Outlines of zoology . the life history,so the predominance ofactivity is exhibited inthe permanent medus-oids, Geryonia, wherethe sedentary hydroidstage is omitted, and theembryo becomes at oncemedusoid. Finally, themedusoids themselvesmay become colonial,and we have active float-ing colonies, like thoseof the Portuguese man-of-war, which show, on adifferent plane, as muchpolymorphism as Hydrac-tinia. The same general con-clusions apply to the jelly-fish and jelly-fish present astrong resemblance tothe medusoids, but are distinguished from them by theirusually greater siz
Outlines of zoology . the life history,so the predominance ofactivity is exhibited inthe permanent medus-oids, Geryonia, wherethe sedentary hydroidstage is omitted, and theembryo becomes at oncemedusoid. Finally, themedusoids themselvesmay become colonial,and we have active float-ing colonies, like thoseof the Portuguese man-of-war, which show, on adifferent plane, as muchpolymorphism as Hydrac-tinia. The same general con-clusions apply to the jelly-fish and jelly-fish present astrong resemblance tothe medusoids, but are distinguished from them by theirusually greater size, as well as by greater complexity andseveral anatomical differences. It is in accordance withthis increased complexity that the alternation of active andpassive forms, though as real, is less obvious. But evenhere we find one type {Pelagia) always locomotor, another{Aurelia) whose early life is sedentary, and others (Lu-cernarians) which in their adult life are predommantlypassive, and attach themselves by a Fig. 67.—Diagram of a typicalHydrozoon polyp.^-After Allman. EC, Ectoderm; EN., endoderm; C, thecavity of the gut (coelenteron); G., a re-productive bud ; T.,2. tentacle ; H., hypo-btome or oral cone ; M., mouth. 142 PHYLUM CCELENTERA. The sea-anemones and their numerous allies may beregarded as bearing a relation to the jelly-fish, somewhatsimilar to that which the hydroid polyps bear to theswimming-bells (Fig. 65). They are, however, much morecomplicated in structure than the hydroids. Solitary formsare much commoner than in the hydroids, but the colonialtype is nevertheless very frequent. The colonies may besupported by an organic framework only, but very commonly-there is a tendency to accumulate lime in the tissues, whichresults in the formation of corals. It should be noted,however, that various quite distinct polypoid types mayform corals. Thus, while the most important reef-buildingcorals are included in the Anthozoa, the Millepore-coralsare hydroids. Fi
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
