5G 



HARDWICKE'S SCIENCE-GOSSIP. 



municated to us by a professed naturalist of great 

 practical experience, who has studied the subject 

 carefully. 



Fig-. 2". HyilrjE strides, attached to the roots of duckweed. 

 After Roesel. 



The species of the marine Hydrozoa most nearly 

 akin to the fresh-water Hydra is the Tuhularia 

 hidivisa described by Sir John Dalyell in his " Re- 

 markable Animals of Scotland." 



From the brief sketch we have given of a " naked, 

 free, solitary polype," we pass on to a compound 

 form, in which the polypes are "protected by cells, 

 developed according to regular patterns from a 

 rooted and ramified horny tubular polypary." If 

 we picture to ourselves a common Hydra, which, 

 instead of casting off its polype-buds in the regular 

 way, has retained Ihem all, with all their buds, and 

 their buds' buds for a dozen generations, all attached 

 to and communicating with each other by their 

 nutritive canals, we shall have before us a rude 

 "archetype" of the compound Hydrozoa, Hydroid, 

 Hydriform, or Hydra-like polypes. 



" The seas which wash our own shores " (we quote 

 from a lecture by Professor Owen) "are tenanted 

 by numerous forms of minute polypi, having essen- 

 tially the same simple organization as the Hydra, 

 but which are protected from the dense briny ele- 

 ment by an external horny integument. Now these 

 likewise develop new polypes by gemmation ; but 

 as the external crust grows with the growth of the 

 soft digestive sac, the young polype adheres to the 

 body of the parent, and, by successive gemmations, 

 a compound animal is produced. Yet the pattern 

 according to which the new polypes and branches of 

 polypes are developed is fixed and determinate in 

 each species ; and there consequently results a par- 

 ticular form of the whole compound animal or indi- 

 vidual by which the species can be easily recognized. 

 This compound hydriform polype-animal, or associ- 

 ation of polypes, resembles a miniature tree, but 

 consists essentially of a ramified tube of irritable 



animal matter (m, fig. 2S), defended by an external, 

 flexible, and frequently jointed horny skeleton (a) ; 

 and is fed by the activity of the tentacula {d), and 

 by the digestive powers of the alimentary sacs (g) 

 of a hundred polypi, the common produce of which 

 circulates through the tubular cavities for the benefit 

 of the whole community. 



Fig. 28. Diagrammatic sketch of Campanularia dichotoma. 

 After Owen. 



" The peculiar external horny defence prevents 

 the exercise of the gemmiparous faculty from 

 effecting any other change than that of adding to 

 the general size, and to the number of the pre- 

 hensile mouths and digestive sacs, of the compound 

 coralline. It is equally a bar to 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 ovigerous 

 vesicles, or ' ovicapsules.' These are sometimes 

 appended to the branches, sometimes to the axilla;, 

 as at h, i, k, fig. 2S : they are at first soft, and 

 have a still softer lining membrane, which is thicker 

 and more condensed at the bottom of the vesicle : 

 it is at this part that the ova or germs are developed 

 (//), and for some time these are maintained in 



