ON THE STRUCTURE OF THE ROTIFERA.
137
We have also appended the author's Systematic arrangement
of the Rotifera.
ON THE SYSTEMATIC POSITION OF THE ROTIFERA.
Formerly placed among the Infusoria, of which they formed the second
class in the classification of Bhrenberg, it has been latterly admitted by
all systematic writers, that the ROTIFERA possess nothiug in common
with the animals truly belonging to that class, or the polygastrica of the
Berlin Professor: but that, in respect of their complex structure, they
represent a higher type of organization. Opinions, however, are still
divided on the question, whether the Rotifera, as Burmeister believes,
belong to the Crustacea, or should be referred to the Annelids, in accordance with the opinion of Wiegmann, Wagner, Milne-Edwards, Berthold,
V. Siebold, and others.
If the truth were always with the majority, we should undoubtedly be
obliged to place the Rotifera in the class of Annelida; hut I believe,
nevertheless, that Burmeister is in the right, though opposed to all the
other observers above named. I also conceive that the Rotifera are much
more closely allied to the Crustacea than to the worms; and, by comparing
the conditions of organization of the two classes, shall venture to support
this view in the following observations.
It may first he mentioned, that so far back as 1824 Nitzsch asserted,
that the Rotifera resembled the Entromostraca ; and, what certainly
deserves attention, Ehrenberg himself, though placing them with the
Infusoria, frequently remarks upon their resemblance to the Crustacea
and Entomostraca; thus, in his great work, p. 410, he says, that the
" spicules, beards, and set»" of many species might be compared with the
arms of the Daphnice; and in p. 411 he remarks, that many Rotifera carry
about their eggs attached to them, " like the Crustacea ;" and speaks to
the same effect in many other places. Dujardin also notices similar resemblances with Cyclops, Cypris, &c.; as, for instance, at pp. 574 and 575
of his work.
If the systematic position of the Rotifera were to be determined mainly
from their externalfigure,the result would certainly be more in favour of
the Crustacean than of the Annelidjtype. No annelid has articulated motile
organs, whilst, on the contrary, the possession of such organs in a perfectly symmetrical form is a fundamental character of the Arthropoda.
The majority of the Botifera are not furnished, it is true, with a pair of
feet, although they have a single annulated, or jointed foot, containing no
part of the viscera, but which is applied solely to the purpose of locomotion. Furthermore, if the rest of the conformation of the body be
regarded, it is obvious at once that a Euchlanis, Salpina, and, in short,
all whose cuticle has acquired the hardness of a lorica, are more closely
approximated to a crustacean than to a worm. For in the whole vermiform division I am unacquainted with any form whose cuticle is indurated
to the same extent.
The muscular structure also approximates, in many Rotifera, more
nearly to the Arthropoda than to the Annelids. In no animal of the
latter class have genuine transversely striped muscles hitherto been seen ;
that is to say, muscles whose contents are divided into minute quadrangular particles, like those of the muscles of vertebrate animals.
That the motions of the body of many species recall in a striking
manner those of the Crustaceans has already been noticed.
If the nervous system be considered, its similarity with' that of the
138
ON THE STRUCTURE AND
lowest Crustaceans cannot escape recognition. In the Rotifera it consists
simply of a cerebral ganglion, with branches radiating from it; there is
no abdominal chord, nor any chain of ganglia. But is the nervous system
of the Lophyropoda more developed? In the Daphnire, even, we are
acquainted only with a cerebral ganglion and nerves proceeding from it;
and, consequently, know of no grounds upon which to establish the law,
that a central nervous system, consisting of a ganglionic ring surrounding
the pharynx, and of a chain of abdominal ganglia proceeding from it,
belongs to the Crustacea as a fundamental character.
Moreover, the manner in which the sensitive nerves terminate peripherally in the Rotifera corresponds precisely with what I have described,
regarding this point, in the Crustacea and Insecta; and nothing like which
is at present known to exist in the class " Venues." Lastly, I shall not
repeat at length, but merely remark, that the eye-spots apparent on the
nervous centre of the Rotifera approximate most closely to the similar
structures of the Crustacea, as Ehrenberg has not failed to indicate.
The disposition and texture of the alimentary canal, in an inquiry into
the systematic position of the Rotifera, afford no decisive evidence in
favour of one view or the other, since many Annelids also have a complex
horny masticatory apparatus ; still, with respect to this I would remark,
that the masticatory apparatus of young Daphnice (I have examined, for
this object, the deep yellow-red young of a very large species, Daphnia
inaxima ?) presents a pretty close similarity with that of many Rotifers,
inasmuch as the two opposed jaws expand into a plate, which is toothed
with numerous transverse ridges, exactly like the corresponding plate in
Lacinidaria.
