THE OCEANIC HYDROZOA. 



SECT. I. MORPHOLOGY OF THE HYDROZOA. 



The body of every Hydrozoon is essentially a sac, composed of two membranes, an 

 external and an internal, wbich have been conveniently denominated by the terms ectoderm 

 and endoderm. The cavity of the sac, which will be called the somatic cavity, contains a 

 fluid, charged with nutritive matter in solution, and sometimes, if not always, with suspended 

 solid particles, which performs the functions of the blood in animals of higher organization, 

 and may be termed the somatic Jlaid. The ectoderm is commonly ciliated, at any rate while 

 young- ; the endoderm is also very generally ciliated, though not always, nor in all parts. 

 The cilia of the endoderm, aided by the contractions of the walls of the body, are the sole 

 means provided by nature for the circulation of the nutritive fluid in the Hydrozoa; the cilia 

 of the ectoderm, similarly aided by contractility, constitute the only respiratory mechanism. 



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 simplicity of organization ; and it is but rarely that it is even disguised to any 

 considerable extent. I know of no Hydrozoon in which the two primary membranes, but 

 little altered, cannot be at once detected in the walls of almost every part of the organism. 



This important and obvious structural peculiarity could hardly escape notice, and I find 

 it to have been observed by Trembley, Baker, Laurent, Corda, and Ecker, in Hydra ; by 

 Rathke, in Coryne ; by Frey and Leuckart, in Lucernaria ; and it is given as a character of 

 the Hydroid polypes in general {Hydrce, Corynidce, and Sertulariadce) in the second edition 

 of Cuvier's ' Lefons.' I pointed it out as the general law of structure of the Hydroid 

 polypes, BipJiyda, and Fhysophorida, in a paper' sent to the Linnean Society, from Australia, 

 in 1847, but not read before that body until January, 1849 ; and I extended the generaliza- 

 tion to the whole of the Hydrozoa in a ' Memoir on the Anatomy and Affinities of the MeduscB,' 

 read before the Royal Society in June, 1849. 



' ' Observations upon the Anatomy of tlie Biphydce, and the Unity of Organization of the 

 Diphydte and Fhysophorida.' An abstract of this essay was published iu the ' Proceedings of the 

 Linnean Society' for 18t9. 



