SOME OBSERVATIONS ON ACINETARIA. 645 
attack and suck small ciliate infusorians, the tentacles of the 
proboscis behaving in the same way as the ordinary acinetan 
tentacle. 
It is probable that the food of the species of Ophryoden- 
dron which live upon crustacea is derived entirely from 
ciliate protozoa in this way, as has been shown by Milne 
(loc. cit.). Ophryodendron sertulariz, in its method 
of feeding would seem to resemble Ophryodendron abie- 
tinum, though it is probable that it is not so exclusively 
parasitic in its diet as Ophryodendron abietinum. 
I found Ophryodendron sertulariw growing on 
Sertularia pumila in 30 fathoms at Plymouth in July 
crowded with the nematocysts of Sertularia, and the nemato- 
cysts were very common wn Ophryodendron sertularie 
collected from the shore at Millport in November. On the 
other hand, there were very few nematocysts in some Ophryo- 
dendron of the same species collected from the shore at Ply- 
mouth in July. 
On examining fixed preparations of Ophryodendron 
abietinum by far the greater number of individuals is found 
grouped around the opening of the hydrotheca of the hydroid, 
but a few are found scattered over the main stem. In the 
case of the latter, it is at first sight rather difficult to believe 
that the tentacles of the Ophryodendron can reach the ecto- 
derm of the hydroid, but it must be remembered that the 
tentacles of the hydroid in the living condition are usually 
held back well over the base of the hydrotheca, and that the 
proboscis which in the living resting form of Ophryodendron 
measured about 66 uw inlength, when expanded measured 332 uu. 
In an Ophryodendron abietinum which was drawn 
while feeding, it was noticed that the tentacles of the Ophryo- 
dendron were wrapped around the tentacles of the hydroid. 
After a short time the proboscis of the Ophryodendron was 
retracted, and the nematoblasts with their contained nemato- 
cysts could be seen sticking for some time in the aperture of 
the tentacles, giving the tentacles a curious knobbed appear- 
ance. It is this appearance that is possibly responsible for 
the figures of knobbed tentacles in Ophryodendron. 
