ACTINOZOA. 



243 



mesenteries, are developed in pairs in the interspaces between the six 

 pairs of the latter, and additional pairs may be subsequently developed 

 in a similar manner in cycles ; but no new mesenteries are produced 

 in the chambers included between the two laminae which form each 

 pair of " principal " mesenteries. 



On the other hand, according to the views of Milne-Edwards and 

 Haime, the mesenteries of the Sea - anemones, and of the ordinary 

 Zoantharian Corals, are developed in a primary cycle of six, to which a 

 second cycle of six is soon added. Should a further development of 

 mesenteries take place, a third cycle of twelve is produced, the new 

 laminae being formed simultaneously in the intervals between those 

 already in existence ; and any further production of mesenteries is 



Fig. 122. — Embryo of a Sea-an- 

 emone {Actinia mesembryanthe- 

 mum), in which the first eight septa 

 have been developed (after Lacaze- 

 Duthiers). The numerals indicate 

 the order in which primitive septa 

 make their appearance. 



Fig. 123. — Calcareous spicules 

 (" dermosclerites ") of the Gor- 

 gonidce, greatly enlarged. A, Gor- 

 gonia radula ; b, Sclerogorgia 

 suberosa; c, Melitheea ochracca. 

 (After Kolliker.) 



supposed to take place in obedience to the same general law. So far 

 as the later cycles of septa in the Hexacoralla are concerned, the general 

 law has been laid down by von Koch that each new septum is produced 

 in the interspace between two older ones, and the septa of each succes- 

 sive cycle are produced, approximately, simultaneously. 



In certain of the Actinozoa, such as the Sea-anemones, and the 

 Ctenophora, the body remains permanently soft ; but most of the 

 animals belonging to this class produce hard structures, which vary 

 much in different cases, and are known by the general name of 

 the " corallum." The simplest form of the corallum is that of de- 

 tached microscopic " spicules " of carbonate of lime, which are 

 more or less largely developed in the soft tissues, but do not unite 

 with one another to form a coherent skeleton. Such spicules are 

 of common occurrence in the Alcyonarians, and their shape (fig. 

 123) varies greatly in different types. They are most abundantly 

 developed in the ccenosarc, but may also be present in the walls 

 of the polypes. Though usually permanently separate, the spicules 

 may become so far fused with one another as to give rise to a 

 coherent external skeleton, as occurs in the Organ-pipe Corals 

 (Tubipora). 



