402 W. K. BROOKS ON THE LIFE-HISTORY 



on "Segmentation and the formation of the Gastrula" that he was induced to study the 

 subject by his belief thai a planula stage did not exist and that the published accounts 

 were wrong. He says, however, that in his studies of the embryos of Tubularia, Aglao- 

 phenia and several PlumularidsB, which were entered upon in this frame of mind, he was 

 unable to find anything like the formation of a g-astrula by invagination; that the paper 

 by Ciamician, in which the invaginate gastrula of Tubularia is figured, is a conglomera- 

 tion of errors, and that in all the forms which he himself has studied, the embryo be- 

 comes a planula like that which Schultze had described for Cordylophora; but while his 

 appeal to nature leads him to these facts he says, "I hold that while the hydroid planula 

 does in fact originate by delamination, without a segmentation cavity, nevertheless the 

 planula is just as truly a gastrula as it would be if it originated by epibole or in any other 

 manner," and that Balfour's view that the planula stage of development represents a free 

 swimming ancestral form in the history of the Ccelenterata, in which the mouth and the 

 digestive cavity were absent, is untenable. 



If we believe that the gastrula stage of the higher metazoa is the representative of 

 an ancestral form like the adult hydroid, we certainly should not expect to find a gas- 

 trula stage in the embryology of the hydroids themselves; and the analogy of the ani- 

 mals above the hydroids is no reason for supposing that the planula is a modified gastrula 

 if we believe that these forms art' the descendants of an ancestral form which was itself 

 a divergent branch from the ccelenterate stem. The planula stage is certainly dominant 

 among the sponges, and the so-called gastrula is here beyond doubt a secondary larva. 

 Kowalevsky has shown that the embryo of Lucernaria is a planula (40) and Fol states 

 (22) that his examination of the embryos of Pelagia has shown the need for a renewed 

 examination of the alleged gastrula stage, while Wilson (67) shows that the Iienilla em- 

 bryo is certainly not a gastrula. and as there is not a single observation of a gastrula in 

 the Hydromedusae, we may, so far as this group is concerned, continue to speak of the 

 larva as a planula. 



Hamann says that since we have a planula in some hydroids. and instead of this, an 

 actinula in others, we are compelled to believe that the life-history of the lower Coelen- 

 terates is considerably modified and does not give us the primitive condition of things; 

 but his own observations show that the actinula of Tubularia is not the equivalent of the 

 planula of other hydroids, but that it is preceded by a planula stage, although this is 

 not free but contained within the medusa-hud. and not being locomotor has no cilia. 



Merejkowsky, who has given us a minute account of the planula of Obelia (50), says 

 that he found a few embryos of Irene, in what seemed to him to be an invaginate gas- 

 trula stage, but he made no minute study of them and did not rear them. Irene is very 

 closely related to Eutima, and it is interesting to note that, in Euthna, after the endoderm 

 and the digestive cavity are formed, and before the appearance of the mouth, there is an 

 ectodermal invagination which is possibly what he has seen in Irene, although the study 

 of the late stages shows that it is not a mouth, but an ectodermal adhesive gland. At 

 the stage which is shown in fig. G, there is a mouth-like aperture f at the small or pos- 

 terior end of the planula, and in the living animal it is easy to s^e that this is formed by 

 an invagination from the surface. In a specimen which has been killed with osmic acid 

 and stained with picro-carmine, fig. 5, it is still more conspicuous, and is seen to be a 



