xii INTRODUCTION. 



bones" imbedded in the flesh of the fish; the points of the interspinous bones are attached 

 to the spinous processes of the vertebrae, each by a ligament, their heads are firmly united 

 to the bases of the fin rays ; this arrangement between the median fins, the interspinous bones, 

 and the vertebral spinous processes, may easily be seen by anyone, who will take the trouble 

 to look out for it, when he is eating a fried Perch for breakfast or dinner. 



The caudal fin or tail is the chief organ of motion in a fish; by a rapid succession of 

 oblique lateral impulses the fish is enabled to dart through the water at a very quick pace. 

 There are two distinct types of tail in fishes, one being much more common than the other: 

 in one type this organ consists of two equal or nearly equal lobes, which are attached to 

 the spinous processes of the posterior part of the vertebral column ; as is the case in all the 

 British fresh-water species of fish, with the exception of the Sturgeon. This symmetrical tail 

 is said to be homoccrcal ; from ofio'^, "the same," and Kepico<i, "the tail." The other type of tail, 

 which occurs in the Sturgeon, Sharks, Dog-fishes, &c., exhibits an unsymmetrical form, for 

 the lobes are unequal, while the vertebral column runs right into the upper portion of the 

 tail; this structural arrangement is designated hy the terva. keterocercal ; from erepo^, "different," 

 and KepKo<;, "the tail" (See this form of tail in the plate of the Sturgeon.) 



Skeleton of Perch. 

 The interspinous bones are seen between the vertebral column and the dorsal fins. 



The skeleton is either osseous or cartilaginous. Most of the British fresh-water species 

 have osseous skeletons, and belong to the sub-class Teleostei ; others, as the Lampreys, have 

 cartilaginous skeletons throughout life ; in some fishes, as in the Sturgeon, the skeleton is 

 partly cartilaginous; in the Lancelet, a salt-water fish of the lowest type, there is no true 

 skeleton, the vertebral column being merely a gelatinous notochord. The vertebra of a bony 

 fish is cup-shaped at both ends, the margins being attached by ligaments. In the cavities 

 formed by the junction of the vertebrae there is a quantity of jelly-like substance, imparting 

 to the spine great flexibility : this lubricating gelatinous substance passes from one intervertebral 

 cavity into another through minute pores which perforate their centres. The spinal column 

 consists of two parts, an abdominal, and a caudal. The spinal cord passes through the upper 

 or neural arch of the vertebrae for the whole length of the body of the fish : the abdominal 

 vertebrae possess also a superior spinous process, and two transverse processes for the attachment 

 of the ribs. In the caudal vertebrae there are no transverse processes, but this portion of 

 the column possesses an inferior or hccvial arch, as well as inferior spinous processes. 



The bones of a fish's skull are numerous, and the structure of the head is very complex; 

 the cranium of osseous fishes, when its parts are complete, is made up of no fewer than 

 twenty-six bones. In the median portion of the cranium there is a cavity which contains the 

 brain and the auditory apparatus. The other parts necessary to be noticed for the discrimination 

 of species are: — (i) The gill-cover, consisting of the operculum, praeoperculum, suboperculum, 

 and interoperculum. (2) The upper portion of the jaw, called the maxillary. (3) The prae- 



