POLYZOA AND TUNICATA. 
231 
animals. In the specimens which you pick up on the 
beach nothing more than this would be seen, however 
closely you might examine them, hut in our sample just 
taken out of the water, we shall be able to shew much 
more. The broad orifice surrounded by spines is covered 
during life with a very delicate clastic membrane, stretched 
across it like the head of a drum, and towards the front 
of this there is a slit with two lips in the form of a cres- 
cent. From the lips of the slit a case or tube of the 
thinnest possible skin passes through the middle of the 
cell, within which plays a polype to and fro, capable of 
protruding itself to a great extent, as well as of retreating 
far into the depths of its cell, according to its pleasure. 
In the latter state the animal is bent upon itself with a 
double fold, somewhat like a person lying in bod with his 
knees tucked up ; but when it wishes to expand itself, 
two pairs of muscles, which are seen in the form of threads 
on each side, draw the body forward, and the skin that 
envelopes it turns inside out, just as we draw off a stock- 
ing. It gradually protrudes from the slit as it unfolds, 
and then displays a number of long tentacles which are 
regularly packed side by side in a bundle ; hut when these 
are quito protruded, they suddenly expand into the shape of 
an elegant bell or cup. The appearance of the animal is 
now very beautiful ; it is as clear as spun glass ; the 
sprightly motions which it exhibits increasing its beauty. 
Each of” the thread-like tentacles seems to have a double 
row of black teeth, like the cogs of a watch-wheel which 
run rapidly along in ceaseless course ; those of one side of 
the tentacle running towards the tip, and those of the 
other side towards the base. Striking and beautiful, 
