244 FISHES 



becomes much narrowed or entirely closed. In Elasmobranchs 

 this tube has a small opening on the surface of the skull through- 

 out life, but in other fishes it is closed in the adult. There is 

 no middle or external ear in fishes, for the middle ear or 

 tympanum is represented by the spiracle and no external ear is 

 formed. The auditory sac contains calcareous matter secreted 

 by its wall and known as otoliths ; the function of these is to 

 convert the sound vibrations into movements of solid matter 

 which have more effect in stimulating the terminations of the 

 auditory nerve. In the dog-fish the calcareous particles are 

 small and separate, in the bony fish they are consolidated into 

 large concretions which increase in size during growth by the 

 addition of layers to the exterior ; the deposits of successive 

 years can often be distinguished, and in this way the age of the 

 fish when it was killed can be ascertained. 



The skin may be supposed to possess the sense of touch all 

 over its surface as a result of the presence of terminations of 

 sensory nerves, without any special organs, but there are also 

 definite sense-organs in the epidermis composed of groups of 

 sensory cells united into spindle-shaped bodies called end-buds ; 

 these are irregularly distributed over the body, on the fins and 

 lips, and also in the epithelium of the mouth and pharynx. In 

 this latter position they form the organs of the sense of taste. 

 But the most peculiar and special sense-organs of fishes are the 

 sense-organs of the lateral line. These are somewhat similar 

 in structure to those already mentioned, but they are usually 

 sunk beneath the surface and contained in tubes, grooves, or 

 closed canals ; in all cases they are developed on the surface 

 of the epidermis and afterwards enclosed in invaginations. The 

 most constant part of this system is the lateral line, which is 

 a tube in the skin extending from the auditory region along each 

 side of the body to the root of the tail. In the dog-fish, as in 

 most Teleosts, this tube is closed and communicates with the 

 surface of the epidermis by pores at regular intervals ; it has 

 thus a structure very similar to that of a tube-railway, the pores 

 corresponding to the stations at which the tubular tunnel com- 

 municates by vertical shafts with the surface of the ground. 

 The sense-organs are in the epithelium lining the tube, on its 

 inner surface, alternating with the pores ; in Teleosts the tube 

 perforates a series of modified scales which make a conspicuous 



