172 



ZOANTHARIA. 



stages occur in the ovary, and the larva is dehisced into the body cavity as 

 a two-layered ciliated planula. 



The laws affecting the formation of the first twelve tentacles and septa 

 appear to be nearly the same as for the Malacodermata. The hard parts 

 begin as a rule to be formed when twelve tentacles have appeared, at which 

 period also the fixation of the larva takes place. On fixation the larva 

 becomes very much flattened. 



The first parts of the corallum to appear are twelve of the septa, which 

 arise simultaneously in folds of the enteric wall in the chambers between 

 the mesenteries, and correspond therefore with the tentacles and not, as 

 might be supposed, with the mesenteries. Each septum is formed by the 

 coalescence of three calcareous plates which originate in separate centres of 

 calcification. The concrescence of the three produces a Y-shaped plate 

 with the single limb directed inwards and the two limbs outwards (fig. 81). 

 The theca does not arise till after the septa have become formed, and is at 

 first a somewhat membranous cup quite distinct from the septa. The 

 columella is formed still later by the coalescence of a series of nodules which 

 are formed in a central axis enclosed by the inner ends of the septa. 



After the formation of the theca the 

 septa become divided into two cycles by 

 the predominant growth of six of them. 

 On the coalescence of the septa with the 

 theca the space between the two limbs of 

 the Y becomes filled up with calcareous 

 tissue. The law of the formation of the 

 third cycle of septa (12 24) has not been 

 worked out, so that it is not possible to 

 state whether it follows the peculiar prin- 

 ciples regulating the growth of the tentacles. 



The whole of the skeletal parts occupy 

 a position between the epiblast and hypo- 

 blast, and are exactly homologous in this 

 respect with the skeleton of the Alcyonidse. 

 By Lacaze Duthiers they are however be- 

 lieved to originate in the hypoblast, but 

 from the observations of Kowalevsky there 

 can be little doubt that they arise in the 

 connective tissue between the two embry- 

 onic layers which is probably epiblastic in 

 origin. 



A peculiar larva, probably belonging to the Actinozoa, has been described 

 by Semper 1 . It has an elongated form and is provided with a longitudinal 

 ridge of cilia. There is a mouth at one end of the body and an anus at 

 the opposite extremity. The mouth leads into an oesophagus, which opens 



FIG. Si. LARVA OF ASTROIDES 



CALYCULARIS SHORTLY AFTER IT 

 HAS BECOME ATTACHED. (After 



Lacaze Duthiers.) 



The figure shews the develop- 

 ment of the Y-shaped septa in the 

 intervals between the mesenteries. 

 The position of the latter is in- 

 dicated by the faint shading. The 

 theca has become developed ex- 

 ternally. 



1 " Ueb. einige tropische Larven-formen." Ztif.f. wiss. Zoo/., vol. xvn. 1867. 



