2 THE OCEANIC HYUROZOA. 



Professor Allman, in his valuable 'Memoir on Cordylophora' (' Philos. Trans.,' 1853), has 

 adopted and confirmed this morphological law, introducing the convenient terms " ectoderm and 

 endoderm," to denote the inner and outer membranes; and Gegenbaur (' Beitriige zur niiheren 

 Kenntniss der Schwimmpolypen/ 1854, p. 42), has partially noticed its exemplification in 

 Jpolemia and Bhizopliym ; but it seems, singularly enough, to have failed to attract the attention 

 of the other excellent German observers, to whose late important investigations I shall so 

 often have occasion to advert. 



The peculiarity 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 towards the external world ; the former, in relation with the nutritive 

 liquid — the yelk. The inner layer, as Remak has more particularly shown, undergoes but 

 little histological change, and, throughout life, remains more particularly devoted to the 

 function of alimentation, while the outer gives rise, by manifold differentiations of its tissue, to 

 those complex structures which we know as integument, bones, muscles, nerves, and sensory- 

 apparatus, and which especially subserve the functions 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 tegumentary tissues, to 

 the more important masses of muscular fibre, 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 ccecal diverticulum of the somatic cavity. 



Thus there is a very real and genuine analogy between the adult Hydrozoon and 

 the embryonic vertebrate animal ; but I need hardly say that it by no means justifies the 

 assumption that the Hydrozoa are in any sense " arrested developments" of higher organisms. 

 All that can justly be aCBrmed is, that the Hydrozoon travels for a certain distance along the 

 same great highway of development as the higher animal, before it turns off to follow the road 

 which leads to its special destination. 



The entire double-walled body of the Hydrozoon, whether it be a minute, simple, oval sac, 

 as in the embryonic condition, or such a vast and complex mass as a tree of Plumularia, an 

 Ayahiia three feet long, or a Uldzostoma of still more massive proportions, will be termed, 

 in the course of the ensuing pages, a liydrosoma. 



The simplest condition of this hydrosoma is that observable in the common fresh- water 

 Hydra, one end of whose body is expanded into a kind of disc, whereby the creature 

 adheres to its support, wliile the opposite extremity presents a widely-open mouth, opening 

 into a cavity which extends through the whole length of the animal, and surrounded by a 

 circle of long tentacular organs. Here, then, the body exhibits only three distinct morpholo- 

 gical constituents : a disc of attachment — which, with its homologous organs in other Hydrozoa, 

 may be termed the liydrorliiza; a sac for the digestion and (as there is, in this case, no dis- 



