DEVELOPMENT. 107 



Against the direct testimony of so able and conscientious an observer as Haeckel, I sliould 

 not consider myself justified in insisting on a hypothesis which I have had no opportunity of 

 verifying by direct examination; but yet I can scarcely avoid seeing that Ilaeckel's description 

 of the structure of these pouches is in some points favorable, rather than contradictory, to my 

 view ; thus the currents of nutritive fluid which he has observed flowing in ramified channels 

 through the mass of the ova appear to me to be explicable only on the admission that these 

 currents are contained within a ramified spadix, for the supposition that the generative elements are 

 directly bathed in the fluid of the gastro-vascular canals is so completely at variance with the 

 analogy of these parts in all the other Hydroida, that we can scarcely bring ourselves, without 

 very strong evidence, to accept it. If we admit the presence of a true spadix penetrating the 

 pouch, and surrounded by the ova or spermatozoa, we have all the parts needed to establish a 

 detailed homology between the leaf-like pouches of Geryonia and the prominent sacs of Obelia, 

 and these last are, without any doubt, true zooids, strictly homologous with the sporosacs of 

 Clava. 



It is more difficult to recognise a zooidal origin in the generative pouches of the Cunina which 

 Haeckel has shown to be produced as a bud from the Gerijonia, and there seems no reason 

 why we should not, with Haeckel, regard the Cimince as truly sexual medussR. What may be 

 the subsequent history of these Cunina, is as yet entirely unknown ; and until this shall have 

 been determined, the significance of Haeckel's beautiful discovery of the relation between the 

 Geryonidans and ^ginidans must remain but partially recognised. 



In the genetic phenomena of the Hydroida, so far as these have been accurately determined, 

 one fact stands out in prominent relief, and its recognition is of great importance in enabling 

 us to perceive the true import of these phenomena, and the mode in which they are associated 

 in the life of the hydroid. I again refer to the fact that in every hydroid the groups included 

 between every two acts of embryonal development (the groups connected by horizontal brackets in 

 the above forumlae) are exactly similar in the nature and succession of their heteromorphic elements, 

 — in other words that the life series of the hydroid may be represented by definite groups of 

 zooids exactly repeated after each generative act.^ It is plain, too, that each of these groups 

 — which we may conveniently designate as the " periods" of the series — exactly corresponds to 

 the " individual" which constitutes the proper logical element of the sjjecies in animals which do 

 not present the phenomenon of alternation, the period here repeating itself by true generation, 

 and this repetition continuing itself indefinitely like a circulating decimal, so as to represent the 

 indefinitely extended life of the species, while the life of the individual is expressed by each period 

 singly. It is further evident that the conception of the individual involved in the above view 

 is in no respect invalidated by the fact that one or more of its zooidal elements may become free, 

 and enjoy an independent existence. 



For the views of Hydroid individuality, embodied in the above paragraph, we are indebted to 

 Prof. Huxley, who first assigned to our conception of the biological individual its proper limits 

 when he defined it as " the total result of the development of a single ovum" — a most 

 important determination by which alone the genetic phenomena of the Hydroida can be 

 properly understood and brought into comparison with those of the higher animals. At the same 



^ Tlie mere number of zooids in two or more of tliese groups may of course vaiy, depending as 

 this does on the accident of abundant or deficient nutrition and the like. 



