elongated, and bears a resemblance to certain torpedos, except that the
head forms a somewhat blunt termination to the spindle-shaped body. The
tail bears a caudal fin, and _Spadella_ and _Krohnia_ have a single pair,
and _Sagitta_ two pairs, of lateral fins; all of which are flattened
horizontally.
The body is externally divisible into three regions—head, trunk, and
tail—and these correspond with the arrangement of the internal organs.
[Illustration: FIG. 101.—_Sagitta bipunctata._ _a_, Vesicula seminalis. ×
4. (After Hertwig.)]
The head is surrounded by a fold of skin, forming a hood, {187}which is
most prominent at the sides (Fig. 102, _g_); within the hood the head bears
from two to four rows of short spines, and outside these a right and left
row of sickle-shaped hooks, the free ends of which in a state of rest
converge round the mouth, but when disturbed these hooks can be widely
divaricated.
The cavity of the body, or coelom, is divided into three distinct chambers
by the presence of two thin transverse walls or septa, one situated between
the head and the trunk, the other between the trunk and the tail (Figs.
104, 105). In the head, this cavity is much reduced by the presence of
special muscles which move the spines, hooks, etc.; and in the small
species, such as _Spadella cephaloptera_, the other two cavities are almost
entirely occupied by the digestive and reproductive organs[223]; but in the
large species, e.g. _Sagitta hexaptera_, a considerable space is left
between the internal organs and the skin, and this is occupied by a
coelomic fluid. If the skin of one of these larger species be punctured the
fluid escapes and the animal shrivels up. A longitudinal partition or
mesentery, with numerous pores in it, runs through these spaces, dividing
the body-cavity into a right and left half; in the region of the trunk this
mesentery supports the alimentary canal.
In addition to certain muscles in the head, which move the hooks, etc.,
there is a muscular lining to the body-wall. This is divided into two
dorsal and two ventral bands, much in the same way as in Nematodes. The
muscle fibres are striated.
The mouth, situated either terminally—_Spadella marioni_[224]—or below the
head, leads into a pharynx; this passes into an intestine lined by a single
layer of ciliated cells with a few glandular ones intermingled. The
intestine runs straight through the body without loop or coil, and opens by
an anus situated at the junction of the trunk and the tail. In most cases
the anus is ventral or on the lower surface, but Gourret asserts that in
_Spadella marioni_ it is on the upper surface.
There are no special respiratory, excretory, or circulatory organs, unless
a glandular structure described by Gourret in the head of _Spadella
marioni_ be a real kidney.
The nervous system consists of a supra-oesophageal ganglion {188}or brain
situated in the head, and of a ventral ganglion lying in the trunk; both
these nerve centres are embedded in the epidermis, and are connected with
one another by means of two stout peri-oesophageal nerves (Figs. 102, 104).
The brain also gives off a pair of nerves to the eyes, another pair to the
olfactory organ, and a pair which ultimately meet one another and so form a
ring; on this are certain ganglia giving off nerves which supply the
muscles of the head. Both the chief ganglia give off numerous nerves, which
divide and split up into a network of fibres which permeate the whole skin.
The sense organs are comparatively simple. A pair of very small eyes lie in
the skin of the head; they are of complex structure, and to some extent
remind one of the simple eyes of certain Crustacea. Behind the eyes and
also on the upper surface of the animal is an unpaired organ which is
usually described as olfactory in function (Figs. 103, 105). This is a
ring-shaped modification of the epidermis drawn out into different shapes
in the various species. The modified epidermal cells bear long cilia. The
remaining sensory organs found in the group consist of clumps of modified
cells scattered in round groups over the surface of the body and of the
fins. The central cells of each group bear long tactile hairs, and are
surrounded by supporting cells.
[Illustration: FIG. 102.—Head of _Sagitta bipunctata_. A, Dorsal view; B,
ventral view. × about 33. (From Hertwig.) A, _a_, spines; _b_, nerves to
lateral cephalic ganglia; _c_, hooks; _d_, cephalic ganglion; _e_,
olfactory nerve; _f_, optic nerve; _g_, hood; _h_, commissure to ventral
ganglion; _j_, olfactory organ: B, _a_, _c_, and _g_ as in A; _k_, mouth.]
The Chaetognatha are hermaphrodite, and carry the female organs in the
trunk, the male in the tail. In a mature specimen the two ovaries occupy
almost all the space in the trunk between the alimentary canal and the
skin, and each is supported by a narrow lateral mesentery. The ovary is
traversed by a oviduct which often contains spermatozoa; it is not clear
how the eggs make their way into the oviduct, which seems to have {189}no
internal opening and to act largely as a receptaculum seminis. The oviducts
open externally on the upper side at the base of the lateral fin, close to
the junction of the tail and the trunk.
The cavity of the tail is divided into two lateral chambers by the
extension backward of the median vertical mesentery. In each of these a
testis and a vas deferens are found. The testes are solid ridges formed by
the growth of the lining cells of this part of the body-cavity; the cells
mature into spermatozoa, which break off and float freely in the coelomic
fluid. At the breeding season the whole tail may be crowded with masses of
spermatozoa, which are kept in a more or less regular circulation by the
ciliated cells lining the body-wall. The vas deferens opens internally into
the space where the spermatozoa lie, and at the other end into a vesicula
seminis, which opens to the exterior. The position of the latter structure
varies, and is of some systematic value.
The eggs are laid in the water and as a rule float at the surface of the
sea. _Spadella cephaloptera_ is, however, an exception to this rule, as it
attaches its eggs by means of a gelatinous stalk to sea-weeds. The
segmentation of the ovum is regular, and gives rise to a two-layered stage
or _gastrula_, which opens by a pore, the _blastopore_. This does not,
however, become the mouth, but closes up and the mouth arises at the
opposite pole. Perhaps the most interesting feature of the development of
_Sagitta_ is that the cells destined to form the reproductive organs
separate from the other cells of the embryo at a very early date, whilst it
is still in the gastrula stage. There is no larval form, but the young
hatch out from the egg in a state resembling the adult in all respects but
that of size.
