CCELENTERATA. 169 



to a mass of fatty material filling up the enteric cavity, which becomes 

 eventually absorbed. 



In the majority of the Zoantharia so far investigated, includ- 

 ing species of Actinia, Sagartia, Bunodes, Astroides, Astraea, the 

 segmentation, which is often unequal 1 and not accompanied by 

 the formation of a segmentation cavity, results in a solid two- 

 layered ciliated planula. In these forms the impregnation takes 

 place in the ovary, and the early stages of development are 

 passed through in the maternal tissues. 



One end of the planula becomes somewhat oval and develops 

 a special bunch of cilia. At the other end a shallow depression 

 appears, which becomes deeper and forms an involution lined by 

 epiblast. This involution is the stomodaeum, and becomes the 

 so-called gastric cavity. The true enteric cavity lined by hypo- 

 blast is for some time filled with yolk material. The larva 

 always swims with the aboral end directed forwards. 



Between the two embryonic layers a homogeneous membrane 

 is formed, similar to that already described in the Alcyonidae. 



The further development of the larvae especially concerns the formation 

 of mesenteries, tentacles and calcareous skeleton. With reference to this sub- 

 ject the observations of Lacaze Duthiers are especially valuable and striking. 



In the adult it is usually possible to recognise 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 investiga- 

 tions 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 



1 I have this on the authority of Kleinenberg. The existence of an unequal 

 segmentation probably indicates an epibolic gastrula. 



