140 ZOANTIIARIA. 



. In the adult it is usually possible to recognize in the tentacles a 

 symmetry of six. There are six primary tentacles, six secondary, twelve 

 tertiary, twenty-four quaternary, etc. In the hard septa of the skeleton 

 the same law is followed up to the third cycle, but beyond that, in the 

 cases where the point can be verified, there appear to be only twelve septa 

 in each additional cycle. The observations of Lacaze Duthiers have shewn 

 that this symmetry is only secondarily acquired and does not in the least 

 correspond with the succession of the parts in development. 



His observations were conducted on three species of Zoantharia without 

 a skeleton, viz. Actinia mesembryanthemum, Sagartia, and Bunodes gem- 

 macea; while Astroides calycularis served as the type for his investi- 

 gations on the corallum. It will be convenient to commence with his 

 results on Actinia mesembryanthemum which served as his type. 



The free cylindrical embryo, with the aboral end directed forwards in 

 swimming, first becomes somewhat flattened and the mouth elongated. A 

 bilateral symmetry is thus brought about. Two mesenteries now make 

 their appearance transversely to the long axis of the mouth, which divide 

 the enteric cavity into two unequal chambers. The mesenteries consist of 

 a fold of hypoblast with a prolongation of the epiblast between the 

 two 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 

 featui*e 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 distinctly differentiated dorsal 

 and ventral surfaces. The supremacy of the first formed tentacle is 

 not confined to the Actinozoa, but as has already been indicated, is 

 also found in the Scyphistoma (p. 137) of the Acraspeda. 



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 belong to the cycle of larger tentacles. The 