[Illustration: FIG. 103.—_Spadella cephaloptera._ Dorsal view. x 30. (From
Hertwig.) _a_, Cephalic ganglion; _b_, commissure to ventral ganglion; _c_,
olfactory organ; _d_, alimentary canal; _e_, ovary; _f_, oviduct; _g_,
testis; _h_, vesicula seminalis.]
HABITS.—The Chaetognatha are essentially pelagic, and {190}resemble many
other creatures that dwell at the surface of the ocean in being almost
completely transparent. Most species have been taken far out at sea, but
some are perhaps rather more numerous near the coast, and one species,
_Spadella cephaloptera_, is littoral. They swim by means of muscular
movements of the whole body; the fins have no movement of their own, and
seem to serve as balancers, and not as locomotory organs. Although usually
found at the surface of the water, many species have been taken at
considerable depths. Chun[225] states that they are found in countless
numbers at depths of from 100 metres to 1300 metres. The commonest species
at these depths are _Sagitta hexaptera_ and _Sagitta serratodentata_.
_Sagitta bipunctata_, according to the same authority, confines itself to
the surface. Whether the change of depth is diurnal, or whether it has any
relation to sexual maturity, or to any other cause, has not been
satisfactorily determined.
The food of the Chaetognatha consists of floating diatoms, Infusoria, small
larvae, and such Copepods as _Calanus finmarchicus_, and small Amphipods as
_Phoxus plumosus_.[226] At times they also devour small larval or
post-larval fishes, and owing to their incredible numbers, they doubtless
do considerable damage to sea fisheries. It is also recorded that they eat
one another, and specimens have been taken which have ingested the whole
body of another _Sagitta_ except the head, which hangs out of the mouth of
the eater, and gives it the appearance of a double-headed monster.[227] It
has been said that they attack hydroid polypes, but here at any rate they
do not have it all their own way. Masterman[228] has figured the apical
group of five polypes of _Obelia_, three of which are engaged in ingesting
as many young _Sagitta_.
They exist in incredible numbers; Grassi describes the surface of the sea
at Messina on certain days as being literally covered with them, and they
must form the food supply of numerous animals which prey upon the pelagic
fauna. The immense number of individuals is probably accounted for to some
extent by the fact that they lay eggs all the year round, and pass
{191}through a very short and rapid development. They are not known to be
phosphorescent.
CLASSIFICATION.—The features of the Chaetognatha which have most systematic
value are the size of the adult, the relations of the length to the
breadth, and of the three divisions to one another; the size, number, and
position of the lateral fins, and of the hooks and spines on the head; the
thickness of the epidermis, and the structure of the olfactory organ; and,
finally, the form of the reproductive organs.
Strodtmann,[229] who gives the latest and most complete account of the
species of Chaetognatha, arranges them under three genera, which he
characterises as follows:—
(i.) _Sagitta_ Slabber.—Two pairs of lateral fins, two rows of spines on
the head. The lateral thickening of the epidermis absent or insignificant.
Under this genus are included nine definite species and five others—_S.
gracilis_ Verrill, _S. elegans_ Verrill, _S. darwini_ Grassi, _S. diptera_
d'Orbigny, and _S. triptera_ d'Orbigny—whose position, owing to the
inadequacy of their description, is of doubtful validity.
[Illustration: FIG. 104.—_Sagitta hexaptera._ Ventral view. × 4. (From
Hertwig.) _a_, Mouth; _b_, hooks; _c_, anterior septum; _d_, alimentary
canal; _e_, commissure from the brain to the ventral ganglion; _f_, ventral
ganglion; _g_, ovary; _h_, oviduct; _i_, posterior septum; _j_, testis;
_k_, vesicula seminalis.]
The distribution of the other species may be mentioned. _S. hexaptera_ is
the largest Chaetognath known, and reaches in the adult stage a length of 7
cm. It is very widely distributed, being found in practically all the
temperate and warm seas, usually at the surface of the water, though at
times it is found at a depth of one metre, or even deeper. _S. lyra_,
Mediterranean, very rare. _S. tricuspidata_, widely distributed. _S.
magna_, Mediterranean and Madeiran, living at the surface. _S. bipunctata_,
the most frequently described form, smaller than the preceding species, 1-2
{192}cm. in length, widely distributed, and as a rule living near the coast
line. _S. serratodentata_, Mediterranean. _S. enflata_, on the surface of
the sea, Mediterranean and Madeiran. _S. minima_, a very small species, 1
cm. in length, Mediterranean. _S. falcidens_, Atlantic, off the coast of
New Jersey.
(ii.) _Krohnia_ Langerhans.—A single lateral fin extending on to both trunk
and tail segment, no lateral epidermal extensions behind the head, only one
row of spines on the head. Trunk longer than the tail.
_Krohnia_ has but two species: _K. hamata_ Möbius, with a length of 3-4
cm., found in the North Atlantic and at considerable depths, 200 to 300
fathoms; and _K. subtilis_ Grassi, 1.5 cm. long, with an extraordinary
slender body and a relatively large head, found at Messina, but very rare;
as a rule only one specimen has been found at a time.
(iii.) _Spadella_ Langerhans.—A single pair of lateral fins; these are
situated on the tail segment. Behind the head a thickening of the epidermis
extends down each side of the body to the fin, or even farther. Two rows of
spines on the head. Small animals, not longer than 1 cm.
[Illustration: FIG. 105.—_Spadella draco._ Dorsal view. × 12. (From
Hertwig.) _a_, Cephalic ganglion; _b_, commissure between the cephalic
ganglion and the ventral; _c_, eye; _d_, olfactory organ; _e_, alimentary
canal; _f_, ovary; _g_, oviduct (the line goes a little beyond the duct);
_h_, testis; _j_, vesicula seminalis.]
_S. cephaloptera_ Busch is the smallest species of Chaetognatha, attaining
at most a length of .5 cm. The body is not so transparent as in other
species, and is of a yellowish colour. It has been found from the Orkney
Islands to the Mediterranean. Strodtmann is of the opinion that the three
species _S. mariana_ Lewes, _S. batziana_ Giard, and _S. gallica_
Pagenstecher differ from the above-named only in size, or that their
description is too indefinite to permit of accurate {193}characterisation.
