336 
wards the base of the cirrus, and at the distal end of the row a 
transverse line of smaller hooks pointing upwards. In the anterior 
ramus of cirrus IV the median segments are provided with two or 
three hooks along the anterior median line pointing downwards. 
— The segments of the posterior cirri carry on their distal half 
two pairs of anterior spines; the distal pair is far longer than the 
lower; in many segments also a rudimentary third pair is indic* 
ated in the middle of the segment. 
The rami of cirrus I are very unequal; the shorter ramus has 
8 segments, and is half as long as the other ramus, which has 
16 segments. In cirrus II the shorter ramus measures 3 A of the 
longer one; the numbers of segments are 6 and 8. In cirrus III 
the rami are subequal, although both of them have 1 1 segments. 
All cirri of the animal are comparatively short. 
The penis is annulated, siender, and one and a half time as 
long as cirrus VI. 
Among the ykastø-species the most nearly related seems to be 
Acasta Dofleini, in which the carino-lateral compartment has the 
same rudimentary character as in the present species; but with 
this character the agreement seems to come to an end. Far nearer 
the relationship with Balanus arcuatus Hoek (1913) seems to be, 
and I should indeed have hesitated in regarding them as anything 
else and more than varieties, had not the present species been a 
distinet Acasta with its cupshaped basis. Special interest is afforded 
by the armature of the cirri, the hooks of which in cirrus III and 
IV seem to agree in every detail in the two species. This throws 
an interesting light on the affinities of the Acasta- groups. 
The present species is the more interesting because it also 
adds to the list of Acasta- species which are not inhabiting sponges. 
Of such species Acasta purpurata Darwin and Acasta hirsuta Broch 
are hitherto known from gorgonarians, Acasta antipathidis Broch 
from antipatharians, and now also the present species inhabiting 
madreporarians, Acasta madreporicola. In all these species the coral 
entirely surrounds the cirriped but for the aperture; whether the 
growth of the surrounding coral tissues puts a limit to the lifetime 
of the cirriped is at present an open question. 
An interesting phenomenon in the present species is afforded 
by the calcareous concrements which occur in great abundance in 
