40 LIFE IN THE SEA [CH. 



This little animal inhabits sea- weeds of various colours 

 and it nearly always varies in hue so as to match the 

 colour of the alga on which it rests. But in the dark, 

 after the sun has set, all varieties of Hippolyte, 

 whether brown or green or red, become a beautiful 

 transparent blue ; and this nocturnal blue is regularly 

 assumed in the conditions of the laboratory. This 

 periodicity of habit in Convoluta and Hippolyte is 

 fully described in Keeble's volume on 'Plant- Animals' 

 in this series of Manuals. 



The annual changes in the temperature and 

 salinity of the sea are of much more importance in 

 their influence on marine life than are the changes of 

 tide. If we consider in the meantime only the higher 

 marine animals, the fishes, molluscs, and Crustacea, 

 we can easily make out three phases in their vital 

 condition throughout the year. A phase of partially 

 suspended animation coincides with the time when 

 the sea temperature is about its minimum ; then 

 there is the reproductive phase which is assumed in 

 the spring, when the temperature is rising rapidly 

 from its lowest point; and then follows the growth 

 phase which coincides with the rise towards the 

 maximum and the period immediately following. 

 The dark and cold winter months, December to 

 February, are those during which the activities of 

 marine animals are at their lowest ebb. Many of 

 the fishes and crustaceans such as the plaice, flounder 



