34 LIVERPOOL MARINE BIOLOGY COMMITTEE REPORT. 



two hundred fathoms vegetable life almost completely dis- 

 appears. 



This extraordinary variability in the external conditions of 

 life is undoubtedly the great cause of the extreme diversity 

 shown by the fauna exposed to it, and is perhaps best 

 realised by comparing it for a moment with the environment 

 of the deep sea fauna. Here we have animals living for 

 the most part in absolute darkness, in water that is never 

 more than a few degrees above the freezing point, on a sea 

 bottom that is perfectly uniform over areas of very great 

 extent, and largely dependent for food on other animals 

 which, having died at or near the surface, have slowly sunk 

 to the depths below. 



Turning now to the special characters of the shallow water 

 animals themselves, perhaps the most general feature they 

 present is the power of fixing themselves in, or to, the sea 

 bottom in order to resist the tides and storms, which would 

 otherwise destroy them, or carry them out to sea. 



This fixation is brought about in very various ways. It 

 may be either a temporary one, such as is effected by the 

 muscular loot of a limpet or chiton, or by the base of a 

 sea-anemone ; or a permanent one, as in barnacles, oysters, 

 ascidians, etc., where the animal is immoveably attached 

 to rock or other firm support. 



In other cases, the requisite anchorage is obtained by 

 burrowing in the sand or mud of the shore or sea bottom. 

 These burrows may be simple holes, such as those made by 

 many bivalves, or may be lined by tubes secreted by the 

 animal. Similar tubes may be formed for protection by 

 animals that do not burrow, and are then attached to foreign 

 bodies, either by their basal ends, as in many hydroid 

 zoophytes and worms, or along their entire length, as in 

 Serpula. 



It is a very noteworthy fact that, in all cases in which the 



