POLYPI. 127 



and returns back into the digestive cavity. The movements of the 

 fluid appear to depend on a delicate ciliated epithelium which lines 

 tlie cavities of the tentacles, as well as the tubular cavities of the 

 stem and branches of the compound polypes.* 



The peculiar external horny defence prevents, as I have just ob- 

 served, the exercise of the gemmiparous faculty from effecting any 

 other change than that of adding to the general size, and to the 

 number of prehensile mouths and digestive sacs, of the compound 

 coralline. It is equally a bar to propagation by spontaneous fission ; 

 so that the ordinary phenomena of generation by ova or germ-masses 

 are more conspicuous in the composite than in the simple Hydrozoa. 

 At certain points of these ramified polypes, which points are constant 

 in, and characteristic of, each species, there are developed little 

 elegant vase-shaped or pod-shaped sacs, which are called the ovige- 

 rous vesicles or " ovicapsules." These are sometimes appended to the 

 branches, sometimes to the axillae, as at h, i, k,Jig. 61, of the ramified 

 coralline : they are at first soft, and have a still softer lining mem- 

 brane, which is thicker and more condensed at the bottom of the 

 vesicle: it is at this part that the ova or germs are developed, h, 

 and for some time these are maintained in connection with the vital 

 tissue of the polype by a kind of umbilical cord, k, I. In all the com- 

 pound Hydrozoa, the ovicapsules are deciduous, and having performed 

 their functions in relation to the development of the new progeny, 

 drop off like the seed-capsules of plants. This phenomenon afforded 

 to the early botanists an additional argument in favour of the relation 

 of these ramified and rooted animals to the Vegetable Kingdom. 



The species of the marine Hydrozoa which is most nearly akin to 

 the fresh-water Hydra, is the Tubularia indivisa, beautifully figured 

 in his fine work on the "Remarkable Animals of Scotland," by Sir 

 John G. Dalyellfj who devoted thirty years to the careful examination 

 of the animals of the Frith of Forth. In this species each individual 

 is distinct, like the Hydra; it propagates its kind by gemmation, and 

 ha?, also, great powers of reparation, reproducing its polype head and 

 double crown of tentScles many times in succession. It also propagates 

 by ova. These are formed in ovicapsules, aggregated in groups which 

 proceed from the space between the long and the short tentacula; and, 

 in proportion as the ovicapsules are developed, the tentacula begin to 

 decay, and the whole flower-like head falls. When cast off, the head 

 does not lose its vitality, but moves, for a few days, perhaps, and 

 crawls off with the ovicapsules to some distance from the parent. 

 A locomotive polype, which is called by Sir J. G. Dalyell a " hydra," 



* CVIII. p. 107. t CIX. pis. 1, 2, and 3. 



