216 
developed, but not dismissed; they remain within the fine chitinous 
tubes, which grow continually in their distal end and may reach 
a considerable length. 
It is very probable that a certain amount of movement ef the 
water is necessary in order that the chitinous tube may be torn 
asunder and the frustule deliberated. But it seems to me to be a 
somewhat hasty inference, when Billard finds the direct cause 
of the occurence of the phenomenon in the current or other move- 
ments of the water. Moreover this theory is contradicted by his 
own experiments: in the case mentioned above, where the hydroids 
were kept in unmoved water, frustules were developed nevertheless, 
only not deliberatéd. Billard confirms the same opinion in his 
note on spontaneous fission in Perigonimus (Billard 1911), and 
in a foot-note in the same paper he appeals to a paper of H al- 
lez (1905), whose experiments have given a corresponding result. 
The experiments of Hallez, however, are so brutal, and the 
conditions under which he placed his colonies so far from any nat- 
ural condition, that no proof may be taken from his results. Hallez 
placed some colonies of "Obelia flabellata" in a jar provided with 
a system of tubes by which the water in the jar might be moved 
in various directions and at various speeds, and moreover he whirl- 
ed round the colonies with a velocity of 30—160 turnings in the 
minute. As a matter of course several peculiar things happened 
to the unfortunate colonies, and one of the results was the  form- 
ation and, deliberation of frustules which were able to stick to 
solid objects, when they found a quiet spot. I won't deny that such 
experiments may have some interest, though I impute more import- 
ance to experiments, in which natural conditions are imitated In 
the aquarium. In any case the experiments of Hallez yield 10 
proof of the theory of Billard, that the development of frustules 
should be a direct result of the movement of the water. I would 
rather explain the formation of frustules on the colonies in question 
as the result, not of the strong movement itself, but of other ab- 
normal conditions, caused by the movement, starvation for instance. 
The colonies of Laomedea longissima in which I observed the 
spontaneous fission during my stay at Plymouth, had been placed 
in a jar in which the water was kept moving, that is true. But it 
was not at all a strong movement. The diameter of the plunger” 
