66 



COMPARATIVE ANATOMY. 



Amongst the Ganoids, the lowest condition is met with in 

 those forms in which the hyaline primordial skull, immovably 

 fixed to the vertebral column, is still retained. These forms are 

 spoken of as Cartilaginous Ganoids. The appearance of 

 definite bones, however, divides them sharply off from the Elas- 

 mobranchs, and proves their skull to be at a much higher stage of 

 development. These bones have the form of richly sculptured 

 plates and shields, and are developed partly from the mucous 

 membrane of the mouth (as for instance the parasphenoid, which 

 lies along the base of the skull), and partly from the outer skin 

 (compare the chapter on the dermal skeleton, p. 32). 



Cop 



FIG. 52. CRANIAL SKELETON OF STURGEON (Acipenscr) AFTER REMOVAL OF THE 



EXOSKELETAL PARTS. 



WS, vertebral column ; SpN, apertures for spinal nerves ; Psp, spinous processes ; 

 Ob, neural arches ; G, notochord ; GK, auditory capsule ; PF, AF, postorbital 

 and antorbital processes ; Orb, orbit ; //, optic foramen ; x, vagus foramen ; 

 Na, nasal cavity ;^R, rostrum ; *, proininent ridge on the basis oranii ; Ps, TV, 

 Ps", parasphenoid; PQ, palatoquadrate ; Qu, quadrate; Md, mandible; De, 

 dentary; Ar, articular; Hm, hyomandibular ; tiy, symplectic ; Ih, interhyal ; 

 hy, hyoid ; / to V, first to fifth branchial arches, with their segments the 

 double pharyngo-branchial (a), the epibranchial (b)', the cento-branchial (c), and 

 the hypobranchial (d) ; Cop, basal elements of the visceral skeleton ; Hi, ribs. 



This dermal skeleton attains to a much more considerable 

 development in a second group of these Fishes the bony 

 Ganoids and gives rise to a strong armour composed of numerous 

 pieces lying on the roof of the skull (Fig. 53). The ossifications are 

 not restricted to the outer surface, but extend into all parts of the 

 skull, as, for instance, the trabecular regions and the lower jaw ; 

 the cartilage thus becomes greatly reduced. 



The branchial skeleton in Ganoids consists of four or five more 

 or less strongly ossified gill-arches, decreasing in size antero- 

 posteriorly. In bony Ganoids the surface which looks towards the 

 throat is beset with teeth. A gill-cover, often supported by several 

 bony pieces, is always present. 



In the form of their skull, the Dipnoi show many points of con- 

 nection with Eiasmobranchs, Ganoids, Teleosteans, and Urodeles. 



