“190 
generically distinct from all the forms that had been re- 
cognised at the time of its discovery, but must be referred 
to a new Order or Sub-class: it is chiefly interesting, 
however, to the biologist from the light which it throws 
on the history and affinities of the tribe to which it 
belongs. Its occurrence was first recorded in 1868 by 
the elder Sars, who gave it the name of Flalilophus mira- 
dilis, but did not at that time enter upon the details of its 
structure, In 1869 Allman described a new Polyzoon, 
under the name of Rhabdopleura Normanni, which had 
been dredged up from a) water in Shetland, and which 
presented some remarkable peculiarities. Its polypides 
(according to Allman) were of the Hippocrepian type, 
having the tentacles disposed in the form of a horse-shoe, 
instead of circularly, an arrangement which had only 
been noticed so far amongst the fresh-water division of 
the Polyzoa, Another anomalous character was the pre- 
sence of a rigid, chitinous rod, extending throughout the 
creeping portions of the polyzoarium, to which the poly- 
pides were attached at intervals by means of a long 
- flexible cord. It now appears that the Shetland Polyzoon 
belongs to the same genus as the Lofoten form just men- 
tioned. Allman, however, having only access to speci- 
mens preserved in spirit, was unable to master all the 
details of the structure or to apprehend fully the signifi- 
cance of the organism as a whole. For a complete 
knowledge of Rhabdopleura we are indebted to the 
careful observations of the younger Sars, who studied the 
living animal ; while to his father we owe a most inte- 
resting interpretation of the facts which the son had 
established, 
Without entering into minutia, I shall endeavour to 
describe briefly the characteristics which mark out the 
Rhabdopleura as unique, and invest it with so high an 
interest, not only for the student of the Polyzoa, but also 
for the philosophical biologist. In the first place, it may 
be stated broadly that we find in this form the Polyzoan 
type in a rudimentary and half-developed condition. It 
clearly represents a very early stage in its evolution, if 
evolution be the method of Nature. The points which 
separate it most strikingly from its congeners are not the 
equivalent of the ordinary differences that occur amongst 
the members of the same class ; they might rather be re- 
garded as surviving features of another and very different 
type, from which it has diverged, and are strictly transi- 
tional in character. Rhabdopleura is a Polyzoon, and yet 
not a// Polyzoon. A large portion of its structure, while 
clearly taking the Polyzoan direction, differs widely from 
that of all known Polyzoa. Some of the features which 
we should regard as most characteristic of this class are 
altogether wanting. And organs in which the Polyzoan 
type is most distinctly traceable, appear in a simpler and 
more rudimentary condition than in any other known 
form. Ina word, two types of structure seem to blend 
in this remarkable animal, one, as it were, fading away, 
and the other dawning. 
The polyzoarium in Ahabdopleura bears a striking re- 
remblance to that of a Hydroid, and might belong to a 
Coryne or Eudendrium. It consists of a number of erect, 
chitinous tubes, distinctly annulated, which are united by 
a creeping, tubular stem. Each of the erect tubes 
(zocecia) contains a polypide, and every polypide is at- 
tached by a contractile cord to a dark-coloured, cylindri- 
cal rod, which pervades the creeping portion of the poly- 
zoary. The polypide differs from those of the normal 
Polyzoa in the following important particulars :— 
1. It is without any sort of attachment to its cell, in 
which it lies quite free. In all other known Polyzoa a 
membrane (the endocyst) lines the cavity of the cell, and 
envelopes the polypide, to which it is attached above, at 
the base of the tentacular crown. When the animal re- 
treats into its cell, it draws in with it the anterior portion 
of this membrane, which securely closes the aperture. 
Between the endocyst and the body of the polypide is a 
NATURE 
ee enone Strate varies ee ted <a 
[Fuly 3, 1873 
space (the perigastric cavity), in which the nutritive fluid 
is confined. But in Rhabdopleura the endocyst is alto- 
gether absent, or appears in a perfectly elementary condi- — 
tion, as a “thin, glassy skin,” immediately surrounding | 
the digestive apparatus. There is nothing to close the 
orifice of the cell, and the surrounding water passes 
freely into its interior. There is no perigastric cavity 
or fluid. The polypide is as free and unattached as a 
Hydroid in its calycle; and its only connection with 
the colony is through the contractile cord already re- 
ferred to. 
2. The digestive system is of the Polyzoan type, but of 
much lower grade than is found elsewhere. There is little 
specialisation of parts; the stomach and intestine consist 
of a simple tube, wider towards its: upper extremity and 
narrowing off rapidly towards the posterior end, which is 
bent abruptly upon itself. The intestine is not separated 
from the true stomach by any valve, but is an immediate 
continuation of it, and passes off from its lower extremity 
in a straight line to the anal orifice. 
In the normal Polyzoa, on the contrary, the stomach is 
divided into two well-defined regions ; and the intestine, 
which is marked off by a distinct valve, takes the origin 
between the upper portion and the large, sub-globular sac, 
in which it terminates below. We have in Rhabdopleura 
the bent tube and the two orifices (oral and anal), but be- 
yond this, perfect simplicity of structure. 
3. The tentacular apparatus exhibits some remark- 
able features. It differs essentially from that of the 
marine, and also from that of the fresh-water Polyzoa, 
though it most nearly approaches the latter. It consists 
of two symmetrical lobes or arms, which extend out 
dorsally from the anterior part of the body, diverging to 
each side ; and each of which bears a double row of 
ciliated tentacles. These lobes are very flexible, and 
exhibit great mobility, bending slowly in various direc- 
tions ; and in this respect they contrast strikingly with 
the unchanging lophophore of the fresh-water Polyzoa, 
The single tentacular crown, which belongs to all the 
other known members of the class has here disappeared ; 
and instead of the circular verticil of the marine, and the 
crescentic but continuous series of the fresh-water species, 
we have here two series, borne on distinct flexible and 
movable appendages. 
4. In habdopleura, the complicated muscular 
system concerned in the protrusion and retraction of 
the polypide, which is so characteristic of the Polyzoa, 
and on which their lively and rapid movements depend, 
is suppressed along with the endocyst. Retraction is 
effected solely by means of the cord that passes from 
the body to the rod pervading the creeping stem. It 
is a very slow and sluggish process, the polypide exhi- 
biting none of the sensitiveness and vivacity of its 
kindred. | Under extreme provocation it retires very 
deliberately ; an ordinary Polyzoon disappears with the 
speed of light, on the slightest alarm. This sluggishness, 
as our author remarks, is accounted for “ by the want of 
special retractor-muscles, and. by the slightly developed 
contractile elements, not distinguishable as evident 
muscular fibres, in the contractile cord.” 
Still more remarkable is the mode in which the pro- 
trusion of the polypide is effected. In the absence of the 
usual muscular appliances, it is difficult, at first sight, to 
imagine how the creature can raise itself from the lower 
extremity to the aperture of its tubular dwelling, It 
appears, however, that a special and most singular organ 
exists for the purpose, and that here also the Rhabdopleura 
departs altogether from the customs of its race, This 
organ consists of a large and prominent shield or disc, 
which projects from the anterior end of the body between 
the oral and anal orifices, and is thickly covered with 
cilia. It evidently corresponds with an anomalous 
Structure (known as the efistome), which occurs only 
amongst the freshwater Polyzoa, and the function of 
