THE TROPHOSOME. 27 



ectoderm, but being itself overlaid by a soft naked expansion of the cocuosarc, for wliicli it thus 

 forms an internal framework, recalling tlie sclerobasic corallum of certain Actinozoa. 



Ilydrorkiza and Ili/droccmliis. — In almost every case the general colony, or hydrosoma, is 

 attached to some foreign body, such as rocks, shells of mollusca and Crustacea, sea-weeds, floating 

 timber, &c., to which it is fixed by some part of its siu'face (woodcuts, fig. 2, e, and fig. 3,/). 

 In many cases this is effected by a definite organ of attachment, as in the fresh-water 

 Hydra, where, by means of a disc-like expansion of the end diametrically opposite to the 

 mouth, the animal can attach itself to the stems and leaves of aquatic plants, from which it 

 can again spontaneously free itself; or, as in a great number of marine Hydroid.\, in 

 whose young state a disc occupying a similar position (PI. XIII, figs. 12 — 16) also 

 becomes an organ of fixation, differing, however, from the corresponding organ in Hydra 

 by its not admitting of spontaneous detachment, and by its being usually replaced, as 

 the animal grows older, by adherent tubular offsets, or stolons, given off from the same 

 part, lor the definite organ of fixation the term hydrorhiza, as suggested by Huxley, may be 

 employed ; while for the whole of that portion of the hydrophyton which intervenes between the 

 hydrorhiza and the hydranth (woodcuts, fig. 2, d, and fig. 3, e) it will also be very useful, 

 especially in descriptive zoology, to have a distinct name, and that of hydrocauliis may therefore 

 be conveniently used to designate it. 



In many cases, however, all trace of a definite hydrorhiza disappears as the animal grows 

 old, and continues to comphcate itself by the formation of new buds and branches ; and we then 

 find fixation effected by some part of the general surface of the hydrosoma, as in certain creeping 

 Campanularians, &c., in which a greater or less extent of the hydrocaulus itself becomes the 

 medium of attachment. Sometimes it is the hydrocaulus which is suppressed, and the hydranth 

 will then be sessile on the hydrorhiza, as in Hydradima (PI. XV). 



It is occasionally very uncertain whether the part which fixes the hydrosoma ought to be 

 regarded as a true hydrorhiza or as an adherent hydrocaulus. Most usually, however, some 

 peculiarity of structure or of form will justify a decision. Thus, in the Campanidaria represented 

 in woodcut, fig. 2, the tendency of the adherent portion to form a network of inosculating 

 branches, so very different from anything exhibited by the free stems, will fully entitle us to 

 regard this adherent network as a true hydrorhiza, and to place it in a category distinct from that 

 of the free hydrocaulus. 



Again, it is by no means always easy to say where the hydranth ends and where the hydro- 

 caulus begins. In by far the majority of cases the distinction is easy enough, as in the Avhole of 

 the calyptoblastic hydroids, and in Tahidaria, Corymorpha, and many others among the gymno- 

 blastic genera, in all of which the line of demarcation is indicated by a marked change of form, 

 and frequently of structure. In some other cases among the Gymnoblastea, however, the hydranth 

 passes so imperceptibly into the hydrophyton that it is difBcidt to say how much we ought to give 

 to the one, and how much to the other. 



The limit of the perisarc, or common chitinous investment, will often help us in this. 

 Thus in Clava (PI. I), where the hydranth possesses a very much elongated form, one might easily 

 be led to regard as hydrocaulus what is really part of the hydranth. Here, however, if wc 

 consider the whole of the naked portion of the trophosome as belonging to the hydranths, 

 we shall have a distinct though rudimental hydrocaulus in the very short, narrow tubes, 

 invested by a perisarc, which arise from the upper surface of the hydrorhiza, but which have 



