215 
on the same stem. — From the primary stolon frequently another 
hydranth is developed, being the primary hydranth of another stem. 
— The primary stolon may grow further ahead; in a couple of 
examples from .the glass-plate the stolon, which bears two hydranths, 
has a comparatively considerable length and has undoubtedly in- 
creased after the formation of the first hydranth. — Billard states 
(p. 46) that the primary stolon does not send out lateral stolons. 
This is not correct. I have seen two examples of development of 
lateral stolons, one of which is represented in fig. 6. 
In opposition to Allman who was of the opinion, that the 
spontaneous fission did not occur at the same time as sexual indiv- 
iduals were produced by the colonies, Billard states that the 
formation of frustules is specially intense at the same time as med- 
usæ are dismissed from the colonies. — As far as my own obs- 
ervations in this respect are concerned, the facts are as follows: 
The colonies of L. longissima which I observed at Plymouth sent 
out frustules and medusæ at one and the same time. The material 
of L. geniculata from Frederikshavn, on the other hand, shows 
the interesting fact, that some colonies have developed large num- 
bers of frustules but have no traces of gonothecæ, while other 
Colonies, collected at the same place and on the same day, pos- 
Sess abundance of gonothecæ- filled by medusa-buds, but in these 
Colonies no frustules are present. 
If we ask of the cause, why such fission takes place, we will 
find that the opinions are very different. — As mentioned above 
Allman thought, that the ability of producing frustules by fission 
Was a specific character of the species in which it was observed 
by him, — Mereschkowsky saw the formation of frustules on 
a branch, placed under a cover in very little water; he sought 
the cause in the circumstance that the water became more and 
MOre salt owing to the evaporation, or in the lack of oxygen. 
Billard gives another explanation. He calls attention to the 
facts 1) that all the species in which the reproduction by fission 
is known are found in abundance at localities, where there is a 
Strong movement in the water (except in the case of Leptoscyphus 
tenuis in which special circumstances play a part); 2) that if a 
Colony is placed in a jar, to which there is carried a current of 
air, but in which the water is kept quiet, frustules may well be 
