318 FISHES AND FISHING 



in jars of clean sea water of the proper salinity and 

 temperature, occasionally changed. They are, how- 

 ever, very susceptible to sudden changes of salinity or 

 temperature, and care must be taken not suddenly to 

 change them from chilly and saline water of the open 

 sea to the warmer and fresher water of a harbour ; 

 such errors and consequent disappointment can gener- 

 ally be guarded against by the use of a little care. 



Demersal eggs are occasionally laid either loose or 

 in adhering masses on the sea bottom, and left to 

 hatch (as in the case of the herring) ; but are more com- 

 monly laid either in masses or in an even layer, and 

 cared for by the parent (usually the male) during the 

 process of incubation. Many shore fishes, and some 

 normally living in deeper water, construct nests, or 

 guard or carry about their eggs during development. 



Instances of the latter habit may be found in the 

 pipe fishes and sea-horses, in which the male carries 

 the eggs in a pouch or groove in the belly ; and in some 

 catfishes and perciform fishes, in which they are carried 

 in the mouth or throat. 



The line between what may be termed nest-building 

 and the mere guarding the eggs is not very easy to 

 draw. 



In some cases (e.g., sticklebacks and some wrasses) a 

 more or less elaborate nest of seaweed or other material 

 is formed ; in others (Lepadogaster and some blennies 

 and gobies) an empty shell (Fig. 206) or the hollow 

 bulbous root of a seaweed is selected ; again, the fish 

 may overturn a shell, so as to form a hollow beneath it, 

 and then conceal the chamber so formed with a cover- 

 ing of sand, or may be contented with a crevice in a 

 rock or overhanging stone. We have known the hollow 

 at the foot of a recording thermometer lifted daily 

 from the harbour where it lay, tenanted by a small 



