No. 3.] MORPHOLOGY OF THE ACTINOZOA. 



309 



embryos with the depression and the fully formed gastrula sur- 

 prised me somewhat, but still the appearances presented by 

 optical sections seemed so clear, and the gastrula so similar 

 to an invaginate gastrula, that I believed that I had missed 

 or overlooked the intermediate stages. Sections, however, 

 explained the matter at once, and demonstrated that instead 

 of with invagination we have to do with delamination. I make 

 special mention of this mistake, since, as I shall shortly endeavor 

 to show, other observers have probably fallen into the same 

 error. 



As to what becomes of the yolk-spherules occupying the cen- 

 tral cavity, I have no evidence. It is possible that they may be 

 absorbed by the endoderm cells as soon as they have reached 

 their final differentiation, or they may pass to the exterior 

 through the blastopore and be lost to the larva. 



Beyond this stage I was unable to rear the embryos. A few 

 Actinozoan larvae obtained by skimming are almost certainly 

 those of Metridium, but I prefer to leave them undescribed for 

 the present, partly on account of the uncertainty of the identifi- 

 cation and partly because they belong to later stages which may 

 be more satisfactorily studied in the larger embryos of Rhodactis 

 and Atdactinia. 



Between the latest stage of Metridium and the earliest which 

 I possess of Rhodactis, there is an hiatus, during which there 

 occurs the formation of a very important and characteristic 

 structure, the stomatodaeum. Still, from what occurs in other 

 forms, we may readily imagine how its formation takes place in 

 Metridium. CeriantJms apparently has a stage almost if not 

 quite identical with the last stage of Metridium described, and 

 in this form the lips of the "gastrula " mouth bend in to form 

 the two-layered stomatodaeum. It seems probable that the 

 same thing occurs in Metridium. 



A comparison of the methods of formation of the germ-layers 

 in the Actinozoa, so far as they are known, is of considerable 

 interest. Our knowledge is derived from the study of only a 

 relatively small number of forms ; nevertheless, it suggests some 

 important ideas. The amount of food-yolk seems to have a very 

 important influence on the details of the development, but the 

 general mode of formation of the germ-layers is the same in the 

 majority of forms. Amongst the Alcyonaria we have accounts 