He recognises three other distinct species: _S. pontica_ Uljanin, from the
Black Sea; _S. marioni_ Gourret, from the Gulf of Lyons; and _S. draco_
Krohn, Mediterranean and Madeiran, and from the Canaries.
Much confusion has been introduced into the classification of the
Chaetognatha by Grassi,[230] who calls some—but not all—of what other
writers term _Sagitta_, _Spadella_, and _vice versâ_. The following table
was compiled by Strodtmann,[231] but I have incorporated in it two species
recently described from Amboyna by Béraneck,[232] and called by him
_Sagitta bedoti_ and _Spadella vougai_ respectively:—
CHAETOGNATHA
I. Two pairs of lateral fins; two rows of spines on the head; slender
forms.
(i.) Number of spines in posterior row greater than in anterior.
_a._ Border of hooks smooth, their point not curved.
α. No interval between the two fins on each side. 3.5 cm. long; 4-7
anterior spines, 8-11 posterior spines; olfactory organ lying
entirely on the trunk. The anterior nerves of the ventral ganglion
lie close to one another as far as the head.—SAGITTA LYRA.
β. A distinct interval between the two fins on each side.
_aa._ Adult animals large; hooks 6-7; anterior spines 3-4;
posterior spines 5-7; tail ¼ or ⅕ of the total length; lateral
areas relatively larger.—SAGITTA HEXAPTERA.
_bb._ Greatest length 1-2 cm.
αα. Thickening of the epidermis behind the head; prominently
projecting vesiculae seminales; olfactory organ very long;
hooks 8-10; anterior spines 4-6; posterior spines
10-15.—SAGITTA BIPUNCTATA.
ββ. No epidermal thickening; two caeca on the anterior end of
intestine; length 1 cm.; hooks 6-9; anterior spines 3-4;
posterior spines 7-8; point of the hooks somewhat bent
round.—SAGITTA MINIMA.
γγ. Epidermis thin; no caeca; hooks 8-9, their ends not bent;
anterior spines 3-4; posterior spines 7-8; length 2 cm.; small
head; trunk proportionately thick.—SAGITTA ENFLATA.
δδ. Hooks 11-14, usually 12; length 1.8 cm.; anterior spines
6-7; posterior spines 18.—SAGITTA FALCIDENS.
{194}εε. Hooks 7 on each side; length 1.3 cm.; anterior spines
8-10, posterior spines 18-22; no olfactory organ.—SAGITTA
BEDOTI.
_b._ Edge of hooks toothed and their point bent round; hooks 6-8;
anterior spines 6-8; posterior spines 10-12; length 1.5 cm.; slender;
conspicuously projecting vesiculae seminales.—SAGITTA SERRATODENTATA.
(ii.) Number of the spines in posterior row smaller than in anterior.
_a._ Anterior spines 3; posterior spine 1; hooks 8; length 3.5
cm.—SAGITTA TRICUSPIDATA.
_b._ Anterior spines 4; posterior spines 3; hooks 10-13; length 4.1
cm.; tail ⅕ of the total length.—SAGITTA MAGNA.
II. One pair of lateral fins lying on the trunk and tail; one row of
spines; body slender; epidermis not thickened.
(i.) Hooks 8-9, bent like an elbow at the point, serrated in the young;
20-25 spines in a row; ovary reddish; length 3-4 cm.—KROHNIA HAMATA.
(ii.) Hooks 8, broad at their base but very sharply pointed; spines in
a curved row, about 18, with a constriction below like the neck of a
bottle; body thin; length 1-1.5 cm.—KROHNIA SUBTILIS.
III. One pair of lateral fins, these lie on the tail; body relatively
very broad in consequence of the thickening of the epidermis lying behind
the head; two rows of spines; greatest length 1 cm.; tail and trunk
usually the same length.
(i.) A great extension of the epidermis behind the head, consisting of
very large cells; amongst these, at the level of the ventral ganglion,
lies a bundle of stiff hairs; tactile organ on papillae; hooks 9-10;
anterior spines 6-8; posterior spines 12-18.—SPADELLA DRACO.
(ii.) Lateral extension of the epidermis not so conspicuous, and the
cells composing it smaller. Tactile organs in little depressions.
Transverse as well as longitudinal muscles in the trunk. Adhesive cells
on the ventral surface of the body. No interval between the lateral
fins and the tail fin. Two papillae on the head-hood elongated into
club-shaped tentacles. Hooks 8-9, slightly serrated; anterior spines
3-4; posterior spines 3-4.—SPADELLA CEPHALOPTERA.
(iii.) Similar to the last-mentioned species, but the tail segment is
larger than the trunk; in the above it is of the same size. No adhesive
cells. The fins are covered with papillae, and with a number of
serrated spines pointed at both ends.—SPADELLA PONTICA.
(iv.) Tactile organs and adhesive cells are unmodified epidermal cells.
Anus dorsal. Orifice of oviducts ventral. No olfactory organ. Epidermis
colourless. Lateral fins without rays. A pair of ganglia at the
postero-lateral angle of the brain.—SPADELLA MARIONI.
(v.) Tactile organs well developed on the head, trunk, and fins; tail
segment a little shorter than the trunk. Body short, length 3-4 mm.
Hooks 9; anterior spines 4-5, posterior spines 6-7.—SPADELLA VOUGAI.
ROTIFERA, GASTROTRICHA, AND KINORHYNCHA
BY
MARCUS HARTOG, M.A., Trinity College (D.Sc. Lond.)
Professor of Natural History in the Queen's College, Cork.
