168 ZOANTHARIA. 



communication with the spaces between the mesenteries 1 . During the next 

 stage the young Alcyonium also acquires eight tentacles, which arise as 

 hollow papilke opening into the eight mesenteric chambers. By this stage 

 also the matter filling up the mesenteric chambers is nearly absorbed. 



Between the epiblast and hypoblast there is formed an homogeneous 

 membrane, which penetrates in between the two layers of hypoblast 

 which form the mesenteries. On the outer side of this membrane, and 

 therefore presumably derived from the epiblast, is a layer of connective- 

 tissue cells, which eventually gives rise to the abundant gelatinous tissue 

 (ccenenchyma) in which the skeletal elements are deposited. In Sympodium 

 coralloides Kowalevsky (No. 168) has shewn still more completely the deriva- 

 tion of the stellate mesoblast cells from the epiblast. He finds that the 

 calcareous spicula develop in these cells as in the mesoblast cells of 

 sponges. The branched gastrovascular canals in this tissue are out- 

 growths of the primitive enteric cavity. A layer of circular muscles is 

 formed at a late period from the epiblast, but the longitudinal muscles of 

 the mesenteries on the inner side of the homogeneous membrane are 

 regarded by Kowalevsky as hypoblastic. 



A ciliated planula 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 

 Alcyonium. 



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 

 blastosphere. A normal invagination obliterating the segmenta- 

 tion 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 (stomodaeum) which as in the Alcyonidae 

 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 



1 The German abstract is very obscure as to the formation of the mouth. 



