OF THE HYDROMEDUS^E. 413 



The union of the sexes is so important to animals which are not locomotor that, among 

 the Arthropoda, a group which includes far more than half of all the animals known to 

 us, the sessile barnacles are almost the only hermaphrodites. 



If the medusae have been formed by the specialization of members of a community, 

 and if the sessile hydroid eormi are, as this hypothesis requires, very old and primitive, 

 we should certainly expect, to find them exhibiting the power to produce from a single 

 cormus medusas of both sexes, for Hydra, which is one of the most primitive hydroids, 

 is hermaphrodite. The polymorphism hypothesis gives no explanation of the remarka- 

 ble fact that, with the exception of Hydra, all the numerous descendants, often many 

 thousand in number, of any particular planula, are of one sex; but we can readily under- 

 stand how this might be the case if the fixed hydroid cormi have been produced as I 

 suppose, for if the sexes are distinct in adult medusa?, the larva of any particular me- 

 dusa must be either a male or a female, and there would be nothing strange in the fact 

 that its gemmiparous offspring should resemble it in this respect. 



Section VI. 



A Review of the Literature on the Relation between the Hydra and 

 the Medusa and on the Origin of Alternation of Generations. 



The fundamental similarity between a hydra and a medusa is so obvious that it hardly 

 seems necessary to dwell upon it, but the history of opinion upon the subject shows that 

 this has been by no means uniform, although nearly all naturalists now agree that a sin- 

 gle hydra is directly comparable or homologous with a single medusa, and that the va- 

 rious hydromedusse are also directly comparable with each other; that both the hydra 

 and the medusa are in that stage of individuality to designate which HaeekcTs term 

 ''person" is now almost universally employed. 



The general plan of structure is very much alike and the history of such forms as the 

 Geryonidas and xEgiimhe, where the hydra-like larva becomes directly transformed into 

 the adult, shows that a medusa is little more than a hydra with sense organs and a loco- 

 motor apparatus. The hydroids are not furnished with sense organs, although there 

 is every reason to believe that the sense organs of the Narcomedusse arc modified ten- 

 tacles, homologous with the solid tentacles of hydroids; and I know of only one writer 

 who does not regard the cavity of the sub-umbrella of the medusa as the homologue of 

 the space which lies on the oral side of the circlet of tentacles in such a hydroid as Eu- 

 tima, PI. 38, fig. 10. The mouth of the hydroid is homologous with the mouth of the 

 medusa, and where this is mounted upon a proboscis or manubrium, this structure is di- 

 rectly comparable with the proboscis or pendent stomach of the medusa. 



The ectoderm of the peristome of the hydroid, or the area included between the bases 

 of the tentacles, is homologous with the ectoderm of the sub-umbrella and proboscis of 

 the medusa; while the convex dorsal or aboral surface of the hydroid corresponds to the 

 convex ex-umbrella of the medusa. 



The oral tentacles of such a hydroid as Tubularia or Pennaria are to be compared will: 

 those of Margelis and I regard them as strictly homologous structures, while the zone 



