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PROCEEDINGS OF THE 



[April, 1857. 



head without tentacula, the medusa-buds being developed on that 

 portion of the stem included between this polyp-head and the bot- 

 tom of the case. They afterwards become free by passing be- 

 tween the lip of the horny case and the abortive polyp-head. 

 This is at least true for the Campanularians. Among the Ser- 

 tularians proper, especially such genera as Sertularia, Aglaophenia, 

 and Thuiaria, where the Medusae appear to be always abortive, 

 more investigation is needed — at least my limited access to Eu- 

 ropean authors has made me acquainted with no researches that 

 entirely clear up the question of the relation of their planules to 

 the fleshy parts of the polyp-stem.* But these groups differ, also, 

 by the more complicated development of the polypidom among 

 the Sertularina, and the position of the medusa buds. In the 

 group just named, there is a considerable tendency to an observ- 

 ance of some of the rules of vegetable growth in the medusa-bear- 

 ing capsules, as was first noticed by Edward Forbes. Each of 

 these capsules, like the flower-bud in plants, may be considered 

 as in some sense a modified branch of the polypidom, wherein all 

 the lateral polyps undergo their full development into sexual 

 Medusa?. Analogically, also, with the position of the flower-bud 

 in plants, though never at the extremity of a branch, so far as I 

 am aware, the medusa-capsules are generally at or near the axils, 

 so to speak, of branches or of individual polyps which may be 

 taken into the consideration as the analogues of the leaves in 

 plants. Among the Tubularina on the other hand, the disposition 

 is freer, and the medusa-buds are found among the tentacula, or 

 beneath them, of the individual polyps, and sometimes scattered 

 over the whole ramified stem of the polypidom. f 



The researches which have of late years been made by authors, 

 and which to a small extent I have had the opportunity of repeat- 

 ing, appear to me to add still another distinctive character to con- 

 firm this division of the Hydroids into two groups. It is the 

 manner in which the Medusa disk is developed. Among the 

 Corynidae, Tubularidse, &c, the Medusa emerges first as a bud, the 

 outer coveringof which becomes thatof the disk of the free swimming 



* In Hydractinia however which belongs, I think, to the Tubularina, the me- 

 dusa-buds which are naked, are also developed on the stalk of an abortive un- 

 tentaculated polyp. 



fl do not here investigate the question whether both these modes of distribu- 

 tion may not in the end be referred to single common plan. They are at least 

 different modifications, and as such I use them. 



