is8 



CORALS 



but may have been formed by the zooids of some other group 

 of animals. It must be admitted, therefore, that, although 

 in most cases the structure of the dried Polyzoan coral is 

 sufficient to determine definitely that it is a Polyzoon, there 

 are some of them which exhibit no characters of the skeleton 

 that can be regarded as conclusive of their zoological 

 affinities. 



The only definite proof that a given coral is a Polyzoon 

 must be obtained bv an observation of the structure of 



the polyps which construct the 

 coral, and a few words must 

 therefore be written to explain 

 the essential features of the 

 anatom}' of this 

 animals. 



\Mien 

 colony of 



group 



of 



an 

 a 



expanded living 

 Polyzoon is ex- 

 amined, the polyps are seen to 

 protrude and to display a crown 

 of long ciliated tentacles ar- 

 ranged to form a funnel, at the 

 base of which is a centrally 

 placed mouth (Fig. 74). By 

 such characters they might 

 be mistaken for Coelenterate 

 polyps, but further examina- 

 tion reveals a second opening 

 just above the crown of ten- 

 tacles, and a bent tube or 

 alimentary canal is seen through 

 the transparent body wall which connects these two openings 

 to the exterior. The presence of this complete alimentary 

 canal is quite sufficient to distinguish the Polyzoan polyps 

 from the polyps of any other group of animals that form 

 corals, but there are many other anatomical characters, 

 which it is not necessary to describe in this book, by which 

 the Polyzoa differ from other coral-forming organisms, and 

 exhibit what is usually regarded as a much higher t3'pe of 

 organisation. 



Fig. 74. — Diagram to illustrate 

 the structure of a Polyzoan polyp. 

 p., the protrusible part of the polyp 

 with a crown of tentacles surround- 

 ing the mouth ; z., the thick outer 

 wall of the non-protrusible part of 

 the polyp or zooecium, which is 

 frequently calcareous ; a., the anus. 

 The bent alimentary canal is seen 

 leading from mouth to anus and 

 attached to the base of the zooecium 

 bv a band of muscles. 



