80 STRUCTURE OF FISHES. 



middle portion, the form of which is that of a pyramid with three 

 sides, with its summit directed forward, presents posteriorly the 

 cranial box, or skull, (Fig. 42, a.) in which is lodged the appa- 

 ratus of hearing as well as the brain. Its middle part is hollowed 

 out to form the orbits, (b.) and in front, we find pits which be- 



SKELETON OF A PERCH. 



long to the olfactory apparatus, (c.) There are bones which 

 correspond to those of the heads of mammals, but most of these 

 bones, in fishes, are composed of several pieces, which never 

 run together into one, as happens at an early age in the mam- 

 malia and birds. 



9. At the anterior extremity of this portion of the head, we 

 find the upper jaw, which is sometimes immoveable, though it 

 generally preserves great mobility : on each side there is an 

 inter-maxillary bone, (d.) placed near the middle line, and a 

 lower jaw bone which extends laterally, and moves upon the 

 first. Besides these parts, we find a very considerable apparatus 

 designed to afford attachment to the branchiae, or to protect them, 

 composed in part of the hyoid bone, which is covered on each 

 side by a sort of cover or door, called operculum-, (/.) or gill-cover. 



10. The vertebral column, (r.) which is continuous with the 

 head, is divided into two distinct portions, one dorsal, and the 

 other caudal. The body of the vertebrae has a peculiar form ; it 

 is hollowed before and behind by a conical cavity ; these two 

 hollow spaces are sometimes joined so as to form a hole, and the 

 double conical cavity arising from the junction of two neighbour- 

 ing vertebrae, is filled by a soft substance. The ring, destined to 

 form a passage for the spinal marrow, is surmounted by a spinous 

 process, and on each side there is a more or less distinct trans- 

 verse process, which, over the cavity of the abdomen, extends 

 outwardly, and articulates with the corresponding rib, but in the 



9. Is the upper jaw moveable or not? What is the operculum ? 

 10. What are the characters of the vertcbrsB in fishes? 



