20 THE OCEANIC HYDROZOA. 



the columella of the urn ; and it will be so convenient to have a special name for these 

 organs set apart for the production of generative buds, that I will term them, gonoblastidia. 



Among the MedusidtB, IVillsia (itself possibly only a zooid) possesses stem-like gonoblastidia 

 terminated, as in Ilydractinia, by an enlarged end, abounding in thread-cells. 



I have already spoken of the detachment of the medusiform gonophores of many 

 Hydrozoa. But in many Cahjcophoridm, and, perhaps, in some P/ii/sophorida, the organism 

 undergoes a still further subdivision in the natural course of its development. 



This process will be found described in detail below, as it occurs in Abi/Ia. It will be 

 seen that each segment of the coenosarc, provided with a ] olypite, its tentacle, reproductive 

 organ, and hydrophyllium, as it acquires a certain size, becomes detached, and leads an 

 independent life — the calyx of its reproductive organ serving it as a propulsive apparatus. 

 In this condition it may acquire two or three times the dimensions it had when detached, and 

 some of its parts may become wonderfully altered in form. 



It is by no means improbable that some of the FlnjsophorideR may undergo a similar 

 disruption, whole groups of organs, as in Apokmia, or at any rate the gonoblastidia, 

 becoming detached.' Eschscholz (p. 159), who observed the ready separation of the latter 

 in Phjsalia, imagined that they developed directly into young Physalice. 



The preceding section was written before Professor Allman's valuable papers, cited at p. 17, came into 

 my hands. I have adopted his terminology as far as possible ; but I have been compelled to make 

 certain modifications in it, for reasons which will be stated in a note at the end of this general intro- 

 duction to the study of the Hydrozoa. 



SECT. II. SPECIAL MORPHOLOGY OF THE HYDROZOA. 



I have now briefly described all the organs" of the Hydrozoa and their leading modifica- 

 tions, or, in other words, I have given some account of the general morphology of the class. 

 The various modes in which these organs are grouped together is the subject matter of 

 the present section on the special morphology of the group. Of these modes there are, 

 at most, si.\. or seven, and each, when clearly expressed, defines the plan of structure 

 common to the members of one of the several groups or orders into which the zoologist 

 divides the class — for a scientific classification is, after all, nothing more than a conve- 

 nient mode of expressing the facts and laws established bj' the morphologist. 



The first type, or plan of structure, is that exhibited by the order of the Hydrida, 

 containing the single genus Hydra, whose hydrosoma consists of only a single polypite, 

 surrounded towards the distal end by a circlet of simple filiform tentacula, and expanded 

 at the proximal end into a discoid hydrorhiza. The reproductive organs are simple 

 spermaria or ovaria developed in the walls of the polypite, and the ectoderm developes 

 no hard cuticular layer. 



' See Leuckart, Z. U., pp. 69, 70. 



^ I have reserved the consideration of the Lithocysts for a future occasion, when the Medusida 

 observed during my voyage will be described. 



