966 



PISCES. 



Fig. 499. 



500. 



Cartilaginous skull and anterior portion of vertebral column of the Sturgeon ( Accipenser sturio). 



a, cartilaginous axis of spine ; b, c, transverse apophyses and ribs ; f,f, g, g', g", g", g"", cartilaginous 

 cranium ; i, k, ocular and nasal cranial cavities ; m, n, pedicle by which the mouth is connected with the 

 cranium; 26, bone supporting the lower jaw ; 18,22, palatine cartilages ; o,p,s, cartilages representing 

 the superior maxilla. 



were only a moiety of the posterior division 

 of the fin of the preceding genus. 



The posterior extremities or ventral fins are 

 attached to a zone similar to that which supports 

 the pectorals (Jig. 500, 14). The pelvic zone is, 

 however, very incomplete, the superior or iliac 

 portion being quite deficient, so that it has no 

 connection with the spine, but is simply im- 

 bedded among the muscles at the posterior part 

 of the abdomen. 



Externally the pelvis supports the first ray 

 of the ventral fin (jig. 500, 15), 

 which is very large, and likewise a 

 long stem (16) composed of nume- 

 rous articulations, to the commence- 

 ment of which the succeeding fin 

 rays are appended. Inferiorly the 

 former is prolonged in the male 

 Rays into a very curious club- 

 shaped apparatus called the "clasp- 

 er," of the nature of which we shall 

 have occasion to speak further on. 



Skeleton of Dermapterygii. In 

 the cyclostomatous Fishes, such as 

 the Lamprey, the skeleton is of still more simple 

 structure than in the plagiostomatous genera. 

 The cranium exhibits through life a soft carti- 

 laginous texture; nevertheless it is not difficult 

 to identify the different pieces of which it con- 

 sists, and to point out their analogies with those 

 of the osseous Fishes. 



The spine consists of a soft cartilaginous 

 stem, which passes along the entire length of 

 the body. It is enclosed in a strong membra- 

 nous investment, from which prolongations are 

 given off that perform the office of spinal apo- 

 physes ; but the only indications of distinct 

 vertebra exist in the presence of slight and 

 almost imperceptible rings of osseous substance 

 distinguishable upon the surface of the cartila- 

 ginous stem above mentioned. 



The Cyclostomata have neither pectoral nor 

 ventral fins, so that in this respect they are the 

 most imperfect of all Fishes: even the vertical 

 fins situated above and beneath the tail are 

 The pectoral fins of the Sharks f/g.496, 497) only supported by a few soft and scarcely sen- 

 and Sturgeons (fig. 498) are formed after the sible fibres representing the fin-rays, 

 same plan as that of the Skate, only upon a The most remarkable part of the skeleton of 

 considerably smaller scale, representing as it the Lampreys (Petromyzon) is the cartilagi- 



Slteleton of Ray. 



1, snout; 2, 3, 4, 5, branchial arches; 6, conso- 

 lidated anterior vertebra ; 1, humeral apparatus ; 

 8, 9, 10, 11, carpus; 12, fin rays ; 13, posterior 

 detached vertebra ; 14, pelvic apparatus ; 15, clasp- 

 er ; 16, tarsus ; 17, fin rays of posterior extremities. 



