THE YOUNG SCIENTIST. 
115 
tant division of the mollusca. The Fohj- 
zoa are chiefly marine, and the common 
"sea-mat," often erroneously treated as 
a sea-weed, is a well-ltnown form. A spe- 
cies of another order often piclced up on 
our coasts is the Sertularia, or Sea-Fir, 
composed of delicate branching stems of 
a horny-loolcing substance, which, under 
a pocicet lens, is found to contain an im- 
mense number of small cells inhabited 
by Polyps. It is instructive to compare 
the two and note how much more ad- 
vanced in structure is the Polyzoon than 
the Polyp. 
Polyzoa were formerly associated with 
the Polyps, to which they bear a strong 
superficial resemblance ; but they are of 
a much higher degree of organization, as 
will be seen by comparing what has been 
said in a former article on the Hydra, 
with the description which we now pro- 
ceed to abridge from Dr. AUman's splen- 
did monogragh on the fresh-water kinds. 
In order to get a general conception of a 
Polyzoon, the Professor tells us to imag- 
ine an alimentary canal, consisting of 
oesophagus, stomach, and intestine, to be 
furnished at its origin with long ciliated 
tentacles, and to have a single nervous 
ganglion on one- side of the oesophagus. 
We must then conceive the intestine bent 
back till its anal orifice comes near the 
mouth ; and this curved digestive tube to 
be suspended in a bag containing fluid, 
and having two openings, one for the 
mouth and the other for the vent. A 
system of muscles enables the alimentary 
tube to be retracted or protruded, the 
former process pulling the bag in, and 
the latter letting it out. The mouth of 
the bag is, so to speak, tied round the 
creature's neck just below the tentacles, 
which are the only portions of it that are 
left free. The investing sack has in 
nearly every case the power of secreting 
an external sheath, more or less solid, 
and which branches, forming numerous 
cells, in which the members of the family 
live in a socialistic community, having, 
as it were, two lives, one individual, and 
the other shared in common with the 
rest. 
The whole group of tubes and cells, 
whatever may be the form in which they 
are aggregated, is called the Polyparij, 
or, as Dr. Allman prefers, the Coence- 
cium (common house); the creature he 
names a Polypide* (polyp-like) ; and the 
disk which bears the tentacles {Lopho- 
phore (crest-bearer). There are some 
more hard words to be learnt before the 
student can enjoy himself scientifically 
among the Polyzoa, and we shall be com- 
pelled to employ some of them before we 
have done ; but we will now endeavor to 
* Polyzoon is preferable, as avoidiner confusion 
with polypite, used for another class of objects. 
describe what was presented to our 
view by the specimen obtained from 
the Hampstead Pond, which was a branch 
of Plumatella repens. 
When all was quiet, the mouths of 
the bags belonging to each cell were 
slowly everted, and out came a numerous 
bundle of tentacles, which were either 
spread like the corolla of a flower, or 
permitted to hang dishevelled like the 
snake-locks of Medusa. We will suppose 
these organs symmetrically expanded, 
and that we are looking down upon them 
with a magnifying power of sixty diame- 
ters, the light having been carefully ad- 
justed by turning the reflecting mirror a 
little on one side, to avoid a direct glare. 
The tentacles, each of which curves with 
a living grace, and displays an opaline 
tint in its glassy structure, do not form a 
complete circle, for at one place we dis- 
cern two slightly diverging arms of the 
disk, or frame (Lophophore) from which 
they grow. 
These arms support tentacles on each 
side, and leave a gap between, so that the 
whole pattern is crescentic, or crescent- 
shaped, and not circular. Extending as- 
far as the points of the arms, and carried 
all round the crescent, is an extremely 
delicate membrane, like the finest gauze,, 
which unites all the tentacles by their 
basal portions, and makes an elegant re- 
treating curve between every two. Each 
tentacle exhibits two rows of cilia, which 
scintillate as their vibrations cause them 
to catch the light. The motion of the 
cilia is invariably down one side and up 
the other, the current or pattern being-^ 
carried on from one tentacle to the other, 
all through the series. This character- 
istic, and the facility with which each 
cilium can be distinguished, gives great 
interest and beauty to the spectacle of 
this wonderful apparatus, by which water- 
currents are made to bathe the tentacles 
and assist respiration, and also to carry 
food towards the mouth, over which a 
sort of finger or tongue is stretched to 
guard the way, and exercise some choice 
as to what particles shall be permitted to- 
pass on. This organ is called the epis- 
tome, from two Greek words, signifying 
" upon the mouth." 
If the cell is an old one, it may be 
covered with so much extraneous matter 
as to obscure the economy within ; but we 
are fortunate in having a transparent 
specimen before us, through which we can 
see all that goes on. The alimentary tube, 
after forming a capacious cavity, much 
longer than it is broad, turns round and 
terminates in an orifice near the mouth, 
and just below the integuments. When 
refuse has to be discharged, this orifice is 
protruded; and after the operation is 
over, it draws back as before. Long 
muscles, composed of separate threads or 
