438 



VERTEBRATE LIFE AND ORGANIZATION 



Finray- 

 Myomeres 



Lateral 

 line 



Fin spine 



Kidney 



Swim bladder 



ctres 



Haemal- 

 arches 



Urogenital tract 

 Wolffian duct 



Spleen^ 

 Fat body - 

 Intestine 

 Pelvic fms 



Heart 

 Pericardia] cavity 



Liver 

 _, ., V Gall bladder and 



Pleuropcrjtoncal tile duct 



cavitv 



Pyloric caeca 



Figure 22.10. The visceral organs of the perch. 



An obvious way in which they differ from the cartilaginous fishes is in 

 having an ossified internal skeleton and in retaining more of the primi- 

 tive bony scales and plates. The internal skeleton consists of cartilage 

 replacement bone that has developed embryologically in association 

 with cartilaginous rudiments, which it gradually replaces. The bone in 

 the scales and plates, although histologically similar to the preceding 

 type, is dermal bone. It develops in the dermis of the skin and is not 

 preceded by cartilage. The deeper portions of the dermal plates in the 

 head and shoulder region become intimately associated with the internal 

 skeleton; thus the skull and pectoral girdle of these fishes, and of the 

 terrestrial vertebrates which have descended from them, contain both 

 types of bone. 



The jaws are formed partly by the ossified mandibular arch of the 

 visceral skeleton (cartilage replacement bone), and partly by dermal bone 

 encasing this arch. The hyoid arch lies close behind the mandibular, 

 and its hyomandibular may take part in the suspension of the jaws. 

 There is no room for a postmandibular gill slit, and even the spiracle, 

 when present, does not open to the surface. Typical branchial arches 

 lie behind the hyoid, but the gill region is covered by a flap containing 

 dermal bone (the operculum), so the gill slits have a common opening 

 just anterior to the pectoral fin. 



The soft parts of most bony fishes, the perch for example (Fig. 

 22.10), show a peculiar mixture of primitive and highly specialized 

 characters. Most need not concern us, but one of great interest is the 

 swim bladder. In the perch, this is a median, membranous sac lying 

 in the dorsal portion of the coelom. The bladder is filled with gases 

 similar to those dissolved m the water (nitrogen, oxygen, carbon diox- 

 ide). It functions primarily as a hydrostatic organ, adjusting the specific 

 gravity of the body so that the fish can stay at various depths with a 

 minimum of effort. Gases may be secreted into the bladder or absorbed 

 from it, as conditions warrant, through specialized capillary networks 



