2i6 PHYLUM CHORD ATA 



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

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

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

 method of growth. 



Exercise 22. 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 

 alimentary canal and consists of the framework of the jaws and 

 gill arches. These two portions of the skull have had a very dif- 

 ferent origin and are different in character and appearance. The 

 cranium forms the entire dorsal half of the skull, and its con- 

 stituent 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. Each visceral arch is a paired structure consisting of a 

 right and a left side which meet in the midventral plane. Each 

 side is also made up of several segments which fall approximately 

 into a dorsal and a ventral half. 



The visceral arches may be divided into two groups: an 

 anterior group, consisting of the first two arches, which form the 

 skeleton of the jaws, the tongue, and the gill cover ; and a pos- 

 terior group, consisting of the last five arches, which support the 

 gills and are called the gill or branchial arches. Identify these 

 two groups. 



We shall first study the anterior group. The first visceral arch 

 is the mandibular arch. The dorsal and ventral halves of it, to- 



* For a study of the skull a large fish like a cod is a much more convenient ob- 

 ject than a perch. 



