Encyclopaedia Britannica > Volume 21, ROT-Siam

R 0 T I F E R A
a Polyzoan larva, while Enteroplea is probably a male Rotifer, and,
like the .other males, in a reduced condition. There is no reason for
considering this mastax as the homologue of either the gastric mill of
Crustaceans on the one hand or the teeth in the Chretopods’ pharynx
on the other; it is merely homoplastic with these structures, but has
attained a specialized degree of development. Both the pharynx
and the oesophagus which follows it are lined with chitin. The
msophagus varies in length and in some genera is absent {Philo-
o/inadx), the stomach following immediately upon the pharynx,
d he stomach is generally large ; its wall consists of a layer of very
large ciliated cells, which often contain fat globules and yellowish-
green or brown particles, and outside these a connective tissue
membrane; muscular fibril lie have also been described. Very
constantly a pair of glands open into the stomach, and probably
represent.the hepato-pancreatic glands of other Invertebrates.
Following, upon the stomach there is a longer or shorter intestine,
which ends in the cloaca. The intestine is lined by ciliated cells!
In forms living in an urceolus the intestine turns round and runs
foi ward, the cloaca being placed so as to debouch over the margin
of the urceolus. The cloaca is often very large ; the nephridia and
oviducts may open into it, and the eggs lodge there on their way
outwards ; they are thrown out, as are the fecal masses, by an
eversion of the cloaca. Asplanchna, Notommata sieboldii, and cer¬
tain species of Ascomorpha are said to be devoid of intestine or
anus, excrementitious matters being ejected through the mouth (11).
Nephridia.—The ccelom contains a fluid in which very minute
corpuscles have been detected. There is no trace of a true vascular
system. The nephridia (fig. 2, b, ri) present a very interesting
stage of development. They consist of a pair of tubules with an
intracellular lumen running up the sides of the body, at times
merely sinuous, at others considerably convoluted. From these
are given off at irregular intervals short lateral branches, each of
which terminates in a flame-cell precisely similar in structure to
the flame-cells found in Planarians, Trematodes, and Cestodes •
here as there the question whether they are open to the ccelom or
not must remain at present undecided. At the base these tubes
open either into a permanent bladder which communicates with the
cloaca or into a structure presenting apparently no advance in its
development upon the contractile vacuole of a ciliate Infusorian.
Nervous System and Sense-Organs.—Various structures have been
spoken of as nervous which are now acknowledged to have been
erroneously so described (18). There is a supra-cesophageal gang-
non winch often attains considerable dimensions, and presents a
lobed appearance (fig. 2, a and b, g), Connected with this are the
eye-spots, which are seldom absent. Where these are most highly
developed a lens-like structure is present, produced by a thicken¬
ing of the cuticle. In the genus Rotifer and other forms these are
placed upon the protrusible portion of the head, and so appear to
have different positions at different moments. The number of eye-
spots varies from one to twelve or more. They are usually red red-
dish-brown, violet, or black in colour. Other structures are found
which doubtless act as sense-organs. The calcar above-mentioned
generally bears at its extremity stiff hairs which have been demon¬
strated to be in connexion with a nerve fibril. On the ventral sur¬
face of the body just below the mouth a somewhat similar structure
is often developed—the chin. There are besides at times special
organs, like the two lateral organs in Microcodon (fig. 1, d s) which
no doubt in common with the calcar and chin have a tactile, function.
Reproductive Organs and Development.—The Rotifera were
formerly considered to be hermaphrodite, but, while the ovary was
alwa,ys clear and distinct, there was always some difficulty about
the testis, and various structures were put forward as representino-
that organ. One by one, however, small organisms have been dis°
covered and described as the males of certain species of Rotifers
n'1lti1n at the present time degenerated males are known to occur in
all the families except that of the Philodinadm. The male Rotifers
are provided with a single circlet of cilia (a peritroch), a nerve
ganglion, eye-spots, muscles, and nephridial tubules all in a some¬
what reduced condition, but there is usually no trace of mouth or
stomach, the main portion of the body being occupied by the testi-
cular sac. There is an aperture corresponding with the cloaca of the
emale, where the testis opens into the base of an eversible penis.
