PISCES 101 



tral and dorsal positions and may be greatly modified or 

 wanting in places. They are supported by bony or car- 

 tilaginous rays. 



7. Exoskeleton. — The integumentary units are scales of various 



sorts, and are found in every degree of exaggeration or degen- 

 ation. In general they may be said to be placoid, ganoid, cy- 

 cloid, or ctenoid in form and structure. Sometimes the scales 

 fuse together to form a coherent armor. 



8. Endoskeleton. — The endoskeleton consists of appendicular 



and axial elements. The appendicular skeleton is mentioned 

 under "Fins." The axial skeleton consists of a bony or 

 cartilaginous cranium and a vertebral column more or less 

 completely organized, consisting of bone or cartilage. A 

 notochord persists in some of the more primitive orders. 



9. Lateral line of sensory organs. — These strictly aquatic sense 



organs are found arranged in linear tracts along the sides and 

 over the head. Their exact function is not definitely known, 

 but they are probably associated with the perception of vi- 

 brations in the water. 



10. Olfactory organs. — These are paired and end blindly, not com- 



municating with the pharnyx as in terrestrial animals and 

 in the hag-fishes. 



11. Auditory Organs. — These are entirely internal and have no 



communication with the exterior. They serve largely the 

 function of equilibration, though they also perceive vibra- 

 tions. 



12. Eyes. — The eyes are much like those of other vertebrates, ex- 



cept that they are lidless, and have spherical lenses for short- 

 range vision in the water. 



13. Brain. — The brain is small and shows ho flexures. It neverthe- 



less has all of the characteristic features of the vertebrate 

 brain, though there are but ten cranial nerves. 



14. Spinal Cord: Uke that in other vertebrates. 



15. Alimentary Tract. — The pharynx is extensive and perforated 



by branchial clefts. The oesophagus is simple and short, for 

 in the fish there is no neck. The stomach is little differenti- 

 ated. The intestine is short and, in order to increase the di- 

 gestive surface, a spiral valve is often present, especially in 

 all of the more primitive orders. 



