1916. | R. E. Lioyp: Campanulina ceylonensis, 53 
droid is not set directly upon the stolon but upon short inter- 
mediate branches which have the same structure as the stolon. The 
line of attachment of the hydroid to these intermediate branches 
is most conspicuous when the hydroid is contracted, but it is 
visible at all times as a sudden change in the character of the 
endoderm cells. 
The intermediate branch as well as the hydroid lies in the 
hydrothecae, 
The Hydrotheca. 
As a rule the hydrothecae arise directly from the perisarc of 
the stolon. Occasionally, however, they atise from short lateral 
branches which are common to them and to a gonotheca. They 
measure “45 mm. by ‘15 mm. (Annandale). They terminate in a 
conical pointed operculum consisting of a number of acute con- 
verging projections of the perisarc which together closely resemble 
the peristome of a moss capsule. The number of these processes 
is about twelve but they are difficult to count owing to their 
transparent delicacy. 
The method of the opening and closing of the hydrotheca is 
simple and effective, depending as it does upon the elasticity of 
the marginal projections and upon a slight upturning of their 
points. Because of their elasticity they tend to converge so that 
they are always in contact with the stalk of the hydroid even 
when it is fully extended. At first sight it seems that in this con- 
dition the converging points would hinder the withdrawal of the 
hydroid by catching in rugosities projecting from the surface of 
the stalk during contraction, and it also seems likely that the 
points would strangle the neck and so prevent the entry of the 
head into the theca. Such possibilities are prevented by the 
slight upturning of the points. The act of withdrawal is not 
easy to observe. Low degrees of magnification are insufficient for 
the purpose, but when using the high power it is of course neces- 
sary to cover the object which causes immediate and lasting con- 
traction of the hydroid. The difficulty was overcome by using an 
ocular of very high power with a low power lens. Ocular No. 18 
such as is supplied with Zeiss’ oil immersion lens combined with 
an objective 2/3 in. focus gave good results. A portion of a colony 
living undisturbed in a capsule of water was thus magnified about 
200 times so that the act of withdrawal could be observed. 
The Hydroid. 
The principal features of the hydroid are well known. The 
web or membrane uniting the bases of the tentacles being the 
chief characteristic. The fully extended stalk of a large individual 
measures about 2 mm. in length by ‘o6 mm. in thickness. In the 
living animal the ectoderm of the stalk is a very thin layer devoid 
of cell outlines or nematocysts. The outlines of the endoderm 
cells on the other hand are plainly visible even under the low 
power of the microscope In the expanded state each endoderm 
