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retreats within its cell by an independent action； and when protruded
puts forth a circle of tentacles whose motion round the mouth is the
means of securing nourishment。 There are； however； peculiarities in
the structure of the Polyzoa which seem to remove them from
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Zoophytology to a place in the system of nature more nearly connected
with Molluscan types。 Some of them come so near to the compound
ascidians that they have been termed； as an order； 〃Zoophyta ascidioida。〃
The simplest form of polype is that of a fleshy bag open at one end；
surmounted by a circle of contractile threads or fingers called tentacles。
The plate shows； on a very minute scale； at figs。 1； 3； and 6； several of
these little polypiform bodies protruding from their cells。 But the
Hydra or Fresh…water Polype has no cell； and is quite unconnected with
any root thread； or with other individuals of the same species。 It is
perfectly free； and so simple in its structure； that when the sac which
forms its body is turned inside out it will continue to perform the
functions of life as before。 The greater part； however； of these
Hydraform Polypes； although equally simple as individuals； are
connected in a compound life by means of their variously formed
POLYPIDOM； as the branched system of cells is termed。 The Hydroid
Zoophytes are represented in the first plate by the following examples。
HYDROIDA。
SERTULARIA ROSEA。 PL。 I。 FIG。 6。
A species which has the cells in pairs on opposite sides of the central
tube； with the openings turned outwards。 In the more enlarged figure is
seen a septum across the inner part of each cell which forms the base
upon which the polype rests。 Fig。 6 B indicates the natural size of the
piece of branch represented； but it must be remembered that this is only
a small portion of the bushy shrub。
CAMPANULARIA SYRINGA。 PL。 I。 FIG。 8。
This Zoophyte twines itself parasitically upon a species of
Sertularia。 The cells in this species are thrown out at irregular intervals
upon flexible stems which are wrinkled in rings。 They consist of
lengthened； cylindrical； transparent vases。
CAMPANULARIA VOLUBILIS。 PL。 I。 FIG。 9。
A still more beautiful species； with lengthened foot…stalks ringed at
each end。 The polype is remarkable for the protrusion and contractile
power of its lips。 It has about twenty knobbed tentacula。
POLYZOA。
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Among Polyzoa the animal's body is coated with a membraneous
covering； like that of the Tunicated Mollusca； but which is a
continuation of the edge of the cell； which doubles back upon the body
in such a manner that when the animal protrudes from its cell it pushes
out the flexible membrane just as one would turn inside out the finger of
a glove。 This oneness of cell and polype is a distinctive character of
the group。 Another is the higher organization of the internal parts。
The mouth； surrounded by tentacles； leads by gullet and gizzard through
a channel into a digesting stomach； from which the rejectable matter
passes upwards through an intestinal canal till it is discharged near the
mouth。 The tentacles also differ much from those of true Polypes。
Instead of being fleshy and contractile； they are rather stiff； resembling
spun glass； set on the sides with vibrating cilia； which by their motion up
one side and down the other of each tentacle； produce a current which
impels their living food into the mouth。 When these tentacles are
withdrawn； they are gathered up in a bundle； like the stays of an
umbrella。 Our Plate I。 contains the following examples of Polyzoa。
VALKERIA CUSCUTA。 PL。 I。 FIG。 3。
From a group in one of Mr。 Lloyd's vases。 Fig。 3 A is the natural
size of the central group of cells； in a specimen coiled round a thread…
like weed。 Underneath this is the same portion enlarged。 When
magnified to this apparent size； the cells could be seen in different states；
some closed； and others with their bodies protruded。 When magnified
to 3 D； we could pleasantly watch the gradual eversion of the membrane；
then the points of the tentacles slowly appearing； and then； when fully
protruded； suddenly expanding into a bell…shaped circle。 This was their
usual appearance； but sometimes they could be noticed bending inwards；
as in fig。 3 C； as if to imprison some living atom of importance。 Fig。
B represents two tentacles； showing the direction in which the cilia
vibrate。
CRISIA DENTICULATA。 PL。 I。 FIG。 4。
I have only drawn the cells from a prepared specimen。 The polypes
are like those described above。
GEMELLARIA LORICATA。 PL。 I。 FIG。 5。
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Here the cells are placed in pairs； back to back。 5 A is a very
small portion on the natural scale。
CELLULARIA CILIATA。 Pl。 I。 FIG。 7
The cells are alternate on the stem； and are curiously armed with
long whip…like cilia or spines。 On the back of some of the cells is a
very strange appendage； the use of which is not with certainty
ascertained。 It is a minute body； slightly resembling a vulture's head；
with a movable lower beak。 The whole head keeps up a nodding
motion； and the movable beak occasionally opens widely； and then
suddenly snaps to with a jerk。 It has been seen to hold an animalcule
between its jaws till the latter has died； but it has no power to
communicate the prey to the polype in its cell or to swallow and digest it
on its own account。 It is certainly not an independent parasite； as has
been supposed； and yet its purpose in the animal economy is a mystery。
Mr。 Gosse conjectures that its use may be； by holding animalcules till
they die and decay； to attract by their putrescence crowds of other
animalcules； which may thus be drawn within the influence of the
polype's ciliated tentacles。 Fig。 7 B shows the form of one of these
〃birds' heads；〃 and fig。 7 C； its position on the cell。
FLUSTRA LINEATA。 PL。 I。 FIG。 1。
In Flustrae； the cells are placed side by side on an expanded
membrane。 Fig。 1 represents the general appearance of a species which
at least resembles F。 lineata as figured in Johnston's work。 It is spread
upon a Fucus。 Fig。 A is an enlarged view of the cells。
FLUSTRA FOLIACEA。 PL。 I。 FIG。 2。
We figure a frond or two of the common species； which has cells on
both sides。 It is rarely that the polypes can be seen in a state of
expansion。
SERIALARIA LENDIGERA。 PL。 I。 fig。 10。
NOTAMIA BURSARIA。 PL。 I。 fig。 11。
The 〃tobacco…pipe〃〃 appendages； fig。 11 B； are of unknown use：
they are probably analogous to the birds' heads in the Cellularae。
PLATE V。
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