178 PHYLUM C@LENTERA. 
beneficial external partnership or commensalism. 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 Alge 
(Zoochlorellz) 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 ; ¢.g. small horse-mackerels (Carangidz) 
swimming in shelter of large jelly-fish; a small fish (Amphzprion 
bzcinctus) inside a giant sea-anemone (Crambactzs arabica) which has 
a diameter of a foot ; another fish (/zerasfer) 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 (Diadema saxatile); and 
another (Afpogonichthys strombz) that spends part of its time in the 
mantle cavity of the large sea-snail (Stvombus gigas) of the Bahamas. 
The quaint little hydroid Zar sabellarum lives at the mouth of the 
tubes of the worm Sade//a; another hydroid (Sty/actds mznoz) grows 
all over the skin of a rock-perch (A@énous) from the Indian Ocean ; 
Stylactis vermicola was found on the back of the worm Aphrodite at 
the great depth of 2900 fathoms, 
A Fic. 93A. 
A., a minute portion of the branched excretory system of a Plathelminth, showing 
longitudinal duct (/), with cilia (C.), its branches (/7 and ///), and the terminal 
flame-cells ZV); B., one of the characteristic hollow flame-cells, leading into 
the excretory tubule (1), showing the.long cilia (2), the excretory globules (3), the 
nucleus (4), and pseudopodia-like processes (5) passing among adjacent cells. 