{197}CHAPTER VIII
ROTIFERA, GASTROTRICHA, AND KINORHYNCHA
ROTIFERA—HISTORY—EXTERNAL FEATURES—MOVEMENT—ANATOMY—REPRODUCTION—
EMBRYOLOGY—CLASSIFICATION—DISTRIBUTION—AFFINITIES—GASTROTRICHA—
KINORHYNCHA
The Rotifera are microscopic animals, the largest not exceeding one-eighth
of an inch in length. According to Hudson and Gosse,[233] they are first
recorded in an observation of the Rev. John Harris, in 1696, of "an Animal
like a large Maggot which could contract itself into a Spherical Figure,
and then stretch itself out again; the end of its Tail appeared with a
_Forceps_ like that of an _Ear-wig_."[234] This was certainly a Bdelloid
Rotifer.
In 1703 Leeuwenhoek[235] gave a fuller description of a tubicolous form,
probably _Limnias_, and noted the peculiar appearance of the ciliary wreath
as "two wheels thickset with teeth as the wheel of a watch." He also noted
a little later[236] the way in which _Melicerta_ (see p. 206) builds its
tube, and was the first to observe the revivification of certain species
after drying.[237] Joblot, a French professor of mathematics, in 1718
figured and described a large number of new genera and species with more or
less fantastic details. Baker's figures[238] are a considerable advance on
Joblot's, and his descriptions of habits are still fresh and accurate.
Eichhorn found a number of new and interesting forms; and O. F. Müller,
influenced by the new discipline of Linnaeus, not {198}only figured many
species, but gave good short diagnoses of their characters. Ehrenberg in
1838 brought out his magnificent _Infusionsthierchen_, which contains
descriptions and figures of what are now divided into Protophyta, Protozoa,
Rotifera, and Gastrotricha. Dujardin's monograph on the "Infusoires," in
the _Suites à Buffon_,[239] was in several respects an advance on
Ehrenberg, whose power of observation was so great as to render his
mistakes the more inexplicable. But Ehrenberg ever adhered to his errors as
firmly as to his facts.
The occurrence of Rotifers among microscopic plants induced the botanists
Cohn and Williamson[240] to work at their structure; the group has been
studied by men engrossed in other professional cares, such as Gosse,
Bedwell, Moxon, Rousselet, and Maupas. Huxley,[241] Leydig,[242] and
Cohn[243] studied Rotifers in the '50's and early '60's with a precision
the more remarkable when we remember the imperfect methods then available.
This period was closed by the valuable monograph published in Arlidge's
(4th) edition of Pritchard's _Infusoria_,[244] under the supervision of W.
C. Williamson. Leidy began the study of the American Rotifers.
Eckstein[245] gave a careful and interesting account of the species about
Giessen in a richly illustrated paper. In recent times the modern methods
of histological and embryological research have been applied by
Vallentin,[246] Plate,[247] Tessin,[248] and Zelinka,[249] the three
_Studien ueber Rotatorien_ of the last author being indispensable to every
student, and containing a full bibliography.
Hudson and Gosse's Monograph (1886-89) contains a history of the class to
which, as to the whole book, we are deeply indebted; and a full systematic
account of all published species.[250] C. Rousselet has introduced a
method[251] of preparation of Rotifers in microscopic slides which enables
workers to preserve the types they figure and describe for future
identification and comparison. Gunson Thorpe has collected and studied
Rotifera in China and {199}Australia. It would be unfair not to record here
the invaluable services of the late Thomas Bolton, and his son of the same
name, both of Birmingham, and of J. Hood of Dundee, who have found and
widely distributed living specimens of new, rare, and interesting species.
[Illustration: FIG. 106.—_Hydatina senta_, ventral view. (After Plate.)
_al_, Lateral antenna; _bl_, bladder; _ci_, cingulum; _e_, _e_, eggs in
uterus; _fg_, foot gland; _g_, gizzard; _gg_, gastric gland; _gm_,
germarium or ovary; _gr_, ciliated lobes of "groove"; _i_, intestine; _k_,
_k_, kidneys; _m_, mouth; _ns_, nephrostome; _oe_, oesophagus; _rc_, renal
commissure, transverse tube uniting kidneys above mouth; _s_, stomach
overlaid by reproductive organs; _tr_, trochus; _u_, uterus; _vm_,
vitellarium or yolk-gland.]
DEFINITION OF THE CLASS.—We may define Rotifera as a class of minute
bilaterally symmetrical animals, with a chitinous integument, a soft
terminal "disc" fringed by a complex ciliary "wreath," an anterior or
subventral mouth, and a dorsal cloacal aperture, beyond which the body is
usually prolonged into the "foot" or process bearing cement glands, and
serving for attachment, temporary or permanent. The body-cavity has no
epithelial lining, and is traversed by nerves and muscles. The alimentary
canal possesses a chitinous gizzard or _mastax_ of peculiar arrangement,
and it usually opens into a cloaca. The nervous centre consists of a
ganglion on the dorsal side of the pharynx, to which a second one on the
ventral side is sometimes connected to form a complete ring; eyes and
bristle-bearing feelers are usually present as sense-organs. A paired
system of renal tubes serves for excretion, opening through a median
contractile bladder into the ventral side of the cloaca. The sexes are
distinct; but the males (Fig. 107), which mostly lack digestive organs,
occur {200}rarely, and the females are usually viviparous, or carry about
the eggs till they are hatched; while, owing to the rarity of the males,
parthenogenesis is habitual. Fission and budding are alike unknown. The
fertilised eggs are of the kind termed "winter" or "resting" eggs, and
resist conditions adverse to life.
The Rotifera are of cosmopolitan distribution; most of the species inhabit
fresh water, whilst some are brackish, and a few are marine; 84 genera and
about 700 species have been described.
[Illustration: FIG. 107.—Male Rotifers. (After Hudson.[252]) 1,
_Floscularia campanulata_; 2, _Lacinularia socialis_; 3, _Notops
brachionus_; 4, _Synchaeta tremula_; 5, _Asplanchna ebbesbornii_; 6,
_Brachionus urceolaris_; 7, _Salpina mucronata_; 8, _Pedalion mirum_.]
