_ organs 0 
Ce@termintua. The Bryozoa which, while 
arrangement ; such a disposition providing for 
the extension or shortening or lateral inflexions 
of the worm, and enabling the animals so con- 
structed to move about with facility in the cavi- 
ties wherein they reside. 
Nevertheless, at this, its first appearance in 
the animal series, muscular fibre has not as yet 
attained to the perfection of structure that it 
offers in the higher classes. The fibres are as 
yet indistinct, soft, and gelatinous; they ap- 
pear to be deficient in fibrin; neither do they, 
when examined with the microscope, present 
the transverse strie that are so characteristic of 
the muscular tissue in a more advanced condi- 
tion. The little fascicles are, moreover, ex- 
tremely short, and run but a little distance 
before they disappear, and are succeeded by 
others. Their whole appearance, in fact, is 
that of muscle in a rudimentary condition, and 
very accurately resembles the nascent muscular 
tissue when it first becomes apparent in the 
embryo of the vertebrate animal. The nervous 
System accompanying this condition of the 
Muscles is extremely simple; a delicate ring 
surrounds the commencement of the cesopha- 
gus, by which, perhaps, the muscles of the 
mouth are associated during the act of imbibing 
nourishment, and prolonged from this ring are 
two long and thread-like nerves, one running 
along the dorsal and the other along the ventral 
" aspect of the body, and passing quite from one 
extremity to the other, but without any percep- 
tible ganglionic enlargements in their course.* 
Having, therefore, no brains or central masses 
to which sang could be referred, localized 
sense are likewise wanting in all the 
naturalists were ignorant of their more compli- 
cated organization, were, until a recent period, 
confounded with the Polyps, are, from their 
€ntire structure, very justly entitled to a much 
higher position in the scale of animals, and un- 
_ doubtedly belong to the Nematoneurose type. 
‘These little beings inhabit transparent cells of 
very elaborate and delicate construction, from 
the mouth of which they protrude the anterior 
portion of their bodies when in search of food. 
Although from their general appearance the 
Bryozoa might easily be, and in fact have been 
until recently, erroneously regarded as Polyps, 
the differences between the two classes are ex- 
ceedingly striking and important. The Bryo- 
zoa, instead of having the tentacula that sur- 
found the oral aperture quite simple and fila- 
mentary, as the Polyps have, are furnished 
with ciliated tentacula, and from the rapid 
_ ciliary movement which is incessantly going 
on, while the arms are expanded, strong cur- 
Tents are formed in the surrounding water, 
all of which impinging upon the oral orifice 
bring to the mouth such nutritive materials as 
are necessary for the support of the creature. 
The digestive apparatus is no longer a closed 
* The dorsal nervous cord of the Ascaris, first 
described by Cloquet, (Anatomie des vers intesti- 
naux, ) seems to have been overlooked by the learned 
writer of the article <‘ Animal Kingdom,” who has 
arranged the Entozoa as being diploneurose ani- 
mals. 
MUSCULAR SYSTEM. 
535 
sacculus, as in the Polyps, but, on the contrary, 
presents a very elaborate structure, consisting 
of a gizzard, stomach, and distinct intestinal 
canal, terminated by an anal aperture, though 
which the feces are ejected; and, more- 
over, the whole of the digestive viscera float 
loosely in a visceral cavity wherein they are 
suspended. In addition to this, the mouth of 
the cell occupied by the Bryozoon is defended 
by a most delicate and complicated opercular 
apparatus, requiring a very perfect set of mus- 
cles to perform all the required movements 
connected with the protrusion and retraction of 
the body, so that there is abundant reason for 
separating the animals in question from the 
Acrite Zoophytes. 
As relates to their muscular system, rudi- 
mentary in its development as it must still ne- 
cessarily be, many circumstances of great inte- 
rest have been brought to light by the patient 
investigations of Dr. Arthur Farre, to whose 
valuable memoir* we are indebted for the fol- 
lowing particulars, which we give at some 
length, more especially as they will serve not 
only to elucidate this part of our subject, but to 
correct several important errors that have been 
promulgated relative to the Rorirera, an 
important class of animals next to be noticed, 
of very analogous structure. 
For the purpose of drawing the protruded 
Bryozoon into its retreat two distinct sets of 
muscles are provided, one set acting upon the 
animal and the other upon the flexible opercu- 
lum that closes the cell. The muscles for the 
retraction of the animal are contained in the 
visceral cavity, and consist of two bundles of 
delicate thread-like cords, the one set arising 
‘from the bottom of the cell to be inserted about 
the base of the stomach, the other also arising 
from the bottom of the cell and passing up free 
by the sides of the pharynx to be inserted around 
the line of junction between this organ and 
the tentacula. 
The muscles provided for the retraction of 
the operculum consist of six flattened bundles 
of fibres, which act upon the flexible portion 
of the cell and a delicate circle of sete placed 
around its orifice. It is at once evident there- 
fore that the muscular system in the Bryozoa is 
capable of great precision of action, and the 
fasciculiare most accurately adjusted. Neverthe- 
less if the intimate structure of this form of 
muscle be investigated, it is found not to have 
attained to full perfection. ‘ It would appear,” 
says Dr. Arthur Farre, “ as if muscular fibre 
were here reduced to its simplest condition. 
The filaments are totally disconnected, and are 
arranged one above the other in a single series. 
They pass straight and parallel from their ori- 
gin to their insertion, and have a uniform dia- 
meter throughout their course, except that each 
filament generally presents a small knot upon 
its centre, which is most apparent when it 
is in a state of contracticn, at which time the 
whole filament also is obviously thicker than 
when relaxed. The filaments have a watery 
* Dr. A. Farre, on the structure of Ciliobra- 
chiate Polypi. Phil. Trans. part 2 for 1837, 
