384 W\ K - BROOKS ON THE LIFE-HISTORY 



now unite with each other over four interradial areas of adhesion which increase in size 

 and convert the peripheral portion of the digestive cavity into a circular tube and four 

 radial tubes. 



8. The veil is formed around the periphery of the peristome. The solid radial hydra- 

 tentacles disappear and the solid interradial tentacles and the hollow radial medusa-ten- 

 tacles are developed. The larval tentacles do not disappear until all the characteristics 

 of the medusa are accpiired, so that there is a period, before maturity is reached, when 

 the animal is both a hydra and a medusa. 



9. During the hydra period there are no marginal sense organs. 



Literature of the Development of the Geryonid.e. 



In 1850 Leuckart pointed out (47) the fact that the young Geiyonid medusa is 

 epiite different from the adidt, and that its growth is accompanied by metamorphosis; and 

 in 1857 Gegenbaur figured and described (25, p. 247, PI. 8, fig. 12) a young Geryonid 

 under the name Eurybioj>sis anisostyla. Fritz Muller's minute and amply illustrated 

 account of Liriojye catherinensis, published in 1859 (55), is the first in which the absence 

 of an alternation of generations is established. He uives an account of the metamorpho- 

 sis of the medusa, and shows that the young embryo is a double spherule of cells, and 

 that the central capsule has, atfirst.no opening; and he also figures an older embryo with 

 a mouth, but without tentacles, although he supposed that the central cavity was the 

 sub-umbrella, that the mouth was the opening of the umbrella, that the peristome was 

 the veil, and that the embryo has, at this stage, no mouth or digestive tract. In his 

 classic monograph, published in 1866, Haeckel gives beautifully illustrated figures of the 

 metamorphosis of Glossocodon eurybea, (30 b) and Carmarina hastata (3(W); but he 

 falls into Fritz Muller's error regarding the embryo, and describes the endoderm as the 

 sub-umbrella, stating that the digestive tract and ehymiferous tubes are formed, at a la- 

 ter stage, by the differentiation or specialization of the sub-umbral ectoderm. In 1873 

 Fol (22) reared Carmarina fungiformis from the egg, and gave a complete account 

 of its development, illustrated by beautiful figures, showing that the central cavity is the 

 digestive cavity and that its endodermal cells arise by delamination from the blastoderm, 

 that the mouth appears later, and that the ectoderm around it becomes the sub-umbral 

 epithelium, around the edges of which the veil and tentacles are developed. Metschni- 

 koif states (52) that his observations (51) are a year earlier than Fol's, but as Fol'^ paper 

 appeared 'Nov. 18, 1873, while Metschnikoff's was not published until Jan., 1N8I, the 

 discovery belongs to Fol, although Metschnikoff's observations, which were made at 

 Yillafranca in 1870, agree with Fol's in all essential particulars. 



Kowalevsky's Russian paper, which appeared in the same year, 1874, gives a totally 

 different account of the early stages, as observed in Carmarina hastata. According to 

 Metschnikoff's statement (52) and Leuckart's abstract in the Arch. f. Naturgeschichte, 

 Kowalevsky observed the origin of the central capsule by delamination, but decides that its 

 cells become converted into the gelatinous substance of the umbrella, and have 1 nothing 

 to do with the digestive tract, which originates by invagination at a later period. 



We therefore have three irreconcilable statements as to the fate of the central capsule 



