THE HYDROZOA. 113 



the epidermis and the epithelium in more complex animals, 

 may be developed, and sometimes attains a great thickness, 



solution, and sometimes, if not always, with suspended solid particles, which 

 perform the functions of the blood in animals of higher organization, and may 

 be termed the somatic fluid. . . . Notwithstanding the extreme variety of form 

 exhibited by the Hydrozoa, and the multiplicity and complexity of the organs 

 which some of them possess, they never lose the traces of this primitive sim- 

 plicity of organization ; and it is but rarely that it is even disguised to any con- 

 siderable extent. . . . This important and obvious structural peculiarity could 

 hardly escape notice, and I find it to have been observed by Trembley, Baker 

 and Laurent, Corda and Ecker in Hydra ; by Eathke, in Coryne ; by Frey and 

 Leuckart, in Lucemaria / and it is given as a character of the hydroid po- 

 lyps in general {Hydras, Corynidas, and SertularidaS), in the second edition of 

 Cuvier's ' Legons.' I pointed it out as the general law of structure of the hy- 

 droid polyps, Diphydce and Physoplioridce, in a paper * sent to the Linnsean So- 

 ciety, from Australia, in 1847, but not read before that body till January, 1849 ; 

 and I extended the generalization to the whole of the Hydrozoa, in a 'Memoir 

 on the Anatomy and Affinities of the Medusas] read before the Eoyal Society 

 in June, 1849. 



" Prof. Allman, in his valuable memoir ' On Cordylophora ' ('Philosophical 

 Transactions,' 1855), has adopted and confirmed this* morphological law, intro- 

 ducing the convenient terms ' ectoderm ' and ' endoderm,' to denote the inner 

 and outer membranes; and Gegenbaur ('Beitrage zur naheren Kcnntniss der 

 Schwimmpolypen; 1851, p. 42) has partially noticed its exemplification in 

 Apolemia and Ehizophysa; but it seems singularly enough to have failed to 

 attract the attention of other excellent German observers, to whose late im- 

 portation investigations I shall so often have occasion to advert. The pecu- 

 liarity in the structure of the body walls of the Hydrozoa, to which I have just 

 referred, possesses a singular interest in its bearing upon the truth (for, with 

 due limitation, it is a great truth) that there is a certain similarity between the 

 adult states of the lower animals and the embryonic conditions of those of 

 higher organization. 



"For it is well known that, in a very early state, the germ, even of the 

 highest animals, is a more or less complete sac, whose thin wall is divisible into 

 two membranes, an inner and an outer ; the latter turned toward the external 

 world; the former, in relation with the nutritive liquid, the yelk. The inner 

 layer, as Eemak has more particularly shown, undergoes but little histological 

 change, and throughout life remains more particularly devoted to the functions 

 of alimentation, wnile the outer gives rise, by manifold differentiations of its 

 tissue, to those complex structures which we know as integument, bones, mus- 

 cles, nerves, and sensory apparatus, and which especially subserve the func- 

 tions of relation. At the same time, the various organs are produced by a process 

 of budding from one or other, or both, of these primary layers of the germ. 



" Just so in the Hydrozoon : the ectoderm gives rise to the hard tegument- 

 ary tissues, to the more important masses of muscular fibres, and to those 

 organs which we have every reason to believe are sensory, while the endoderm 

 undergoes but very little modification. And every organ of a Hydrozoon is 

 produced by budding from one, or other, or both, of these primitive membranes ; 

 the ordinary case being that the new part commences its existence as a papillary 

 process of both membranes, including, of course, a diverticulum of the somatic 

 cavity. 



" Thus there is a very real and genuine analogy between the adult Hydro- 

 zoon and the embryonic vertebrate animal ; but I need hardly say it by no 

 means justifies the assumption that the Hydrozoa are in any sense ' arrested 

 developments ' of higher organisms. All that can justly be affirmed is, that the 



1 "Observations upon the Anatomy of the Diphydse and the Unity of Organiza 

 tion of the Diphydse and Physophoridse. 11 An abstract of this essay was published 

 in the " Proceedings of the Linnsean Society " for 1849. 



