~ 
ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 415 
the terms which were still wanting to complete the life-series of the 
hydroid. 
Krohn*, having placed in a jar of sea-water some mature specimens of Cla- 
donema (a medusa which, as we have already seen, Dujardin had previously 
proved to be produced by budding from a Coryne-like trophosome, to which 
he gave the name of Stauridium), observed that after a time they had 
deposited eggs, which adhered to the sides and bottom of the vessel. Soon 
after deposition, the segmentation of the yolk commenced ; and in about forty- 
eight hours after the beginning of the cleavage, the ovum had become 
changed into a free-swimming ciliated infusorium-like embryo (planula). 
This embryo was successfully watched by Krohn through all its subse- 
quent stages—the disappearance of its cilia, the fixing itself to the sides of 
the jar, its conversion into a little circular disc, the growth of a short 
column from the centre of the disc, and its final conversion into a hydroid, 
identical with the Stawridiwn from which Dujardin had originally seen the 
Cladonema thrown off. To Krohn then is due the first grand observation 
by which the whole circle of hydroid development, in the case of a free 
phanerocodonic gonophore, has been completed. 
Gosse t+ had seen the medusa, Turis neylecta, Forbes, discharge from the 
generative mass formed in the walls of its manubrium ciliated planule, which, 
after some time, fixed themselves to the glass, and became elongated into 
adherent, branched, stolon-like bodies, which threw up a perpendicular stem, 
on whose summit a circle of four tentacles was developed, and the whole 
became thus changed into a Clava-like hydroid. 
Wright ¢ ‘subsequently watched the development of the ovum in this same 
medusa. His observations agree with those of Gosse, but he has succeeded 
in tracing the development a step further; for he saw the tentacles increase 
in number by the growth of others behind those first formed, giving by their 
scattered disposition a still more Clava-like appearance to the hydroid, while 
he also noticed the formation of a chitinous periderm which clothed the 
creeping stolon. 
Gegenbaur § describes the development of the egg in a medusa, which he 
names Jizzia Kollikeri. He has seen the segmentation of the vitellus, and 
the formation of « ciliated planula, which, after enjoying for a time its loco- 
motive existence, loses its cilia, fixes itself to the side of the vessel, expands 
one extremity into a disc of adhesion, elongates the rest of its body into a 
cylindrical stem, which, after clothing itself with a chitinous polypary, deve- 
lopes a mouth upon its free extremity, and just below this throws out a 
verticil of tentacula, while the expanded base becomes extended into short 
stolon-like prolongations. 
The development of the ova in another medusa, named by Kéolliker 
Oceania armata, was also observed by Gegenbaur ||. He traced the seg- 
mentation of the vitellus, the formation of a ciliated planula, the fixation of 
the planula, and its development into a stolon-like body; but beyond this 
point his observations were not carried. 
Wright {| observed that numerous planule had made their appearance in 
a vessel in which he had placed some isolated specimens of Thawmantias 
tmeonspicua. He believes that these planule were produced by the Thau- 
* Miiller’s Archiv, 1853, p. 420. tab. xiii. 
+ A Naturalist’s Rambles on the Devonshire Coast, 1853, p. 348. pl. 13. 
{ Edinb. New Phil. Journ. July 1859, pl. 8. f. 1. 
§ Generationswechsel, 1854, p. 23. pl. il. figs. 1-9. 
|| Loc. cit. p. 28. pl. 2. figs. 10-16. 
§| Micr. Journ. vol. ii., new ser. 
