22 MORPHOLOGY. 



the animal lives, while the free surface of the cndoderm is turned inwards, and forms the boundary 

 of the gastric cavity and of all its prolongations through the organism. A similar composition may 

 be demonstrated not only in all the rest of the Ilydrozoa, but in the whole group of the Ccelen- 

 ierata. For the important generalisation which thus asserts the composition of every coelenterate 

 animal out of two membranes — a generalisation which forms the basis of the whole morphology 

 of the Ccelenterata — we are indebted to Prof. Huxley, who first enunciated it as a great 

 anatomical truth .^ 



Another character which the Hydroida possess in common with the entire group of Ca;len- 

 terata is the presence of the peculiar bodies known as thread-cells. These bodies, which will be 

 afterwards more particularly examined, are developed in the ectoderm, where they are frequently 

 aggregated in definite groups very characteristic of the species. 



3. Composite character of the Hydroida — Trophosome, and Gonosome. 



The Hydroida, wherever our knowledge of them is sufficiently complete to justify us in 

 arriving at any well-founded conclusion regarding the entire life of the individual are all, with 

 only a single apparent exception, composite animals at some one period of their existence, each con- 

 sisting then of an assemblage or colony of zooids,^ in organic union with one another (woodcut, 

 fig. 2). The colony thus formed constitutes the " hydrosoma" of Huxley. 



It will be shown in the sequel that, except in the solitary — and, perhaps, after all, only 



1 Huxley, " On the Anatomy and Affinities of the Medusae," ' Phil. Trans.,' 1849. 



" For the introduction of the very convenient term " zooid" into the language of zoology, we 

 are indebted to Prof. Huxley, who, in defining tlie " individual" as " the total result of the develop- 

 ment of a single ovum," proposed to designate by the terra zooid all more or less independent forms 

 which may be included as elements in this total result. (See Huxley, " Observations on Salpa," &c., 

 in ' Phil. Trans.,' 1851 ; " Lecture on Animal Individuality," ' Ann. Nat. Hist.,' June, 1853 ; and his 

 review of J. Miiller's " Researches on the Development of the Echinodermata," in ' Ann. Nat. Hist.,' 

 July, 1851. See also Carpenter, 'Principles of Gen. and Comp. Physiology,' 1851, p. 906, and 'Brit, 

 and For. Med.-Chir. Eev.' for Jan. 1848 and Oct. 1849, where the same idea is clearly supported. 

 Dr. Carpenter using the expression " a generation" for all that intervenes between one act of true or 

 sexual generation and another. 



The distinction between a " zooid" and an " organ" is not always easy, and may indeed some- 

 times appear to be arbitrary. I believe, however, that we may define a zooid as a more or less 

 i)}d/vidualised animal organism, ivhich may or may not be capable of independent existence, and which 

 constitutes one of a series tvhose members are related to each other by some form of non-sexual reproduc- 

 tion, and morphologically repeat one another eitlier actually or homologically . In this sense not only 

 are the free medusiform buds of the Hydroida true zooids, but we must also regard as such the fixed 

 hydranths and those fixed gonophores which never attain a developed medusiform structure, as well as 

 the simple generative sacs which are developed on the radiating canals of Obelia, Thaumantias, &c. 

 (see p. 35). 



On the other hand, from the above definition are necessarily excluded all mere organs, however 

 capable they may be for a longer or shorter period of self-maintenance, such, for example, as the 

 Hectocotylus of the Hcctocotylus-forraing Cephalopoda. 



Many zooids may combine to form the true zoological Individual —the logical element of the species. 



