19 



modified by developing their reproductive organs while still 

 attached to the hydriform colony ; this results in their 

 ceasing to become detached, and in some forms they become 

 more and more degenerate, until eventually the sporosacs of 

 Hydractinia, and the simple reproductive organs of Hydra, 

 are reached. 



In the Hydrocorallina and some Gymnoblastea, "poly-" 

 morphism" has added greatly to the complication of the 

 colony. We find various sets of buds developing into 

 differently shaped persons which are specially fitted to per- 

 form certain functions in the colony. In Hydr actinia ^"^^ 

 for example, there are nutritive persons, reproductive 

 persons, tentacular persons, and defensive persons in the 

 one colony. 



On its outer surface, in the hydriform person in many of 

 the Hydromedusae, the ectoderm (the outer layer of cells) 

 forms a horny layer, the perisarc. This may be very slightly 

 developed and confined to the aboral end, or it may cover 

 the entire body and project .beyond the oral region in the 

 form of a calycle or hydrotheca, and cover groups of medusi- 

 form buds as a gonangium (as in the Calyptoblastea). The 

 Hydrocorallina probably diverged from the base of the 

 Gymnoblastea, their ancestors having acquired the property 

 of forming calcareous deposits in the ectoderm, so as to 

 produce a hard, stony corallum. Polymorphism is found 

 here in an advanced condition. 



The Siphonophora diverged from the ancestral Hydro- 

 medusae, and acquired the characteristic of never becoming 

 fixed at any period of their life-history. Probably at an 

 early point in their independent history the hydriform person 

 developed from the ovum commenced to bud while very 

 young, and produced medusiform persons before becoming 

 fixed. The result of this would be that the pulsations of 



* AUman's Gymnoblastic Hydroids, Part ii, p. 220, Ray Soc, 1872. 



