FEEDING OF MARINE ANIMALS 213 



resources. This accumulation of reserve food afifects the 

 vital functions of the animal in such a way as to lead to 

 the appearance of female characteristics, even to the 

 production of eggs ! 



Symbiosis 



In a previous chapter we discussed the case of animals 

 which lived together, such as gall crabs in corals, and hermit 

 crabs in anemones or with anemones growing on their 

 shells. This mutual association has not only to do with 

 defence but with food — thus the anemone in return for the 

 protection it affords the hermit crab is able to seize frag- 

 ments of food broken up by the crab, while in the more 

 extreme case of the pea crab which lives within mussels 

 or similar bivalves or within the large sea squirts, the crab 

 intercepts the food collected on the latticed feeding organs 

 of these animals, scooping into its mouth strings of food 

 mixed with mucus. But there is a much more intimate 

 form of association the object of which is also provision of 

 food for one or both of the parties concerned. We may have 

 two animals or an animal and a plant living in intimate 

 iinion the one with the other to their mutual advantage, 

 and often so dependent one on another that one or both 

 cannot exist if separated from the other. This is the type 

 of association known as symbiosis. 



The commonest type of symbiosis is the partnership 

 between an animal and minute, green, yellow or brown 

 plants — single-celled Algae — which are taken into the 

 tissues of the animal in large numbers. Many of the 

 simplest animals, the protozoa, especially the delicate 

 Radiolarians, always contain these, as do some sponges, 

 corals, and flatworms. The animals are always associated 

 with a particular kind of alga which is called Zoochlorella 

 if green and Zooxanthella if yellow. The advantage to 