EXTERNAL FEATURES.[253]—The body is divided into three regions: (1) the
_head_, ending in the _disc_, which bears the ciliary _wreath_; (2) the
_trunk_, containing the viscera; (3) the _foot_, which only contains
muscles, nerves, and cement-glands. The general form of the BODY varies
greatly: it is spherical in _Trochosphaera_, ovoid in Asplanchnidae,
conical in Scirtopoda, Triarthridae, and _Synchaeta_; moderately elongated
in the majority of the Ploima, among which some forms are very flat, like
_Pterodina_, _Metopidia_, and _Brachionus_; shortly elongated and
cylindrical in _Hydatina_ (Fig. 106), Notommatidae, and many others. In
_Taphrocampa_ it is cylindrical and segmented, while the segments are
{201}telescopic in the Bdelloida, both ends being retractile into the
middle segment. In most attached, tube-dwelling forms the body is ovate,
tapering behind into the elongated stalk-like foot.
The FOOT at the hinder end of the body is usually more or less jointed; in
_Pterodina_ and _Brachionus_ it is long, transversely wrinkled, and
retractile. Usually it terminates in a couple of acute, mobile toes,
perforated at the tips by the ducts of the pedal glands (Fig. 106, _fg_),
whose viscid secretion serves to anchor the animal. In _Rotifer_ there are
three of these toes, which are retractile, and in addition there are in
this genus, as in most of the Bdelloida, toe-like pointed spurs in pairs on
the more proximal joints of the foot. In _Callidina_ the spurs are often
perforated, and the toes are replaced by numerous openings on the last
joint of the foot (Fig. 109, A); while in _Discopus_ the end of the foot
expands into a large disc, with numerous pores for the exudation of the
pedal cement, and there are no spurs. In _Pedalion mirum_ the foot is
represented by two tubular processes ciliated at the apex and at the outer
side near the base (Fig. 117, _f_). These are inconstant in size and form,
that of one side being sometimes reduced or absent, while both are absent
in the closely allied species _P. fennicum_.
In Melicertidae and Flosculariidae the long foot ends in an expanded disc,
which is cupped and ciliated in the larva (Fig. 112, B) and in the
larva-like male (Fig. 107); but in two species it is prolonged into a long
flexible thread which is not contractile. The foot is also elongated in the
Bdelloid genus _Actinurus_ and the Ploimal genus _Scaridium_. It forms a
mere ventral disc in _Apsilus_ (and _Atrochus?_), and is absent in
Asplanchnidae (except _Asplanchnopus_), Triarthridae, and Anuraeidae, and
in the genera _Trochosphaera_ (Melicertaceae) and _Pompholyx_
(Pterodinidae).
The fringed spines of Triarthridae are jointed appendages moved by powerful
muscles; in _Triarthra_ one is median and ventral, the others being
attached to the shoulders. In _Polyarthra_, there are twelve flattened and
serrated spines, a bunch of three being attached to the dorsal and ventral
faces of either shoulder. An easy transition leads to the hollow appendages
of Scirtopoda, which end in a fringe of bristly hairs, themselves feathered
with finer hairs (Fig. 117). These processes are in _Pedalion_ six in
number, two median (respectively dorsal and ventral), two antero-lateral,
and two postero-lateral. As they contain proper muscles, {202}and the
postero-lateral pair contain part of the nephridia and bear the lateral
antennae, they are true outgrowths of the body, and are not homologous with
the spines of Triarthridae.
[Illustration: FIG. 108.—Diagrammatic views of disc of Rotifers. Cingulum
represented by a black line, groove shaded; trochus dotted; the black spot
represents the mouth. 1, Simple disc of _Microcodon_; 2, Bdelloid disc of
_Rotifer_ or _Callidina_, the star represents the ciliated proboscis; 3,
disc of _Hydatina_, groove represented by lobes bearing ciliated styles; 4,
disc of _Melicerta_, the star represents the ciliated ventral cup with
openings into it from the groove; 5, disc of _Conochilus_; 6, disc of
_Stephanoceros_, cingulum (?) of setose lobes, trochus horseshoe-shaped,
mouth central.]
The front of the body constitutes the HEAD, which is scarcely distinct,
though usually separated by a slight neck-like constriction. The DISC,
which terminates the head, varies greatly in shape and in the arrangement
of its parts. Imagine a circular funnel, finely ciliated within, and with
the mouth at the bottom, the prominent rim bearing two zones of cilia, the
inner or anterior being the coarser, and termed the "trochus" or _hoop_;
the outer finer, and termed the "cingulum" or _girdle_, while a very finely
ciliated groove lies between the two zones. Either or both of these zones
may be interrupted on the dorsal or ventral median line, or both; and the
funnel-shaped mouth may be shifted—usually ventrally, so that it forms only
a dilatation of the ciliated groove. Again, the wreath as a whole may be
festooned or lobed; or the lobing may be confined to the area between the
cingulum and trochus, as in most Ploima (Figs. 106 and 108, 3). Very
frequently on these lobes adjacent cilia are fused together during life,
producing "vibratile styles," whose true nature is only revealed after
death. In Microcodonidae the structure of the disc (Fig. 108, 1) nearly
conforms to the primitive type; but the ciliated groove is absent, and the
"trochus" is in two separate half-elliptical bands. In the Flosculariaceae
(Fig. 108, 6) the mouth is also central, the disc is funnel-shaped,
{203}and the trochus is a horseshoe-shaped ridge, with its ends dorsal and
raised into prominent knobs. The margin of the funnel is in Flosculariidae
(Fig. 115) usually lobed, and furnished either with exceptionally strong
cilia, or else with very long bristles which are usually passive. However,
by the retraction of the lobes that bear them they are clasped together
like casting-nets to enclose prey brought into the funnel by the action of
the trochal cilia. An external ring of cilia in _Floscularia mutabilis_ and
_F. pelagica_ serves for swimming. In Apsilidae the margin of the disc
bears neither cilia nor bristles, but is either simple and ring-like, or is
produced into tentacles (Fig. 112, C). The oral funnel is probably
represented in Flosculariaceae by the continuation of the small central
mouth into a ciliated tube (Fig. 115, C, _tf_), open below, and hanging
freely down into the crop.
