on MORPHOLOGY. 



through the intervention of its sexual buds. We are thus Ijrought up to an important point in 

 the developmental history of the Ilydroida, and are enabled to enunciate the following fundamental 

 proposition : 



The fixed plant-like Hydroida give origin to sexual buds, not only in the form of 



CLOSED sacs {the SpOrOSac), which develop within them the generative elements ; BUT ALSO 

 in THAT OF A MORE SPECIALISED FORM OF BUD, WHICH BECOMES A FREE (RARELY FIXED) MEDUSA, 

 AND THIS ULTIMATELY ATTAINS EITHER DIRECTLY {the gomcheme) OR INDIRECTLY [the Uasto- 



cheme) to sexual maturity, and produces ova or spermatozoa. 



But the point to which we thus arrive does not present us with the entire life-series of the 

 medusa-producing hydroid, for the important question still remains. What is the result, immediate 

 and remote, of the development of the ovum produced by the medusa? and how far does this 

 development correspond with that of the ovum produced in a sporosac without the intervention 

 of a true medusiform bud ? 



A considerable number of facts bearing upon this question have also been accumulated ; and 

 the development of the ovum formed in the medusa has been traced, with more or less minuteness, 

 by various observers, so that we are now enabled to present the terms which were still wanting to 

 complete the life-series of the hydroid. 



As the observations which have thus aided in completing our knowledge of hydroid 

 development are of great importance in the present inquiry, it will be necessary to give here some 

 account of those cases in which the development of the egg of the hydroid medusa has been 

 satisfactorily traced. 



Dujardin^ observed that a remarkable little medusa, which he described under the name of 

 Cladonema, was developed as a bud from a hydriform trophosome, to which he gave the 

 name of Stauridium. He had noticed the production of eggs by his Cladonema, and had also 

 seen young Statiridia developed from these eggs, though the planula stage seems to have escaped 

 him. 



Krohn," having placed in a jar of sea-water some mature specimens of Dujardin's Cladonema, 

 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-Uke embryo (planula). 



This embryo was successfully watched by Krohn through all the subsequent 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 Stauridium from which Dujardin had originally seen the Cladonema 

 thrown off. To Dujardin and to Krohn are thus due the first grand oliservations by which the 

 whole circle of hydroid develo])ment, in the case of a free phanerocodonic gonophore, has been 

 completed. 



^ Dujardin, 'Ann. des Sc. Nat.,' ser. iii, vol. iv, 1845, p. 273. 

 - Miiller's ' ArcLiv,' 1853, p. 420, tab. xiii. 



