THE HYDKOZOA. 113 



the epidermis and the epithelium in more complex animals, 

 may be developed, and sometimes attains a great thickness, 



solution, and sonu -times, it' not always, with suspended solid particles, which 

 perform the I'mi'-timis <>! the blood in animals of higher or^'am/at'inn, and may 

 he termed the xnt'itii'jliii<t. . . N"t wit hstandhiL' t he . I ' form 



exhibited by tin' BydrOflOft, and the multiplicity and complexity of the organs 

 whieh some of them possess, they never lose the traces of thi> primitive sim- 

 plieitx of organi/ation ; and it i> hut rarely that it is even di.-iruised to any con- 

 siderable extent. . . . This important, und obvious structural j.<-euliarit\ 

 hardly escape notiei -, an<l I And it to have been observed by Trcmblev, Baker 

 mid Laurent, Conhi and Kcker in lli/drn by Rathke, in Corune ; by Frej and 



droid pol\ ps, Dipkyda and PKy90pk>ri<to. in a paper ' sent t-> the Linn:' 

 ciet\ . from Australia, in 1847, but not read before that body till .January. 1849; 

 and I extended the generalization to the whole of the ////.//"-'"/, in a Memoir 

 on the Anatomy and Affinities of the Medusa] read before the Koyal 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 xur nf.heren Kenntniss der 

 Schwimmpolypen; 1854, p. 4-2) has partially noticed its exemplification in 

 Apolemia and Rhizophysa: 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 Tnembrnnes, an inner and an outer ; the latter turned toward the external 

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

 layer, as Remak kas more particularly shown, undergoes but little histol.. L 'ioal 

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

 of alimentation, while the outer gives rise, by manifold differentiation! of its 

 tissue, to those complex structures which we now 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 .u'erin. 



"Just so in the Hvdrozoon : 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 eudoderm 

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

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

 the ordinary case being that the new part commences its cxi-renceas a papillary 

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

 cavity. 



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

 xoon and the embryonic vertebrate animal : but I need hardly say it 

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

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



1 "Observations upon the Anatomy of the Dlphydte and the Unity of Organic* 

 tion of the Diphydse and Phveophorida>." An abstract of this essay was published 

 in the k - Proceedings of the Lmnaean Society " for 1849. 



