THE OCTOCOKALLA. 143 



course of its development, the finally hexamerous Antho- 

 zoon passes through a tetramerous and an octomerous stage. 



Phenomena analogous to the " alternation of generations," 

 which is so common among the ITydrozoa, are unknown 

 among the great majority of the Actinozoa. But Semper ' 

 has recently described a process of agamogenesis in two spe- 

 cies of Fungio?, which he ranks under this head. The Fiingioe 

 bud out from a branched stem, and then become detached 

 and free, as is the habit of the genus. To make the parallel 

 with the production of a medusoid from a hydroid polyp 

 complete, however, the stem should be nourished by a sexless 

 anthozooid of a different character from the forms of Fungice 

 which are produced by gemmation. And this does not appear 

 to be the case. 



In one division of the CoralUgena — the Octocoralla — 

 eight enteroccele chambers are developed, and as many ten- 

 tacles. Moreover, these tentacles are relatively broad, flat- 

 tened, and serrated at the edges, or even pinnatifid. The 

 Actinozoon developed from the egg may remain simple 

 (JHaimea, Milne-Edwards), but usually gives rise to a zoan- 

 thodeme. * 



The ccenosarc of the zoanthodeme in the Octocoralla is a 

 substance of fleshy consistence, which is formed chiefly of a 

 peculiar kind of connective tissue, containing many muscular 

 fibres developed in the thickened mesoderm. The axial cavity 

 of each anthozooid is in communication with a system of 

 large canals. In Alcyoiiiiun, a single large canal descends 

 from each anthozooid into the interior of the zoanthodeme, 

 and the eight mesenteries are continued as so many ridges 

 throughout its entire length, 2 so that these tubes have been 

 compared to the thecal canals of the Millepores. In the red 

 coral of commerce ( Corallium ruhrurn, Fig. 30), the large 

 canals run parallel with the axial skeleton. A delicate net- 

 work, which traverses the rest of the substance of the cceno- 

 sarc, appears to be sometimes solid and sometimes to form a 

 system of fine canals opening into the larger ones. The 

 anthozooids possess numerous muscles by which their move- 

 ments are effected. The fibres are delicate, pale, and not 

 striated. Nerves have not been certainly made out. 



It is in these Octocoralla that the form of skeleton which 

 is termed a sclerobase, which is formed by cornification or 



1 " Ueber Generations- Weebsel bei Steinkorallen." Leipsic, 1872. 



2 Poucbet and Myevre, " Contribution a 1' Anatomie des Alcyonaires." 

 {Journal <P Anotomie et de la PJiysiologie, 1870.) 



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