170 



ZOANTHARIA. 



limbs of the fold. The larger chamber next becomes divided by two fresh 

 mesenteries into three, and a similar division then takes place in the smaller 

 chamber. The stage with six chambers is almost immediately succeeded 

 by one with eight, owing to the appearance of two fresh mesenteries in the 

 second-formed set of chambers. At the stage with eight chambers there is a 

 marked period of repose. The number of chambers is increased to ten by 

 the division of the third-formed set of chambers, and to twelve by the 

 division of the fourth-formed set. It will be observed that the number of 

 the chambers increases in arithmetical progression by the continual addition 

 of two, alternately cut off from the primitive large and small chambers. 

 The freshly formed chambers are always formed immediately on one side of 

 the primitive mesenteries. The stages with six and ten are of very short 

 duration. The two primitive chambers are necessarily at the ends of the 

 long axis of the mouth. After the division of the enteric cavity into twelve 

 chambers, these chambers become about equal in size, and the formation of 

 the tentacles commences. The law regulating the appearance of the 

 tentacles is nearly the same as that for the mesenteries, but is not quite so 

 precise. One tentacle makes its appearance for each chamber. The most 

 remarkable feature in the appearance of the tentacles is due to the fact that 

 the tentacle surmounting the primitive largest chamber arises before any 

 of the others, and long retains its supremacy (fig. 80 A). This fact, coupled 

 with the inequality of the two primitive chambers, supplies some grounds 

 for speculating on a possible descent of the Ccelenterata from bilaterally 

 symmetrical forms with dis- 

 tinctly differentiated dorsal and 

 ventral surfaces. The supre- 

 macy of the first-formed tentacle 

 is not confined to the Actinozoa, 

 but as has already been indi- 

 cated, is also found in the Scy- 

 phistoma (p. 166) of the Acras- 

 peda. 



After the twelve tentacles 

 have become established they 

 become secondarily divided into 

 two cycles of six respectively 

 larger and smaller tentacles, 

 which alternate with each other. 

 The two tentacles pertaining to 

 the two original chambers be- 

 long to the cycle of larger ten- 

 tacles. The mesenteric fila- 

 ments appear first of all on the 

 primary pair of septa. The 

 increase in the number of ten- 

 tacles and chambers from 12 to 24 has been found to take place in a very 



tl 



FlG. 80. TWO STAGKS IN THE DEYKl.op- 



MENT OK ACTINIA MESEMBRYANTHKMUM. 

 (After Lacaze Duthiers.) 



In the younger ciliated embryo A, viewed 

 from the side, only one tentacle is developed. 

 ///. mouth. 



The older larva B is viewed from the face 

 when 24 tentacles have just become established. 

 The letters shew the true order of succession of 

 the tentacles; but e and/are transposed. 



