ON THE DEN^ELOPMEXT OF THE PLANULA IN A 

 CERTAIN SPECIES OF PLUMULARIA. 



P>y Prof. Ernest Warrek, D.Sc. 



The present brief paper on the peculiar mode of develop- 

 ment of the plannla in a hydroid constitutes a summary of a 

 more detailed account with plates, which will be published at a 

 later date in the " Annals of the Natal Museum." 



The species of hydroid is probably new, and was found in 

 January, 191 1, on an oyster shell in a rock-pool just north of 

 the mouth of the St. John's River, Pondoland. 



The hydroid has a creeping hydrorhiza, which sends upwards 

 pinnafte stems about f inch in height. The hydrocaulus in the 

 region of the pinn?e consists of only hydrothecate internodes, 

 which carry a median sub-calycine nematophore and a pair of 

 supra-calycine nematophores. The pinn?e are peculiarly short, 

 and carry, as a rule, only two hydrothecse. The pinnae arise 

 alternately on the right and left; there is a basal non-thecate 

 internode articulating with a slight protuberance of the hydro- 

 caulus internode, then follows a somewhat short, non-thecate 

 internode with a median nematophore, then a thecate internode 

 with a median sub-calycine and paired supra-calvcine nemato- 

 phores, followed by a non-thecate joint, and terminally there is 

 a second thecate internode. The pinnse usually bear a short 

 pinnule which arises on the distal edge seated on a slight 

 prominence springing just above the first theca. The pinnule 

 carries only one hydrotheca. 



The gonothec?e. both male and female, are frequently borne 

 on the same i^innate stem. The female gonothecte spring from 

 a spot just below the hydrotheca of the main stem, and on one 

 side of the sub-calycine nematophore. Frequently three or four 

 of such gonothecre arise in succeeding internodes, the oldest and 

 largest being the proxim'al one. 



The male g'onothecs arise on the more proximal pinn.x just 

 below the i^roximal theca. 



Here, as in other hydroids. which bear the sexual organs of 

 both sexes, it may be noticed that the female organs are carried 

 on the main axis where the available nutritive supply would be 

 greater, while the male organs arc borne in places where the 

 nutritive supplies are smaller. 



This fact is suggestive that the determination of sex is at 

 least partly dependent on the amount of available nutriment. 



The male gonotheca is small, and consists of an elongated 

 theca with a well-marked " covering plate " at the apex. Inside 

 there is a blastostyle carrying a sperm-mass. 



The female gonotheca when mature is broadly ovate. 

 When young the covering plate (Deckcnplattc) is verv marked. 

 Later the gonotheca becomes inflated and encloses a median narrow 