In all other cases the mouth is displaced, and lies in the groove and on
its ventral side (except in _Conochilus_, where it is dorsal, Fig. 108, 5).
In the Bdelloida the disc is prolonged into two great lobes like
kettle-drums, round the posterior, external, and ventral edges of which run
the trochus, cingulum, and ciliated groove (Fig. 108, 2). All three are
interrupted behind in the median line; ventrally the groove widens into the
oral funnel, the cingulum is continued into a sort of spout-like lower lip
(Fig. 109, C, D, _l_), and the trochus is absent. The body is prolonged
dorsally above the lobes into a two-jointed _proboscis_, ending in a
ciliated cup overhung by two dorsal flaps: this we regard as a detached
portion of the wreath.
This "Bdelloid" type of wreath occurs also in Scirtopoda (Fig. 117), and in
the Ploimal genera _Triarthra_, _Pterodina_, and _Pompholyx_. A simpler
wreath of essentially the same type occurs in Asplanchnaceae and
Melicertaceae; the disc is not prolonged into drum-shaped lobes, but is
thin at the rim, where it bears the triple ciliated zone, interrupted on
the dorsal median line and depressed ventrally into the oral funnel. In the
Melicertidae, moreover, the disc is widened into a great plate-like
extension, often beautifully lobed; and in many of the species a ciliated
cup lies ventral to the lips, and is connected with the groove by a short
ciliated channel on either side (Figs. 108, 4, and 116). Even the simpler
wreath of Asplanchnidae is complicated by stronger lobes on either side
bearing vibratile styles.
The most complex discs are found in Ploima, especially in
{204}_Brachionus_, _Hydatina_, and _Synchaeta_, since the groove is
replaced by a zone of lappets, as above mentioned. In _Proales_ the whole
face of the disc is strongly ciliated. The wreath is reduced in the
parasitic genera _Drilophagus_, _Albertia_, _Balatro_, and the Seisonaceae;
in _Adineta_ and _Taphrocampa_ it is only represented by a general but
scanty ciliation of the disc.
[Illustration: FIG. 109.—_Callidina symbiotica._ (After Zelinka.) A,
Ventral view, with the disc half expanded, proboscis extended; B, lateral
view, proboscis extended; C, ventral view of anterior segments with
expanded disc; D, lateral view of same (proboscis retracted). _a_, Antenna;
_bl_, bladder (enlargement of rectum); _c_, ciliated cup of the proboscis;
_ci_, cingulum; _cl_, cloaca; _cp_, group of pores, the openings of cement
glands; _di_, disc; _g_, gizzard; _gm_, germarium (that of the opposite
side seen at a higher level); _gr_, ciliated groove; _k_, kidney; _l_, lip;
_m_, mouth; _pr_, proboscis; _sp_, spurs of foot; _tr_, large cilia of
trochus, showing vertical movements; _vm_, yolk-gland. The body muscles are
represented by shaded bands.]
The head is very frequently retractile, as a whole, by strong muscles. In
Bdelloida the disc proper is retracted when the animal crawls, while the
proboscis is exserted (Fig. 109). Ciliated patches occurring outside the
region of the disc point to {205}a primitive condition when the whole
surface of the body was ciliated, as does the partial ciliation of the foot
in certain groups. _Synchaeta_ and many Notommatidae possess a pair of
lateral, hollow, ciliated pits on the body, which can be everted to serve
as additional swimming organs; these are termed "auricles."
The _cuticle_ varies much in texture. It may be smooth and flexible, dotted
or shagreened, or in the Loricata firm and of definite shape, constituting
a _lorica_, which may be more or less distinctly divided up into areas or
separated into distinct pieces. In this case it resists decomposition, and
several species are only known by this "skeleton." In _Ploesoma_ it is much
thickened and looks like a honeycomb. A regular alternation of harder and
softer zones effects the annulation of the body in certain genera.
The _hypoderm_ or protoplasmic layer of the skin has no cellular
boundaries, though it contains large and distinct nuclei; it is usually
somewhat granular. It forms the wall of the body-cavity, which contains a
transparent liquid without corpuscles.
The principal _external glands_ are the pedal or cement-glands, which
secrete a viscid substance that sets in water and serves to anchor the
animal. They are formed from an ingrowth of the hypoderm, are usually
paired, and open by fine ducts on or near the apex of the toes, when these
processes of the foot are present (Fig. 106, _fg_). These glands are mostly
absent when there is no foot, as in most Asplanchnidae and in Anuraeidae,
but in _Asplanchna herrickii_ a small gland on the ventral side of the
cloacal aperture appears to represent the last rudiment of the foot.
In addition to these, the ciliated ventral cup below the disc of many
Melicertidae secretes a viscid substance (Fig. 116, _p_); and possibly the
whole surface of the body is secretory in those species of this group, and
of the Flosculariidae, whose tube (Fig. 115, A) is uniform and not made of
pellets. In several other species belonging to Bdelloida and
Ploima-Illoricata a viscid secretion of the surface of the body renders it
"sordid" with adherent particles of dirt.
When the secretion takes the form of a tube, the body can be wholly
withdrawn into it by the contraction of the foot. In _Floscularia_,
_Stephanoceros_, and _Conochilus_ the tube is hyaline and thin-walled; in
_Oecistes_ and _Cephalosiphon_ it is more or less floccose; and in
_Limnias_ it is thin, firm, and annulated. In _Melicerta_ and some species
of _Oecistes_ the tube thus secreted by {206}the body is only formed in a
very young state. In _M. janus_ and _M. pilula_ it is increased by the
successive deposition of ovoid faecal pellets on to the rim. In _M.
ringens_ (Fig. 116) and _M. conifera_ pellets are formed of the excess of
the food particles brought to the disc by the ciliary current; they are
carried through the gutters on either side of the projecting ventral lip or
"chin" into the ciliated glandular cup on that side of the head. Here, as
they revolve, they are cemented together into a pellet which is spheroidal
in the former species, cylindro-conoidal with a basal hollow like a
rifle-bullet in the latter. After a pellet is completed the animal stoops
down and deposits it on the edge of the tube. This may easily be verified
by furnishing a young _Melicerta_ with water containing solid particles of
carmine. _M. tubicolaria_ forms a thick tube which is laminated, the
laminae being directed upwards and outwards, and having diatom shells,
etc., between the layers. In this case we have observed that the faeces are
pellucid, and sometimes are so ejected as to lie in a sheet against the
funnel-shaped mouth of the tube, and we are inclined to believe that the
tube itself is formed altogether in this way. A similar process probably
occurs in _Oecistes crystallinus_ and _Oe. umbella_.
