PHYLUM CCELENTERA. 



beneficial external partnership or commensaltsm. In some 

 other animals it may degenerate into parasitism (see Fig. 



93)- 



Another kind of partnership is illustrated by many sea- 

 anemones and Alcyonarians. Minute unicellular Algae 

 (Zoochlorellae) live within the cells of the animals in close 

 physiological partnership with them (symbiosis). 



A spatial partnership in which one animal finds habitual shelter within 

 or near another is not infrequent ; e.g. small horse-mackerels (Carangidae) 

 swimming in shelter of large jelly-fish ; a small fish (Amphiprion 

 bicinctus} inside a giant sea-anemone (Crambactis arabicd] which has 

 a diameter of a foot ; another fish (Ficrasfer) that goes in and out of 

 the hind -gut of Holothurians ; another that lives among the very long 

 hair-like spines of the Red Sea rock-urchin (Diadetna saxatile] ; and 

 another (Apogonichthys strombi} that spends part of its time in the 

 mantle cavity of the large sea-snail (Strombus gigas] of the Bahamas. 



The quaint little hydroid Lar sabellarum lives at the mouth of the 

 tubes of the worm Sabella ; another hydroid (Stylactis minoi] grows 

 all over the skin of a rock-perch (Minous) from the Indian Ocean ; 

 Stylactis vermicola was found on the back oi the worm Aphrodite at 

 the great depth of 2900 fathoms. 



FIG. 93A. 



, a minute portion of the branched excretory system of a Plathelminth, showing 

 longitudinal duct (/), with cilia (C), its branches (//and ///), and the terminal 

 flame-cells (IV )\ -# one of the characteristic hollow flame-cells, leading into 

 the excretory tubule (i), showing the long cilia (2), the excretory globules (3), the 

 nucleus (4), and pseudopodra-like processes (5) passing among adjacent cells. 



