HYDROIDEA. 



23 



Fig. 7. 



Fig. 8. 



Plumularia. 



Sertularia. 



may be early distinguished within them, and are often arranged 

 along a central axis, each communicating, according to Lister, with 

 the common axis of the zoophyte.* My associate, Dr. Charles 

 Pickering, first pointed out to me, while at sea, in 1838, that a close 

 analogy subsists between the arrangement of the ovules in a vesicle 

 and a contracted branchlet of the 

 zoophyte.f The same subject 

 has been thoroughly investigated 

 by Professor E. Forbes, and the 

 fact of this arrangement fully 

 ascertained. J In consequence of 

 the communication with the axial 

 cavity of the zoophyte, the pulpy 

 chyloid fluid of the main stem 

 and branches is carried into the 

 vesicle and to each ovule, and the developement of the whole pro- 

 moted. On arriving at maturity, the ovules pass out in succession 

 from the sac, which, now empty, falls off. They are carried about 

 for awhile by means of their vibratile cilia, and then perhaps in two 

 or three days they affix themselves. Each now grows and buds, 

 till shortly "a whole grove of Corallines" is formed. 



According to Van Beneden, the Campanularidse, when first deve- 

 loped from the ovule, are like minute Medusae in shape, and have 

 eight eyes, which are lost as the animal attaches itself. In this state, 

 it has no vibratile cilia. This same author has very minutely in- 

 vestigated the TubularidaB, and finds in them the same mode of 

 developement, and eight eyes to the medusa-shaped young, at the base 

 of the tentacles. Dalyell seems to have observed similar facts. He 

 states that the ovules, which in this g'roup are collected about the 

 bases of the tentacles, drop from their attachment for evolution below. 

 Slight prominences soon denote incipient tentacles ; next the nascent 

 animal reversing itself, enjoys the faculty of progression by means of 

 the inverted tentacula, as on so many feet, apparently to select a site ; 

 when again resuming the natural direction, with the extremities up- 

 wards, the lower surface fixes itself below and roots there for ever.|| 



* J. J. Lister, Phil. Trans., 1834, pp. 365-389, pi. ix. and x. 



f Figures 7 and 8 are by Dr. Pickering ; they were drawn from gulf-weed species, in 

 September of 1838, at the time the above-mentioned observation was made. 

 $ Proceedings of the British Association, for 1844. 

 Van Beneden, Mem. sur les Campanulaires, &c. Brussels, 1844. 

 || Rep. Brit. Assoc., for 1834, p. 600. 



