THE PERCH 55 



Count the trunk and caudal vertebrae. Note how the former 

 grade into the latter. The spinal column ends posteriorly with a 

 fan-shaped bone called the urostyle, which supports the caudal fin. 



Exercise 27. Draw a posterior and also a side view of one of the 

 trunk vertebrae ; draw the same views of a caudal vertebra. 



Make a sagittal section of a vertebra. Examine the concavities 

 with the aid of a lens and note the lines of growth. The centrum 

 increases in size by the addition of successive layers of bone to the 

 outside, each of which is a little larger than the previous one and 

 hence overlaps it ; the biconcave shape is the result of this method 

 of growth. 



Exercise 28. Draw the section on a large scale, showing these features. 



The Skull. The skull is composed of two portions : the cranium, 

 which protects the brain and the special sense organs; and the 

 visceral skeleton, which surrounds the anterior end of the alimen- 

 tary canal and consists of the framework of the jaws and gill 

 arches. These two portions of the skull have had a very different 

 origin and are different in character and appearance. The cranium, 

 forms the entire dorsal half of the skull, and its constituent bones 

 and cartilages are for the most part immovably knit together, so 

 that they form a single compact whole. The visceral skeleton 

 forms the ventral half of the skull, and its bones and cartilages are 

 mostly loosely joined with one another and with the cranium. 



The Visceral Skeleton. Without at first removing any of its 

 parts we shall now study the skeleton of the jaws and the gill 

 arches. The visceral skeleton is made up of a series of seven 

 arches called the visceral arches, which surround the mouth and 

 pharynx. In the embryonic teleost, as in the adult elasmobranch, 

 these arches are composed entirely of cartilage. This cartilage 

 is for the most part replaced by bone as the young teleost becomes 

 older, so that the visceral arches become bony structures. In con- 

 nection with these there appear also other bones, which do not 

 develop in the cartilage but in the skin and in connective-tissue 

 membranes covering the skull and which are called membrane 

 bones. Thus, as to the method of origin, two kinds of bone are 

 present in the skull,— cartilage bones and membrane bones. 



