IV. VERTEBRATA: PISCES. 495 



the corium, which, under control of the nervous system, can alter their form and 

 extent and thus produce color changes in the fish, thus adapting them to their 

 surroundings. Destruction of the eyes results in loss of power to change color. 

 Many deep-sea fishes possess phosphorescent organs (p. 121, fig. 87), possibly 

 of use in the assemblage of the sexes. 



The axial skeleton shows many conditions unknown outside the class, 

 and varies in character from group to group, the most important differ- 

 ences consisting in its cartilaginous or bony character. The vertebrae are 

 nearly always amphiccelous, the notochord persisting in the cavities be- 

 tween the successive centra (fig. 513). Neural and haemal arches, com- 

 pleted by the unpaired spinous processes occur. The neural arches 

 extend throughout the column; the haemal are complete only in the tail; 

 in the trunk the hasmal spines are absent and the ha'mal processes, 

 divided into basal processes and ribs, surround the viscera. When 

 ossification is lacking or is incomplete, two pairs of arches may occur in 

 each segement, the anterior being the stronger and alone persisting in 

 fishes with ossified vertebrae; the second is much smaller, so that its 

 elements are not called arches, but inter calaria (figs. 512, 546). No fish 

 has a sternum. 



The great number of visceral arches, and their independence from the 

 cranium, are characteristic of fishes. After removal of these the cranium 

 in all cartilaginous fishes is very simple (fig. 546), but in the teleosts, with 

 the appearance of ossification, it becomes very complicated, since the bones 

 are numerous. There are also great differences between the different 

 families of fishes, some having bones which are lacking in others (figs. 



5 l6 > 547)- 



The large membrane bones of the cranial roof (parietals, p, frontah, f, and 

 nasals, na) and the large ventral parasplienoid (ps) are especially constant. The 

 Tomer (vo), in front of the parasphenoid, is unpaired, while in all other verte- 

 brates it is paired. Most constant of the cartilage bones are the ethmoids (the 

 paired cctetlnnoids, ec, and the sometimes paired mesethmoid) , and the four 

 occipitals. On the other hand the otic and optic regions vary considerably; the 

 otic region usually has five bones (fig. 547) : ptcrotic, pto, often called squamosal; 

 splieiintic, spho, frequently called postfrontal; epiotic, cpo; prootic, pro; and 

 opisthotic, oo, the last sometimes lacking. In the region of the eye the cartilagi- 

 nous sphenoids are rarely well developed, the large parasphenoid taking their 

 place. The same is true of the all- and orbitosphenoids, these sometimes forming 

 an interorbital septum (fig. 516) or leaving a more or less wide interorbital 

 fenestra (fig. 547). 



The character of the visceral skeleton is related to the aquatic life. All 

 fishes have numerous gill arches (five to seven, mostly five), which, since 

 their function gill supporting is similar, are similar in structure. So 

 far as they are not degenerate they consist each of four parts and are 

 connected below by unpaired copula-, these often being fused. The 