The glandular lobate appendages placed upon the stomach in the Cirripeds, which have been explained as " salivary glands," might, perhaps, be
regarded as analogues of the ventricular glands of the Botifera. Similar
organs, however, also exist in many of the dorsibranchiate Annelids ;
many Annelids also, like many of the lower Crustaceans, are alike in the
circumstance that the liver is represented simply by large cells, with
peculiar contents, situated in the wall of the stomach or intestine. Should
any one find an objection to the arthropodons type in the deficiency of an
intestine in some .Rotifers {Notommata anglica, N. Bieholdii, &c), he may
recall the neuropterous larva of Myrmelem, in which, as is well known,
the faeces are also discharged by the mouth ; the rectum being transformed
into a spinning organ. But with respect to the intestinal tract of many
Rotifers (as EucManis, Bteplianoceros, &c), what especially recalls the
condition of that part in the lower Crustacea, is its peculiar bell-like
movement, which is precisely similar to that with which we are acquainted
in the intestine of certain parasitic Crustaceans (Achthenes, Traclieliastes,
&c.)
As to the substance which I have pronounced to be a urinary secretion,
the close relations which obtain, with respect to it, between the Rotifers
and the larva of Cyclops, cannot fail to be recognised; whilst anything
allied to it is wholly wanting in the Annelids.
Lastly, the anatomical and physiological phenomena relating to the
sexual life speak loudly enough in favour of the proposition that the Rotifers should be ranked with the Crustacea. I would not lay much stress
upon the circumstance that they produce two kinds of ova,—the so-termed
"summer" and "winter-eggs" (the latter in Triarthra, in the structure
of their shell present much similarity with the ephippian eggs of Daphnia),
or that many species carry their ova about with them ; for, as regards
these particulars, the genus Ctepsine among the Annelids might be named
as one in which the same thing takes place. The coloured oil-globules
SYSTEMATIC POSITION OF THE EOTIFERA.
139
also, met with in the vitettus of many Rotifers, and indicative of a Crustacean type, may be left out of consideration. But of greater importance,
perhaps, is the striking analogy existing between the male Rotifers—
which in a certain sense may be said to be aborted—and those of many
Crustaceans. Who will not remember the diminutive, male parasitic
Crustaceans, which Noidmann discovered on the female individuals of
AcMheres, Brachiella, Chondracantkus, and Anchorella, and Kroyer in
other Lernaeopoda and Lerneee ?
And the reason that we are only just beginning to become acquainted
with separate males of Rotifers is probably due to the same circumstances,
—appearance at a certain time of year, diversity of figure from that of the
female—as those owiDg to which we have not yet discovered the males,
for instance, of Ergasilus, Polyphemus, Limnadia, Apus, &c.
If the development also of these creatures be regarded, it will be found
in favour of crar view ; for, with respect to several species, it has been
shown that the young, at its liberation from the ovum, has not got the
form of the adult animal, and, consequently, must undergo a metamorphosis. And is not the subsequent diminution, and even complete disappearance of the eyes, which exist in the young condition of the animal,
a farther indication of an approach towards certain Crustacean forms ?
Whilst the structural conditions hitherto mentioned more or less powerfully support the view of the Crustacean nature of the Eotifera, they are,
on the other hand, separated from the Crustaceans by the condition of the
respiratory organs and the presence of vibratile cilia, and approximated to
the Annelids; but in both these respects they equally approach the
Echiuodermata ; for, as has been said above, the proper vibratile organs of
Synapta digitata appear to me to be structures equivalent to the " vibratile organ" of the Rotifer.
But in the determination of the systematic position of an animal, the
question must depend, as it seems to me, upon the fact, whether the sum
of the resemblances is greater than that of the differences, as respects the
animal groups with which the animal might be supposed to be associated.
In applying this law to the subject under discussion, we find that the
number of conditions allying the Rotifers with the Crustaceans far exceeds
that of the properties possessed by them not in common with the Crustacea. I consequently regard it as fully justifiable to rank the Iiotifera
as a special order of Crustacea, and propose, from the distinctiye character
exhibited in them, to denominate them " ciliated Crustaceans" (Winiperkriibse). They necessarily stand at the commencement of the Crustacean
class ; since, in the structure of their respiratory organs, they continue to
be allied with the Annelids. Huxley (1. c.) has regarded them as Annelids
possessing permanently the form of the Echinoderm-larva, and has expressed this comparison in diagrammatic figures (1. c , Plate III.), in
which he places Lacimdaria opposite an Annelid larva, Melicerta opposite
to that of Asterias, Philodina to that of Bolothuria, Brachionus to the
larva of Sipunculus, and, lastly, Stephanoceros to the larva of Echinus.
Although the ingenuity of this attempt must be admitted, still I am
unable to adopt the view of the English observer, but am compelled, from
the considerations above detailed, to declare myself an adherent to Burmeister's view, as the only one agreeing with my own.
CLASSIFICATION OF THE CILIOCRUSTACEA.
It is obvious that the arrangement of the Ciliocrustaceans, or Rotifers,
as proposed by Ehrenberg, must be changed, inasmuch as the principle