The _muscles_ are simple elongated fibres, usually having near the middle a
mass of granular protoplasm containing a nucleus; they may be smooth or
striated. The principal muscles of the body are conspicuously striated in
many active free-swimming forms (_Pedalion_, _Synchaeta_, _Pterodina_,
_Triarthra_).
The muscles of the body-wall are transverse and longitudinal. They are best
seen in Bdelloida. The principal muscles of the body-cavity are
longitudinal; the most conspicuous and constant are the retractors of the
disc and of the foot, protraction of these organs being usually
accomplished by the contraction of the transverse muscles. Special muscles
effect the vigorous springing of the Triarthridae and Scirtopoda; in the
former group the muscles only raise the spines, and their elastic recoil is
the actual mechanism of progression; but in the latter (Fig. 117) special
flexor muscles of the limbs are the effective agents of the leaping
movements.
MOVEMENTS.—The Rotifera vary very greatly in their movements. The cilia of
the disc, and especially of the trochus, are the principal organs of
prehension of food, and also of swimming when {207}the animal is not fixed
by its foot. In some cases, as in Bdelloida, the cilia lash downwards
successively in the longitudinal plane of the body (Fig. 109, C, D); this
motion during fixation produces a hollow vortex ring, like the rings of a
skilled cigarette-smoker, but when the animal is free it determines a
simple forward progression through the water. In other cases the animal
rotates on its long axis, or may even turn somersaults (_Synchaeta_). The
appearance of the spokes of a wheel is a pure illusion due to the greater
visibility of the cilia in their slow recovery than in their instantaneous
down-lash. The finer cilia of the groove and cingulum play a very minor
part in the act of swimming, and in the production of the great vortices at
the edge of the disc when the animal is fixed; they serve to direct the
particles brought by the vortices to the edge of the disc onwards towards
the mouth. It is easy to see that the stream must be in opposite directions
on opposite sides of the groove; its prolongation across the dorsal median
line would be useless, which explains the existence of the dorsal median
gap. At the ventral side we usually find a prominent ciliated lip, whose
cilia work outwards, and carry off the excess of food particles as by an
overflow spout. In many cases among the Notommatidae, Coluridae, etc., the
disc serves as much for creeping over organic débris as for swimming.
We have already noticed the springing bristles and limbs of the
Triarthridae and Scirtopoda respectively; the great foot of _Scaridium_ is
also used for leaping. The Bdelloida have the power of retracting their
disc and progressing in loops like a leech or looper (Geometrid)
caterpillar.
Baker, in a letter addressed to Martin Folkes, Esq., President of the Royal
Society, dated London, 16th January 1744-5,[254] gives the following lively
account of the aspect and movements of _Philodina roseola_ belonging to
this group, with figures, some of which we reproduce from the original
copper-plate engraving:—"I call it a _Water Animal_, because its Appearance
as a living Creature is only in that Element. I give it also for
Distinction Sake the Name of _Wheeler_, _Wheel Insect_ or _Animal_; from
its being furnished with a Pair of Instruments, which in Figure and Motion
appear much to resemble Wheels. It can, however, continue many Months out
of Water, and dry as Dust; in which Condition its Shape is globular, its
Bigness exceeds not a Grain of {208}Sand, and no Signs of Life appear.
Notwithstanding, being put into Water, in the Space of Half an Hour a
languid Motion begins, the Globule turns itself about, lengthens by slow
Degrees, becomes in the Form of a _lively Maggot_, and most commonly in a
few Minutes afterwards puts out its Wheels, and swims vigorously through
the Water in Search of Food; or else, fixing by its Tail, works them in
such a Manner as to bring its Food to it. But sometimes it will remain a
long While in the Maggot Form and not shew its Wheels at all....
[Illustration: FIG. 110.—_Philodina roseola._ (After Baker.) A, B,
Crawling, with extended proboscis, and showing antenna; C, D, E, attached,
with "wheels" extended for catching food; F, attached, with anterior end
retracted.]
"If the Water standing in Gutters of Lead, or the slimy Sediment it leaves
behind, has any Thing of a red Colour, one may be almost certain of finding
them therein,[255] and, if in Summer, when all the Water is dried away, and
nothing but Dust remains, that Dust appears red, or of a dark brown, one
shall seldom fail, {209}on putting it into Water, to discover Multitudes of
minute reddish Globules, which are indeed the Animals, and will soon change
their Appearance, in the Manner just now mentioned....
"A Couple of circular Bodies, armed with small Teeth like those of the
Balance-Wheel of a Watch, appear projecting forwards beyond the Head, and
extending sideways somewhat wider than the Diameter thereof. They have very
much the Similitude of Wheels, and seem to turn round with a considerable
Degree of Velocity, by which Means a pretty rapid Current of Water is
brought from a great Distance to the very Mouth of the Creature, who is
thereby supplied with many little Animalcules and various Particles of
Matter that the Waters are furnished with.
"As these _Wheels_ (for so from their Appearance I shall beg Leave to call
them) are every where excessively transparent, except about their circular
Rim or Edge on which the Cogs or Teeth appear, it is very difficult to
determine by what Contrivance they are turned about, or what their real
Figure is, though they seem exactly to resemble Wheels moving round upon an
Axis....
"As the Animal is capable of thrusting these Parts out, or drawing them in,
somewhat in the Way that Snails do their Horns, the Figure of them is
different in their several Degrees of Extension and Contraction, or
according to their Position to the Eye of the Observer, whereby they not
only appear in all the various Forms before represented, but seem at
certain Times as if the circular Rim of the Wheel or Funnel were of some
Thickness, and had two Rows of Cogs or Teeth, one above and the other below
that Rim."
