CONDITIONS OF LIFE 



249 



habitually shelters within the mantle cavity of a large Sea Snail 

 or Conch, leaving its host at intervals to search for food. 

 Other related species are equally at home within the cavities 

 of sponges {cf. p. 235). A little Goby (Gobius) has been found 

 living inside the gill-chamber of a Shad (Alosa), lying curled 

 up quite comfortably beneath the operculum of its host, and 

 a similar association between small Eels and Devil-fishes 

 {Mobulidae) has also been described. Small Eels are said 

 occasionally to find their way into the body cavities of larger 

 fishes, generally with fatal results as far as the Eel is concerned. 

 Yet another type of association is known as symbiosis, where 

 two animals live together in such a fashion that both receive 



Fig. 96. 

 Candiru {Vandellia cirrhosa), X i^ ; v. Lower view of head, X 3. 



some sort of benefit from the other. In the seas of India, for 

 example, there is a little Scorpion-fish {Minous), and practically 

 all the individuals of this species are more or less covered with 

 a thick fleshy colony of hydroid polyps (plant-like animals 

 sometimes spoken of as Zoophytes). Many examples of this 

 fish have been captured, but very few indeed are without the 

 polyps, and the hydroid itself has never been found living apart 

 from the fish. The benefits derived from this association are 

 mutual, the encrusting polyp growth conceahng the fish from 

 watchful enemies by giving it the appearance of a weed-covered 

 stone, while the hydroid colony is carried about constantly by 

 the fish and thus obtains fresh feeding-grounds without eflfort. 



Finally, there is the type of association known as parasitism, 

 where one animal dwells on or inside another, nourishing itself 



