420 Comparative Morphology of Chordates 



counterpart among Amniota. It consists of minute organs which are 

 sensitive to stimulation by vibrations transmitted through the water. 

 The effective vibrations are of much lower frequency (i.e., "slow" 

 vibrations) than those which ordinarily affect a vertebrate ear. Each 

 organ is a small cluster of specialized epidermal cells which, at their 

 bases, are related to branches of nerves which connect with the pos- 

 terior part of the brain (see p. 164, and Fig. 149). Organs of this kind 

 are arranged at regular intervals along a line extending, on each side 

 of the body, from head to tail at the level of the division between the 

 dorsal and ventral parts of the myomeres, and usually in a canal en- 

 closed within the skin. From this fact the organs derive their name, 

 "lateral-line organs." Rows of the organs may occur elsewhere 

 on the body. On the head is a somewhat complex pattern of rows of 

 these organs (Fig. 150). Similar organs occur also singly and dis- 

 tributed in no particular order. These lateral-line organs may serve to 

 make the fish aware of the proximity of solid objects or of anything 

 which may set up a commotion in the water. 



Classification 



All classifications of fishes rest, in the main, on the condition of the 

 skeleton. At one extreme are sharks whose endoskeleton is entirely 

 cartilaginous; at the other are fishes whose adult skeleton is completely 

 bony. Between the extremes are fishes whose adult skeletons include 

 both cartilage and bone, and in highly varying proportions. Most 

 classifications begin, therefore, by separating fishes into Chon- 

 drichthyes, having (in addition to a possibly persisting notochord) 

 only cartilage in the endoskeleton; and Osteichthyes, having the 

 skeleton more or less ossified. The first of these divisions includes, of 

 modern fishes, only those that are sharklike. The second is vastly 

 larger and highly diversified. 



In the further classification of the Osteichthyes, much stress has 

 been laid on the structure of the pectoral fin. In certain extinct fishes 

 the proximal region of the projecting fin consisted of a thick muscular 

 lobe containing heavy basal skeletal parts. In most modern fishes the 

 muscles which move the fin extend only slightly, if at all, onto the 

 basal region of the projecting fin which, therefore, is thin and bladelike 

 even to its base and is supported by slender rays developed in the 

 skin. Distinction is made, accordingly, between "lobe-fin" fishes, 

 Crossopterygii, and "ray-fin" fishes, Actinopterygii. Other features 

 are consistently associated with the difference in the fins. The Cros- 

 sopterygii are nearly, if not quite, extinct. A South African deep-sea 

 fish (Latimeria), known from only one incomplete specimen, has been 

 adjudged to be a genuine crossopterygian — the only genus known to 



