150 MARINE PRODUCTS OF COMMERCE 



were at least partly ossified (bony) and these hard parts became fossilized more 

 readily. The true fishes, in addition to the vertebral column, have hard skulls; 

 unlike sharks the upper jaw is not attached directly to the skull, but is joined with 

 intermediate bones. There are so-called membrane bones, such as the opercle and 

 subopercle, about the head, and at least one pair of limbs is more or less de- 

 veloped. Most of the true fishes have an air bladder, and a single opening leads 

 to the gills or branchial chamber. The vast majority of living fishes and a con- 

 siderable portion of the extinct ones, as already indicated, belong to this class. 



The direct ancestors of some of the living Teleostomi appear first in the 

 Devonian formation, sometimes estimated as early as 400,000,000 B.C. These 

 primitive forms include the ganoid group in which the skeleton is not fully ossi- 

 fied. To this group belong the familiar living fresh-water gars, paddlefish, stur- 

 geons, and the somewhat despised bowfin or fresh-water dogfish of our waters. 

 The fringfins of North African rivers and the lungfish of Australia, Africa, and 

 South America also belong to this primitive group. The ganoids were once much 

 more numerous. What caused their decline has not been determined. Certainly, 

 the living forms seem well adapted for carrying on the processes of life under 

 present conditions. 



The fossils of teleosts, or "true bones", date back only to the Jurassic period, 

 or perhaps somewhat less than 200,000,000 years. The teleosts, the "youngest" 

 of the major groups of fish, have been regarded as a branch of the early ganoids; 

 now, however, the teleosts greatly outnumber all the other groups of living fishes 

 combined. In general they have a thoroughly ossified, or calcified, skeleton: The 

 backbone seldom enters the upper lobe of the caudal fin, as in the ganoids; the 

 air bladder is very rarely lunglike and generally does not assist in respiration; and 

 the dermal bones of the head, which are at the surface and enamel-coated in the 

 ancestral ganoids, are deep-seated and often covered with skin and scales. 



Such, then, in brief, is the origin and the history of fishes, living and fossil. 

 Some of the recent forms have diverged widely from their ancestors, while others 

 have remained fairly stationary. A discussion of the evolutionary processes in- 

 volved in the changes cannot be given in this short treatise. It can only be said 

 here that every ichthyologist is keenly aware that evolutionary forces are still 

 active. 



Anatomy 



Form or Shape of Body. Fish vary widely in shape or form. The common At- 

 lantic herring, Clupea harengus, may be chosen as an example of the most usual. 

 In this species the body is elongate and somewhat compressed, all lines being 

 curved. This fish serves well as the type of the usual fish form, and with it some 

 unusual forms will later be compared. Uniformity in shape, however, does not 

 always exist within a family, and certainly not within the herring family, from 

 which the fish with the usual form has been selected. There is, for example, 

 the so-called round herring {Etrumeus sadina) which is nearly round, or almost 

 cigar-shaped. Other species, although elongate, are excessively compressed 

 (Odontognathus) . Still another member of the herring family (Pristigaster martii), 

 which also is strongly compressed, has a chest and abdomen so greatly expanded 

 that the ventral outline forms an arc. 



Many common food fishes are shaped more or less like the Atlantic herring, a 



