202 MARINE BIOLOGICAL LABORATORY. 



general surface depressions the sense-organs sink down 

 forming, as they do so, small pits, at the bottom of which 

 the organ lies. The lips of the pit grow upward and 

 inward, and meeting above arch over the pit forming 

 the beginning of a canal. This arch grows away from 

 the pit in two directions until it meets another canal, 

 with which it fuses, or until its energy is spent, when it 

 comes to a standstill and remains a longer or shorter 

 canal open at both ends, possessing somewhere near the 

 middle of its course the canal sense-organ which gave 

 the first impulse to the development of the canal. The 

 sense-organ lies in a pit which represents what we 

 know in the ear canals as the ampulla. The lateral line 

 sense-organs are then supplied with, or lie in, ampullae. 



The canals being formed in short sections, one to 

 each sense-organ, and fusing as they do to make longer 

 canals, we should expect the compound canal at the 

 point of fusion of its two components would retain a 

 pore connection with the surface from which the canals 

 were formed. Such is the case. 



Again, in the division of a canal organ and its canal, 

 we should expect to find the canal retaining its surface 

 communication by means of a single pore, if the division 

 did not progress to completion, and we find this occurs 

 regularly in the development of canals. The reverse of 

 this process also occurs, in which the pore divides first, 

 while the parent canal may remain undivided. 



We shall see later on that the ear canals fuse and 

 retain their communication with the surface by means 

 of a single pore, and that in one instance a canal organ 

 divides, bringing about an incomplete division of the 

 original canal, so that both canal organs communicate 



