154 ALTERNATIONS OF GENERATIONS. 



developed. During the above process the body of the Scyphistoma 

 gradually grows in length and continues to be segmented, so that 

 a series of Ephyras are uninterruptedly formed, of which those near 

 the base are the youngest. The original terminal ring of tentacles 

 of the Scyphistoma gradually atrophies. 



In the further development of the Ephyra3 each of their eight 

 lobes becomes bifid at its extremity. 



As the Ephyrse successively reach this condition they become 

 detached, and by a series of remarkable changes, amounting almost 

 to a metamorphosis, and accompanied by an enormous growth in size, 

 reach the adult condition. 



The alternation of generations in the Acraspeda cannot be quite 

 so simply explained as in the Hydromedusse, though the principle 

 is probably the same in the two cases. 



Actiuozoa. Amongst the Actinozoa there occurs in Fungia a 

 peculiar process which is, as shewn by Semper (171), in many ways 

 analogous to alternations of generations 1 . From the larva a nurse- 

 stock is developed, at the end of which a cup-like coral resembling 

 the adult is formed as a bud. The bud becomes detached and 

 then gives rise to a permanent sexual Fungia. From the nurse- 

 stock there is formed however a fresh bud at the centre of the 

 scar left on the detachment of the old one. The fresh bud even- 

 tually becomes separated from the nurse-stock leaving a small portion 

 of its stem behind ; each succeeding bud similarly leaves a small 

 portion of its stem, so that the nurse-stock eventually acquires a jointed 

 appearance. In the above process we clearly have, as in the Hydro- 

 medusae, a non-sexual form the nurse-stock produced directly from 

 the larva, giving rise by budding to a sexual form ; all the conditions of 

 an alternation of generations are therefore fulfilled. It seems however 

 possible that the nurse-stock itself may eventually become sexual. 



BIBLIOGRAPHY. 

 Coelenterata. General. 



(145) Alex. Agassiz. Illustrated Catalogue of the Museum of Comparative Anatomy 

 at Harvard College, No. ir. American Acaleplise. Cambridge, U. S., 1865. 



(146) O. and K. Hertvvig. Der Organismus d. Medusa; u. seine Stellung z. Keim- 

 bldttertheorie. Jena, 1878. 



(147) A. Kowalevsky. " Untersuchvmgen iib. d. Entwicklung d. Ccelenteraten." 

 Nachrichten d. kaiser. Gesell. d. Frcunde d. Naturerkenntniss d. Anthropologie u. Ethno- 

 graphic. Moskau, 1873. (Russian). For abstract vide Jahresberichte d. Anat. u. Phys. 

 (Hoffman u. Schwalbe), 1873. 



Hydrozoa. 



(148) L. Agassiz. Contributions to the Natural History of the United States of 

 America. Boston, 1862. Vol. iv. 



(149) G. J. Allman. A Monograph of the Gymnoblastic or Tubularian Hydroids. 

 Kay Society, 1871-2. 



1 Vide also Moseley. Notes by a Naturalist of the Challenger, p. 524 and 525. 



