XVII 



EXTERNAL FORM 



4/9 



External Form. The Polyzoa 



may be roughly divided into (1) 

 encrusting forms, usually calcare- 

 ous, but sometimes soft; and (2) 

 erect forms, which are either rigid 

 or flexible. This flexibility can 

 coexist with a highly calcified 

 ectocyst, as in Crisia (Fig. 237), 

 CeUaria, and others in which the 

 branches are interrupted at inter- 

 vals by chitinous joints. The 

 coral-like forms may assume the WliL " 



most exquisite shapes, pre-eminent 

 among which are the lovely net- 

 like colonies of Retepora. Polyzoa ^ 

 of this type are seldom found be- 

 tween tide - marks, where their 

 brittle branches would be liable to 

 be snapped off by the waves. The 

 erect species which occur in such 

 positions are flexible, although 

 flexible species are by no means 

 restricted to the zone between 

 tide-marks. 



Although the form of the 

 colony is very different in different 

 Polyzoa, a pocket-lens will usually 

 show whether a given specimen 

 belongs to the group or not. The 

 surface is nearly always more or 

 less distinctly composed of zooecia, 

 or at least shows their orifices. 

 The entire colony may be built up 

 of these zooecia ; and this is by 

 far the commonest arrangement, 

 both in encrusting and in erect 



forms. In certain genera, how- Fig. 237. Crisia ramosa Harmer, Ply- 

 , i i mouth. A, End of a branch, x 1 ; B, 



ever, and particularly in some another branch, x 20, showing the 



CtenOStomeS (Fig. 238), and in chitinous joints, the tubular zooecia 



v ' . characteristic of Cyclostomata, and 



most of the EntoprOCta, the llldl- the pear-shaped ovicell with a funnel- 



shaped orifice at its upper end. 



