332 



COELENTERATA ANTHOZOA 



CHAP. 



anthococliae can be withdrawn below the general surface of the 

 coenenchym, so that the position of the zooids in the colony is 

 indicated only by star-like holes, or simple key-hole slits in the 

 superficial coenenchym. Such colonies are said to be completely 

 retractile (Fig. 147). 



It is often very difficult to determine whether a particular 

 species is or is not completely retractile, unless observations can 

 be made upon the living colony ; and there are many instances 

 of confusion in the work of systematists due to a species being 

 described as partially retractile in one instance, and completely 

 retractile in another. The complete retraction of the anthocodiae 



Fig. 147. Diagram of a vertical section of a portion of a lobe of Alcyonium to show the 

 mode of retraction of the anthocodiae. 1, Anthocodia of a zooid fully expanded ; 

 2, in the first stage of retraction ; 3, in the second stage ; 4, in the third stage, 

 leaving a shallow prominence or " verruca " on the surface ; 5, final stage, the 

 verruca flattened down and the coenenchym closed, can, Canal system ; d.m.f, 

 dorsal mesenteric filament of a zooid ; si, siphonoglyph. 



may be effected very slowly, and after continuous irritation only. 

 If the colony is killed too quickly, the anthocodiae remain in a 

 state of partial retraction. An example of this may be found 

 in the common British Alcyonium digitatum. Specimens of this 

 species which are put into a bucket of sea water and allowed to 

 roll, about with the movements of a small boat in a rough sea, 

 undergo complete retraction ; but if the same specimens be 

 allowed to expand in the aquarium, and then plunged into spirit, 

 or allowed to dry in the sun, they will die in a condition of 

 partial retraction. 



The phenomenon of dimorphism occurs in some Alcyonaria. 

 A certain number of the zooids of a colony are arrested in their 

 development, and are known as the " siphonozooids." They may 

 be distinguished from the fully formed zooids, which, in these 





