. The Cambridge natural history. Zoology. 332 COELENTERATA ANTHOZOA anthocodiae can be withdrawn below the general surface of the coenenchym, so that the position of the zooids in the colony is indicated only by star-like holes, or simple key-hole slits in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 bein
. The Cambridge natural history. Zoology. 332 COELENTERATA ANTHOZOA anthocodiae can be withdrawn below the general surface of the coenenchym, so that the position of the zooids in the colony is indicated only by star-like holes, or simple key-hole slits in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 sectiou 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 tlie surface ; 5, final stage, the verruca flattened down and the coenenchym closed, can^ Canal system ; ^ 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 " ; They may be di
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