HIGH SCHOOL ZOOLOGY. 17 



the epidermis ; these present several varieties in shape and function, 

 those next the coriiim being cohimnar, those on the free surface cubical, 

 while the intervening cells are intermediate in shape. Certain peculiar 

 cells stand out from the others ; these are the slime-cells present in all 

 fishes, which provide the skin with its covering of mucus, and the cla- 

 vate cells of unknown function, occurring chiefly in fishes with a soft skin 

 like the Eel and Burbot. A few pigment-cells wander out from the corium 

 into the interspaces of the epidermis. Finally certain special epithelial 

 cells are to be found in bud-like groups on the end of some of the papil- 

 lae. Each of these is provided with a delicate hair-Hke process at its 

 tip, and is connected with a nerve at its basal end ; they thus belong to 

 the class of neuro-epithelial cells and they constitute the simplest form 

 of sense-organ in the fish. It is su^jposed that they are alfected by 

 vibrations in the surrounding medium, and that they are tactile in 

 function. That this is so may be inferred from the use to which the 

 barbels or feelers, which are covered with these organs, are put. Else- 

 where on the surface of the fish, groups of similar cells occur, not pro- 

 jecting freely on the surface, but retracted for protection into minute 

 sacs oj^ening by slits on the surface, or projecting at intervals into the 

 cavities of the sensory canals of the lateral line and head, which com- 

 municate with the surrounding medium by distinct pores. The button- 

 like hillocks of neuro-epithelium are generally protected by a bony 

 scale, and it is the fusion of such scales which gives rise to some of the 

 skin-bones of the head. 



10. Skeletal System. — In the course of the above para- 

 gi-aphs reference has been made to bones developed and situated 

 in the skin. When such bones acquire no connection with the 

 deeper parts they are said to belong to the exoskeleton ; the 

 rough teeth in shark's skin, and the scales of a white-fish, 

 e. g., are of this nature. It is evident from what has been said 

 that the catfish is very poorly provided with exoskeletal stiiic- 

 tures j all of its bones belong to the internal or endoskeleton- 



11. The skeletal system is formed chiefly by tissues of the connective- 

 tissue group, viz., fibrous connective tissue (in the form of ligaments and "" 

 membranes), cartilage and bone. Of these the cartilage plays only a 

 transitory part in the development of the catfish's skeleton, both it and 

 the fibrous connective tissue being in great part converted into bone. 

 Teeth are true exoskeletal structures, although they are only found in 



