142 



ZOANTHAKIA. 



tentacles are constantly added in pairs. An odd tentacle is always 

 found at the extremity of the mouth opposite the oldest tentacles. 



In the other species with an embolic gastrula eight tentacles would 

 seem to appear simultaneously at the period when eight chambers are 

 present ; though on this point Kowalevsky's description is not very clear. 

 The presence of such a stage would seem to indicate a close affinity to 

 the Alcyonidse. 



Amongst the sclerodermatous Actinozoa, except Caryophyllium, the 

 embryo closely resembles that of the delaminate Malacodermata. The 

 first 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 calcai-eous 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 iuto 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 be- 

 comes 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 principles 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 Alcyonidfe. 

 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 con- 

 nective tissue between the two embryonic 

 layers which is probably epiblastic in origin. 

 A peculiar larva, probably belonging 

 to the Actinozoa, has been described by 



FIG. 81. LARVA OF ASTROIDES 

 CALYCULAEIS 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 mesen- 

 teries. The position of the latter 

 is indicated by the faint shading. 

 The theca has become developed 

 externally. 