I he males of Flosculana are shown in fig. 1. The male of Pedalion
mira possesses rudimentary appendages. The ovary is usually a
large gland lying beside the stomach connected with a short oviduct
7p,1C,h ,opens mt<? th® cloaca- The ova often present a reddish hue
{Fhilodina roseola, Brachionus rubens), due doubtless, like the red
colour of many Crustacean ova, to the presence of tetronerythrin.
Up to the present our embryological knowledge of the groun is
very incomplete. _ Many Rotifers are known to lay winter and
summer eggs of different character. The winter eggs are provided
with a thick shell and probably require fertilization. Two or three
of them are often carried about attached to the parent {Brachionus,
A otommata), but they are usually laid and fall into the mud, there
to remain till the following spring. The summer eggs are of two
kinds, the so-called male and female ova, both of which are stated
to develop parthenogenetically. They may be carried about in
large numbers in the cloaca or oviduct or attached to the body of
the parent. The female ova give rise to female and the male ova
to male individuals. . Male individuals are only formed in the
autumn in time to fertilize the winter ova.
Habitat and Mode of Life.—The Rotifera are distri¬
buted all over the earth’s surface, inhabiting both fresh
and salt water. The greater number of species inhabit
fresh water, occurring in pools, ditches, and streams. A
few species will appear in countless numbers in infusions
of leaves, &c., but. their appearance is generally delayed
until the putrefaction is nearly over. Species of Rotifer
and Philodina appear in this way. A few marine forms
only have been described—Brachionus rnulleri, B. hepta-
tonus, Synchseta haltica, and others.
A few forms are parasitic. Albertia lives in the intestine
of the earthworm; a form has been described as occurring
in the body-cavity of Synapta; a small form was also
observed to constantly occur in the velar and radial canals
of the freshwater jelly-fish, Limnocodium. Notommata
parasitica leads a parasitic existence within the hollow
spheres of Volvox globator, sufficient oxygen being given
off by the Volvox for its respiration.
Many Rotifers exhibit an extraordinary power of resist¬
ing drought. Various observers have dried certain species
upon the slide, kept them dry for a certain length of time,
and. then watched them come to life very shortly after the
addition of a drop of water. The animal draws itself to¬
gether, so that the cuticle completely protects all the softer
parts and prevents the animal itself from being thoroughly
dried. This process is not without parallel in higher
groups , e.g., many land snails will draw themselves far into
the shell, and secrete a complete operculum, and can remain
in this condition for an almost indefinite amount of time.
The eggs are also able to withstand drying, and are pro¬
bably blown about from place to place. The Rotifera can
bear great variations of temperature without injury.
Since their removal from among the Protozoa various
attempts have been made to associate the Rotifera with
one or other large phylum of the animal kingdom.
Huxley, insisting upon the importance of the trochal disk,
put forward the view that they were “ permanent Echino-
derm larvae,” and formed the connecting link between
the Nemei tidee, and the Hematoid worms. Ray Lankester
proposed to associate them with the Chsetopoda and
Arthropoda in a group Appendiculata, the peculiarities in
the structure of Pedalion forming the chief reason for
such a classification. There is, however, no proof that we
thus express any genetic relationship. The well-developed
coelom, absence of metameric segmentation, persistence of
the trochal disk in varying stages of development, and the
structure of the nephridia are all characters which point to
the Rotifera as very near representatives of the common
ancestors of at any rate the Mollusca, Arthropoda, and
Chsetopoda.' But the high development of the mastax,
the specialized character of the lorica in many forms, the
movable spines of Polyarthra, the limbs of Pedalion, and
the lateral appendages of Asplanchna, the existence of a
diminutive male, the formation of two varieties of ova, all
point to a specialization in the direction of one or other of
the above mentioned groups. Such specialization is at
most a slight one, and does not justify the definite associa¬
tion of the Rotifera in a single phylum with any of them.
Classification. The following classification has been
recently put forward by Dr C. T. Hudson (19).
Class ROTIFERA.
Order I.—Rhizota.
Fixed forms ; foot attached, transversely wrinkled, non-retractile
truncate.
Fam. 1. Flosculariadze. Floscidaria, Stephanoceros.
ram. 2. Melicertada;. Melicerta, Cephalosiphon, Megalo-
trocha, Limnias, AUcistes, Lacinularia, Conochilus.