. Bulletin of the British Museum (Natural History). EVOLUTIONARY TRENDS IN CAPITATE HYDROIDS AND MEDUSAE It is of course well known that the egg may encyst in various species of Hydra, but encystment here may possibly have arisen in response to the need for such a device in fresh water where ponds are liable to dry up periodically. We do not know enough about encystment in the Hydrozoa and Scyphozoa to assess its full significance but as already suggested the occurrence of the plano- convex cyst in widely divergent groups all exhibiting some kind of alternation of generations, may have a beari
. Bulletin of the British Museum (Natural History). EVOLUTIONARY TRENDS IN CAPITATE HYDROIDS AND MEDUSAE It is of course well known that the egg may encyst in various species of Hydra, but encystment here may possibly have arisen in response to the need for such a device in fresh water where ponds are liable to dry up periodically. We do not know enough about encystment in the Hydrozoa and Scyphozoa to assess its full significance but as already suggested the occurrence of the plano- convex cyst in widely divergent groups all exhibiting some kind of alternation of generations, may have a bearing on the evolution of the hydroid phase. Its elaboration from a resting stage, at first merely developing directly into a medusa, then gradually acquiring a polypoid form and budding daughter medusae, and evolving to a feeding polyp with tentacles can be readily envisaged, but is less attractive than the actinula theory for which there is more supporting evidence (but see p. 503). 5. THE DEVELOPMENT OF PERISARC Before considering the development of the perisarc proper it may be appropriate to consider the phenomenon of the secretion of a thin pellicel by the fertilized egg in Corymorpha. This has been noted in Corymorpha palma (Torrey, 1907) and in Corymorpha nutans (Rees, I937fl) and seems to be the last surviving indication that encystment was a regular feature in the ancestral Cor3miorphines, but it has disappeared completely in species like Tubularia in which the egg develops directly into an actinula on the manubrium of the sporosac. The way in which the eggs of Corymorpha attach themselves to the substratum is also significant. In Corymorpha nutans, " The pellicel of the egg is very elastic and is pushed out into broad pseudopodia-like growths on the underside into contact with the substratum to which the pellicel adheres. The so-called ' pseudopodia' then withdraw into the main body and the dilated pellicel shrivels up into a small tube. Several of these may be for
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