ACTINOZOA 877 



This cycle of phenomena is one of those to which the term ' alter- 

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

 together under this designation a number of cases in which genera- 

 tion A does not produce a form resembling itself, but a different form, 

 B ; whilst generation B gives origin to a form which does not re- 

 semble 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 Medusa 3 of both 

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

 tached 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 C'ampanulariida and /Sertulariida, 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 Professor E. Forbes and 

 other eminent zoologists, have now come to be generally adopted. 3 



Actinozoa. Of this group the common sea-anemones 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 always 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 red coral and the Tubipora 

 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-swim- 

 ming gelatinous animals, many of which are beautiful objects for 

 the microscope, are by some zoologists ranked with the Actinozoa. 4 



Of the Zoantharia the common Actinia or ' sea-anemone ' mav 

 be taken as the type, the individual polypites of all the composite 

 fabrics included in the group being constructed upon the same model. 5 

 In by far the larger proportion of these zoophytes, the bases of the 



1 See his treatise on The Alternation of Generations, a translation of which has 

 been published by the Ray Society. 



2 Brit, and For. Med. Chir. Review, vol. i. 1848, p. 192 et seq. 



5 Compare Huxley, Anatomy of Invert ebrated Animals, p. 133; and Balfour, 

 Comparative Embryology, i. p. isi. 



4 Professor Haeckel, led by the study of Ctenaria ctenophora, associates the 

 Ctenophora with the Hydrozoa (Sitzungtber. Jenaische GesellscJtaft, May 16, 1879). 



5 On the anatomy of Actinia and its allies, see O. and E. Hertwig's monograph 

 in vols. xiii. and xiv. of the Jenaische Zeitschrift. 



