414 W. K. BROOKS ON THE LIFE-HISTORY 



which carries the circlet of tentacles of the Campanularian hydroids, or the aboral ten- 

 tacles of Pennaria, is homologous with the bell margin of the medusa with its tentacles. 

 The velum is not represented by any distinct hydroidean structure. 



The digestive cavity of the medusa with its tubes or pouches is quite different from 

 that of a hydroid, although the history of the origin of these parts in medusa-buds, or 

 in the egg-embryo of Liriope or in Cunina, shows that the union of the ex-umbral and 

 sub-umbral layers of endoderm has converted the peripheral portion of the simple diges- 

 tive cavity of the hydroid into radial canals or pouches arranged around the central 

 stomach of the medusa, which therefore does not correspond to the whole stomach of the 

 hydroid, but only to its axial or central portion, while its peripheral portion is homolo- 

 gous with the canal system of the medusa. For a more extended statement of the sub- 

 ject see Koch (38) and Haeckel (78). 



There can be no doubt that this, the generally accepted view, is correct, and the 

 fact that the hydra-larva of the Trachomedusae and Xarcomedusse becomes directly con- 

 verted into a medusa, furnishes very direct and conclusive proof. I doubt, however, 

 whether it could be so satisfactorily established if we were not acquainted with these 

 forms; for many of the phenomena in the life of the Antho- and Leptomedusae which are 

 urged in proof of it are in themselves inconclusive. The retrograde metamorphosis, 

 which according to Van Beneden (8), Hinks (33), Allinan (6) andMerejkowsky (50) 

 often results in the conversion of a medusa into a hydra-like organism, through the dis- 

 appearance of the umbrella and the return to a sessile habit, seems to show that the two 

 forms are mutually convertible; but Merejkowsky confirms Van Beneden's statement 

 that in these cases of degeneration the resemblance to a hydra is entirely superficial. 



The fact that the chymiferous tubes of a medusa-bud are formed by the development 

 of areas of adhesion in the lateral portions of a digestive cavity which is at first contin- 

 uous like the stomach of a hydra, is often adduced as evidence of fundamental similar- 

 ity, but there is no reason for believing that the ontogenetic history of developing buds 

 repeats the phylogenetic record. The history of Cunina and Liriope shows that the 

 peristome of the hydra is the sub-umbrella of the medusa, and if bud-ontogeny were a 

 recapitulation of phylogeny, we should expect the sub-umbrella of a budding medusa to 

 arise as it does in Cunina; but we find, on the contrary, that in the medusa-buds of all 

 the Campanularians and Tubularians, as well as in the Siphonophores, it originates as a 

 bud nucleus, KnospungsJcern, which gives rise by splitting, before the mouth is formed, to 

 the sub-umbrella, which has at first no opening to the exterior. It is therefore unsafe to 

 t rust any of the evidence furnished by bud-embryos ; but the evidence from egg-embryol- 

 ogy is not open to this doubt, for in all cases where there is no reason to suspeel second- 

 ary modification, we may safely regard this as a recapitulation of phylogeny; and the 

 life-history of those few medusa? which develop directly from the egg is therefore of the 

 greatest importance as a basis for comparison of the medusa with the hydra. 



In all these egg-embryos, the "bud-nucleus " is absent, the mouth appears before the 

 bell cavity makes its appearance and this is never closed but is formed by the folding 

 of the body; while the chymiferous tubes arc formed by the modification of a simple di- 

 gestive cavity as we should expect if the hydra and the medusa are representatives of the 

 same fundamental type. 



