206 SYDNEY J. HICKSON. 



parasites and other foreign bodies covered by an bypertrophied 

 growth of the ccenosarc. 



The formation of the corbula of Aglaophenia may be 

 accounted for by a similar explanation. The stimulus of 

 the growing blastostyles causes, not only an increased acti- 

 vity in the growth of the lateral branchlets, but a growth in 

 such a manner as to enclose the blastostyles in a cup. 



Similarly the various kinds of animal galls found in Hy- 

 droids and Alcyonarians are probably caused by a circxim- 

 ferential hypertrophy of the tissues surrounding the parasitic 

 pycnogonid, crab, or mollusc. 



From this evidence, then, it does seem probable that a' local 

 stimulus or local irritation of the ccenosarc of these forms 

 causes a growth of the tissues which gradually folds over 

 the seat of the irritation. 



If this is the case, then, the production of a very rudi- 

 mentary and imperfect umbrella-shaped structure is a phy- 

 siological result of the stimulus caused by the growth of the 

 sexual cells, and the medusa is simply a modification, produced 

 by natural selection, of such a structure. 



If this view is a reasonable one, we get over the principal 

 difficulty in accepting the view that the ancestral Hydrozoan 

 was a colonial Hydra form. 



One of the chief features of the higher Protozoa and of 

 the CcBlenterata is the power they possess of forming large 

 colonial organizations by asexual reproduction. And it is 

 reasonable to suppose that when the primitive Hydrozoan 

 became differentiated off from its colonial Protozoan ancestry it 

 retained the power of forming colonies by fission or gemmation. 



It has seemed to me improbable that Hydra can be closely 

 related to the ancestral type, because it does not possess this 

 power. 



If this view of the origin of medusae is correct, there is 

 no difficulty in believing that the ancestral form was a colonial 

 trophosome, and that medusae of different kinds may have 

 originated quite independently of one another from the 

 Hydroid stocks. 



