CLASS POLYZOA. 



49 



If we take a single cell of this compound growth, and 

 examine the animal contained within it, we find that it 

 has the following struc- 

 ture : The animal con- 

 sists of a little mem- 

 branous bag (fig. 20, 

 en), which is closely- 

 applied to the horny 

 wall of the cell (ec), 

 and which is filled in- 

 ternally with a fluid, 

 in which float the in- 

 ternal organs. From 

 the mouth of the cell, 

 the inner membranous 

 sac can be partially 

 thrust out ; and here 

 are situated the mouth 

 and vent, close beside 

 one another. The 

 mouth (0) is surrounded 

 by a circle of beautiful 

 flexible processes (f), 

 which are termed "ten- 



tacles," and the sides 

 of which are fringed 

 v'ith innumerable vi- 

 brating hair-like fila- 

 ments. These hair-like 



Fig. 20. — Diagram of the animal contained in 

 one of the cells of Fiustra foUacea. ec 

 Outer horny wall of the cell ; e?i Inner 

 membranous wall enclosing the internal 

 organs ; Mouth, surrounded by tentacles 

 (0 ; 9 Gullet ; s Stomach ; i Intestine ; 

 V Vent ; 11 Nervous system ; m Muscle 

 by which the animal can pull itself into 

 its cell. 



processes are m con- 

 stant movement, vibrating to and fro, and by their means 

 currents are set up in the surrounding water, and par- 

 ticles of food are thus conducted to the mouth, When 

 quiescent, or irritated, the animal can draw in the front 

 portion of its body, with the tentacles, into the shelter 

 of the horny cell. 



The mouth conducts by a gullet (g) into a stomach 

 (s) ; and this in turn opens into an intestine (^), which 

 finally terminates in a distinct vent (v), placed on one 



D 



