G(ELENTERATA. 139 



the mesenteries on the inner side of the homogeneous membrane are 

 regarded by Kowalevsky as hypoblastic. 



A ciliated plaimla with delaminated hypoblast is also found in Gorgonia 

 and Corallium rubrum. In the former genus at the time when the larva 

 becomes fixed, the hypoblast is formed of two strata, an outer one of 

 columnar cells, and an inner one of round ciliated cells lining a central 

 enteric cavity. The inner layer is believed by Kowalevsky to become 

 eventually absorbed and to be homologous with the inner granular mass of 

 Alcyoniuni. 



Zoantharia. Amongst the Zoantharia several forms have been 

 investigated by Kowalevsky (147) and Lacaze Duthiers (170), of 

 which some are stated by the former author to pass through an 

 invaginate gastrula stage, while in other instances the hypoblast is 

 probably formed by delamination. 



To the first group belongs an edible form of Sea Anemone found 

 near Messina, Cerianthus, and perhaps also Caryophyllium. In the 

 first of these segmentation results in the formation of a blasto- 

 sphere. A normal invagination obliterating the segmentation cavity 

 then ensues, and the blastopore narrows to form the mouth. The 

 borders of the mouth bend inwards and so give rise to the gastric 

 cavity (stomodseum) which as in the Alcyonida? is lined by epiblast. 

 Simultaneously with the formation of the mouth there appear the 

 two first mesenteries. 



In Cerianthus the segmentation is unequal, the early stages are the 

 same as in the Actinia just described, but the hypoblast cells give rise 

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

 eventually absorbed. 



In the majority of the Zoautharia so far investigated, including 

 species of Actinia, Sagartia, Bunodes, Astroides, Astraea, the seg- 

 mentation, 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 stomodseum, and becomes the 

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

 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 larvse 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. 



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

 tation probably indicates an epibolic gastrala. 



