sharks the lateral muscles of the body 

 and fins resemble those of the osseous Fishes. 

 Their jaws, however, constructed after the same 

 principle as in the Skate, are equally moveable, 

 and their muscles almost similar; only here, as 

 their mouth is situated much nearer the anterior 

 extremity of the skull, the two great muscles 

 coming from the spine to the upper jaw are 

 wanting. 



Fig. 506. 



a, a, a, cranium 



Myology of Shark ( Squalus glaums). After Carus, 



b, rostrum ; c, olfactory organ; d, eye-ball ; e, muscles of eye ; /, upper-lip ; h, j, 

 teeth ; k, lower surface of skull ; I, m, muscular masses which close the mouth, resembling those of 

 the Skate described above; g, broad muscle passing from upper to lower jaw ; p, depressors of lower 

 jaw, as in the Skate ; q, q, q, entrances to the gill-chambers. 



In the Lampreys (Petromyssonidas ) the oral 

 sucker is moved by slips derived from the ante- 

 rior temination of the great lateral muscle (Jig;. 

 507, f) as well as by a set of very strong fasciculi 

 derived from the hyoid apparatus, which, by 

 retracting the interior of the disc, cause the 

 adhesion of the sucker, and move the different 

 parts of the dental apparatus described in a 

 preceding page, (g, h, m.) The action of 

 these will, however, be better understood by 

 inspecting the figures than by any detailed 

 description. 



Tegumentary system. The essential character 

 of the skin, says Agassiz,f is that it completely 

 envelopes an animal, and thus forms a kind of 

 external skeleton which protects it over its whole 



* Cuvier, Le9ons d'Anatomie Comparee. 

 t Agassiz, Recherches sur les Poissons Fossiles, 

 4to. IQ34. 



surface, as the osseous skeleton protects and 

 supports the internal viscera. In the invertebrate 

 races of animals there are no other solid parts 

 except those which are produced by or con- 

 nected with the tegumentary system, but which 

 nevertheless can by no means be compared 

 with the osseous system of the Vertebrata, 

 which is quite peculiar to the latter, and has 

 no analogy whatever with the solid framework 

 of the inferior classes. 



The skin, moreover, (observes the same 

 illustrious author,) is not exclusively limited 

 to the external surface of the body, but pene- 

 trates into and invests the internal cavities, 

 on the inner surfaces of which it likewise pro- 

 duces solid structures of various kinds to 

 which different offices are assigned, as, for 

 example, the teeth and all the corneous pieces 

 which in many classes are met with upon the 

 lining membrane of the digestive tube. It 



