ZOOLOGY FOR MEDICAL STUDENTS CHAP, ix 



open by a clear jelly which is secreted into it by the glandular activity of 

 certain ot its lining cells. 



The sense-organs may be divided into two sets (i) those of the 

 ury canal type which are arranged in rows and (2) those of the 

 umpullary type which occur in clumps. 



Of the first type the main row forms the lateral line which stretches 

 hack along the side of the body practically to the tip of the tail. The 

 -move containing the sense-organs has become covered in to form a 

 tu he with openings to the exterior at intervals. In Acanthias the 

 uroove remains throughout life as an open groove for some distance from 

 its posterior end. The lateral-line canal is continued in the head region 

 by the system of canals shown in Fig. 144. The arrangement and nerve 

 supply of the various canals will be gathered from the figure. The 

 anipullary type of sense-organ is found in a number of clumps in the 

 head region, each clump including a very much greater number of 

 organs than are shown in the figure. The pits at the bottom of which 

 these organs are situated become so deep as to form long unbranched 

 tubes opening on the surface of the head by conspicuous pores. 



As regards the function of these organs of the lateral-line system it 

 must be remembered that it is always impossible to understand exactly 

 the nature of a sense which we do not possess ourselves, but there is 

 some evidence in favour of this sense being akin to that of hearing and 

 ot its serving to detect vibrations in the water of a frequency too low- 

 to produce a continuous sound. 



