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 



?ertbrm the functions of tlie blood in animals of higher organization, and may 

 e ternmd the somatic Jluid. . . . 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 Lucernaria ; and it is given as a character of the hydroid po- 

 lyps in general {Hydra;^ Corynidce^ and Sertularidoe,)^ in the second edition of 

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

 droid polyps, Diphydoi and Fhysophoridce^ in a paper i sent to the Linnpean So- 

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

 and 1 extended the generalization to the whole of the Hydrozoa^ in a 'Memoir 

 on the Anatomy and Affinities of the Meduso&^ read before the Koval 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 ('Beitriige zur naheren Kenntniss der 

 Schwimmpolypen; 1854, p. 42) has partially noticed its exemplification in 

 ApoUmia and Rhizophym; 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. Tlie 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, Avith 

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

 layer, as Eemak has more ^larticularly shown, undergoes but little histological 

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

 of alimentation, while the outer gives rise, by manifold diflferentiations 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 niodification. And every organ of a Ilydrozoun is 

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

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

 process of both membranes, includmg, of course, a diverticuUim of the somatic 

 cavity. 



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

 zoon and the embryonic vertebrate" animal ; but 1 need hardly say it b}- 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 .\natomyof the Diphydfe and the Unify of Organize 

 tion of tlie Dipliydae and Physophorid*." An abstract of this essay was published 

 in the '• Proceediugs of tlie Llnuaeau Society " for 1849. 



