394 Dr. F. Miiller on Balanus armatus. 
could be determined as belonging to a large sulphur-yellow 
Papillina not uncommon here. ‘This Papillina (and perhaps, 
indeed, the whole genus Papillina, Schmidt), however, is no- 
thing but a Vioa which takes up its abode in shells and other 
calcareous structures, penetrates them, and in course of time 
almost entirely destroys them, and, finally growing over them, 
increases into large cake-like masses, which may attain a dia- 
meter of more than a foot. It was therefore doubtful in this 
case whether the Balanus had sought out the Sponge, or the 
Sponge the Balanus, especially as the shells were attacked by- 
the Sponge in several places. 
Subsequently I have frequently found Balanus armatus in 
abundance in a Reniera which, in shape, colour, and form of 
spicules, closely approaches 2. aqueductus, Schmidt, and is 
particularly characterized by its greatly developed fibrous 
framework, which is as readily washed out as that of the: 
common sponge. Very rarely (I have as yet only once found 
three specimens) the Ba/anus occurs in one of our commonest 
sponges, which coats whole rock-walls in the form of a dark- 
red mass beset with steep, jagged, mountain-like protuberances, 
and in its hard structure approaches Reniera digitata, Schmidt. 
On the contrary, it is frequent on an eight-rayed polype, 
Carijoa rupicola, F. Miill.*, which grows at the depth of 
* Carijoa rupicola (fig. 56). The entire stem of the polypary (which 
attains a height of 0°15 metre, rising straight or slightly curved, and is 
about 2 millim. in thickness, is formed by a single polype, which unfolds 
its tentacles at the extremity, and the body-cavity of which penetrates 
the whole stem. The polype can retract itself into the upper extremity 
of the stem. This retractile part is snow-white. The plumose tentacles 
are produced into a thin terminal filament, which appears nodose, like 
the slender lateral appendages. Beneath the circlet of tentacles there are 
some delicate calcareous spicules. The dissepiments surrounding the 
stomachal tube are continued throughout the whole length of the hollow 
stem as eight slightly projecting longitudinal lines: two of these, lying 
close together, bear an undulated membranous border with a thickened 
margin, in which the dark brownish-violet ova are developed, likewise 
throughout the whole length of the stem. The wall of the stem below 
is sometimes as much as 0°5 millim. in thickness; above, it’ becomes 
thinner and flexible; in the lower part the stem appears smooth; in the 
upper, softer portion it is traversed by eight longitudinal furrows. There 
are no projecting calcareous spicules.. The wall acquires firmness by 
closely approximated calcareous spicules, which are deposited in all pos- 
sible directions in planes parallel to the axis. These are straight or 
slightly curved, irregularly. beset with more or less numerous knots: 
some of them are longer (average 0:25, single ones more than 0°5 millim.), 
slenderer, and smoother than the rest; the others, which pass into the 
former by intermediate forms, are shorter and stouter, and beset with 
more numerous and stronger processes. The latter occur here and there 
fused together. bus 
From the stem spring numerous branches, usually four or five, at nearly 
