134: THE MICROSCOPE AND ITS REVELATIONS. 



common all round our coasts, often have a diameter of from 6 to 15 

 inches; while the Rhizostoma sometimes reaches a diameter of from two 

 to three feet. The quantity of solid matter, however, which their fabrics 

 contain is extremely small. It is not until adult age has been attained, 

 that the generative organs make their appearance, in four chambers dis- 

 posed around the stomach, which are occupied by plaited membranous 

 ribands containing sperm-cells in the male and ova in the female, and the 

 embryoes evolved from the latter, when they have been fertilized by the 

 agency of the former, repeat the extraordinary cycle of phenomena which 

 has been now described, developing themselves in the first instance into 

 Hydroid Polypes, from which Medusoids are subsequently budded-off. 



525. This cycle of phenomena is one of those to which the term 

 'alternation of generations' was applied by Steenstrup, 1 who brought 

 together under this designation a number of cases in which generation A 

 does not produce a form resembling itself, but a different form, B; whilst 

 generation B gives origin to a form which does not resemble itself, but 

 returns to the form A, from which B itself sprang. It was early pointed 

 out, however, by the Author, 2 that the term ' alternation of generations ' 

 does not appropriately represent the facts either of this case, or of any of 

 the other cases grouped under the same category: the real fact being that 

 the two organisms, A and B, constitute two stages in the life-history of 

 one generation; and the production of one form from the other being in 

 only one instance by a truly generative or sexual act, whilst in the other 

 it is by a process of gemmation or budding. Thus the Medusce of both 

 orders (the ' naked-eyed ' and the ' covered-eyed ' of Forbes) are detached 

 flower-buds, so to speak, of the Hydroid Zoophytes which bud them off; 

 the Zoophytic phase of life being the most conspicuous in such Thecata 

 as Campanularida and Sertularida, whose Medusa-buds are of small size 

 and simple conformation, and not unfrequently do not detach themselves 

 as independent organisms; whilst the Medusan phase of life is the most 

 conspicuous in the ordinary Acalephs, their Zoophytic stage being passed 

 in such obscurity as only to be detected by careful research. The 

 Author's views on this subject, which were at first strongly contested by 

 Prof. E. Forbes, and other eminent Zoologists, have now come to be 

 generally adopted. 



526. ACTINOZOA. Of this group, the common Sea- Anemonies may be 

 taken as types; constituting, with their allies, the order Zoantharia, or 

 Helianthoid polypes, which have numerous tentacles disposed in several 

 rows. Next to them come the Alcyonaria, consisting of those whose 

 polypes, having only six or eight broad short tentacles, present a star- 

 like aspect when expanded; as is the case with various composite Sponge- 

 like bodies, unpossessed of any hard skeleton, which inhabit our own 

 shores, and also with the Ked Coral and the TuMpora of warmer seas, 

 which have a stony skeleton that is internal in the first case and external 

 in the second, as also with the Sea-pens, and the Gorgonice or Sea-fans. 

 A third order, Rugosa, consists of fossil Corals, whose stony polyparies 

 are intermediate in character between those of the two preceding. And 

 lastly, the Ctenophora, free swimming gelatinous animals, many of which 

 are beautiful objects for the Microscope, are by most Zoologists ranked 

 with the Actinozoa. 



1 See his Treatise on " The Alternation of Generations," published by the Ray 

 Society. 



2 " Brit, and For. Med.-Chir. Review," Vol. i. (1848), p. 192, et seq. 



