OF THE HYDEOMEDUS^. 401 



carmine, they arc flatter than they are in the living animal, and it is easy to see that 

 they are arranged in a single layer to form the walls of the digestive cavity. Fig. 6 is 

 a stained specimen of the same age as the one shown in fig. 5. In a surface view, c, the 

 rounded ectoderm cells are seen and by focussing a little deeper, the polygonal outlines 

 of the granular endoderm cells, e, come into view, while still deeper focussing shows an 

 empty space, g, the stomach, around the edges of which the single layer of endoderm 

 cells is seen in sectional view. 



There can, of course, be no doubt that in most hydroids, the planula is at first solid, 

 and that the digestive cavity does not make its appearance until some time after the 

 cells are specialized into an ectoderm and an endoderm, and I think that the persistence 

 of the segmentation cavity of Eutima as the digestive cavity and the absence of a solid 

 stage must be regarded as a secondary modification of the ancestral history, although it 

 is not impossible that the manner in which the endoderm and digestive cavity are formed 

 in the Geryonidre (see below) maybe the primitive one and the solid stage a secondary 

 phenomenon. 



The fact that a solid stage occurs in so many hydroids, in the Acraspeda (see Kowa- 

 levsky, 40) and in the Anthozoa (see Wilson, 67), as well as in the sponges, would seem 

 however to indicate that the early appearance of the digestive cavity in Eutima and in 

 the Geryonidse is not primitive but secondary. 



I shall not enter upon the discussion of the relation of the embryology of the Hydro- 

 medusa? to the gastrula theory, further than to point out that not a single hydroid gas- 

 trula has been observed; but that, in every species which has been studied, the digestive 

 cavity has at first no opening to the exterior, and that the mouth is formed very much 

 later than the stomach. Most writers believe, it is true, that the planula is a modified 

 gastrula, and that its digestive cavity was originally iuvaginated from the exterior, but 

 this is purely a deductive inference from the analogy of other animals. Thus Claus (14) 

 describes the origin of the endoderm and digestive cavity of ^Equora and Merejkowsky 

 that of Obelia (50) as like that of Eutima, but both these writers state their opinion that 

 the planula has originated through a modification of a primitive invaginate gastrula. 

 Bolnn says (p. 153) that it is natural to derive the Hydromedusse and sponges from an 

 ancestral gastrula, since in no other group is descent from this form so certain. 



No one can question the resemblance between an adult hydroid or sponge, and the gas- 

 trula stage of the ordinary metazoa, and there is every reason for believing that the 

 almost universal occurrence of this larval stage indicates that the coelomatous metazoa 

 are the descendants of an ancestral form which was essentially like the existing ccelen- 

 terates and that these themselves are the divergent modifications of a common type, the 

 gastrula or two-layered metazoon, with stomach and mouth; but it is quite conceivable 

 that the cu'lentei ales themselves may be the descendants of a form with a stomach, but 

 without a mouth, and that the planula stage may be the ontogenetic representative of this 

 just as the gastrula stage is the ontogenetic representative of the adult coelenterate. 

 Most writers have started however with the assumption that, as the hydroids must be the 

 descendants of a gastrada, the planula must be a modified gastrula ; and one writer (32) 

 has, with the greatest simplicity, given us the chain of reasoning which has led him to 

 supply a missing gastrula stage in the life of the hydroids. ilamann says in his section 