DIGESTIVE ORGANS.—The _pharynx_ is usually a narrow ciliated tube, which
varies in length from genus to genus, but in no other important point, save
in Flosculariidae, where it assumes the form of a crop, into which the
mouth hangs freely down as a narrow ciliated tube. At its lower end is an
enlargement, the _mastax_ or gizzard.[256] This is a strong muscular sac
containing the _trophi_ or hard chitinous chewing organs, with an
{210}antero-ventral inlet from the pharynx, and a postero-dorsal outlet
through which the food passes into the stomach either directly or through a
slender gullet (Fig. 106, _oe_). In the ventral wall of the gizzard of most
Ploima is a median piece, the _fulcrum_, from which run forwards and
upwards two pieces, the _rami_, which are hinged on the fulcrum. The
Y-shaped structure formed of these three pieces is called the _incus_
(anvil). At either side of the gizzard and at a higher level is a paired
piece, the _malleus_, so called from its resemblance to a hammer, of which
the _manubrium_ (handle) looks backwards, and is embedded in the side walls
of the mastax, while the toothed claw or _uncus_ looks forwards and
inwards, and is hinged at its inner side with the tip of the ramus. As the
unci and rami are usually strongly toothed, this gizzard forms a very
efficient apparatus for chewing. In some cases, when the pharynx is short
and dilatable, the points of the unci and rami may be protruded for biting,
for clinging to the host (in the parasitic genera _Albertia_ and
_Drilophagus_), or for the prehension of food (Rattulidae, etc.).
[Illustration: FIG. 111.—Diagram of trophi. (After Hudson.) A, Malleate; B,
submalleate; C, virgate; D, forcipate; E, malleoramate (_Melicerta_); F,
incudate (_Asplanchna_); G, uncinate (_Stephanoceros_); H, ramate
(_Rotifer_). _f_, Fulcrum; _i_, incus; _ma_, manubrium (malleus in G); _r_,
ramus; _un_, uncus.]
The type we have just described is termed the "malleate" type (Fig. 111,
A). If all the trophi are slender and scarcely toothed, we have the
"virgate" type (C), which is frequently {211}asymmetrical. In the
"submalleate" type (B) the mallei only are slender; in the "forcipate" type
(D) both the unci and rami are slender and sharply pointed.[257] In the
"malleoramate" type (E) the manubrium is a curious looped structure, while
the uncus is formed of a number of parallel slender elongated teeth; this
characterises the family Melicertidae, and the genera _Triarthra_,
_Pterodina_, and _Pedalion_. In the "uncinate" type (G) the mallei are
simply incurved hooks with a few teeth at the free end, the rami are simple
or absent, and there is no fulcrum; this type occurs in Flosculariaceae
only. In Asplanchnidae the rami are large and hooked, constituting the
"incudate" mastax (F); but here reduced mallei are often present, and in
_Asplanchnopus_ they are almost as well developed as in Melicertidae,
affording a transition to the malleoramate type. In this group too the
mastax has a very peculiar form; it is divided into two chambers, dorsal
and ventral. The dorsal chamber forms a great purse-like sac or crop, with
a framework of four longitudinal bars: into this the gullet and pharynx
open. The ventral pouch is much smaller, and in its base the large rami are
inserted, so that they can be protruded into the crop. This ventral sac
with the rami may even be everted through the crop and the mouth, to
swallow the small Rotifers and Entomostraca which form the food of this
group, or to eject the undigested remains of the food. Two lateral sacs
open at the junction of the ventral pouch and the crop, but whether they
play a part in the deglutition of food or in the disgorging of faeces is
uncertain. The fact that the whole of this apparatus is lined by a
non-ciliated chitinous cuticle justifies our view that it is simply an
enlargement and specialisation of the mastax.
The trophi in Bdelloids also are only represented by the rami, which have
the form of segments of a sphere, excavated on the curved sides for the
attachment of muscles, and transversely ridged on the two flat sides; the
gizzard is here called "ramate" (H).
It will be seen that the characters of the gizzard are very useful for
classification, only breaking down indeed in the {212}Ploima; for though
the majority of these present one or other of the four varieties of the
malleate type, _Triarthra_ and _Pterodina_ (but not the other genera of
their respective families) have the gizzard malleoramate.
The _oesophagus_ is, when present, a contractile ciliated tube in which the
food makes no sojourn on its way to the stomach.
The _stomach_ may be nearly spherical, ovoid, or elongated and cylindrical.
Its walls are formed of large cells, often granular and sometimes brownish,
whence a hepatic function has been assigned to them. Its apertures are both
surrounded by constricting muscular fibres. The _intestine_ may be simple
or divided by a similar constriction into _intestine_ proper and _rectum_.
The whole of the alimentary tract, with the exception of the mastax, is
richly ciliated within. The rectum opens into the slender non-ciliated
cloaca. The intestine is sharply bent upwards and towards the back in the
tubicolous forms, but is nearly straight elsewhere; in _Trochosphaera_ and
_Apsilus_ it is bent ventrally. In Asplanchnaceae and in _Paraseison_ there
is no rectum, the stomach being a blind sac.
The so-called _salivary glands_, usually two in number, open into the
pharynx or mastax; and the paired _gastric glands_ (Fig. 106, _gg_) open
into the oesophagus or stomach. While the prehension of food is usually
accomplished by the ciliary current of the disc and pharynx, we have seen
that a more active swallowing action takes place in Flosculariaceae and
Asplanchnidae, which devour whole Algae, Infusoria, and even other
Rotifers, the long spines of _Triarthra_ not availing as a protection. Many
Ploima put out the tips of their trophi to nibble at débris, or, in the
case of _Diglena_ and _Distemma_, to attack Desmids, or the Infusorian
_Stentor_. But this use of the trophi is most efficient in _Ploesoma_.
Bilfinger[258] writes: "It has the courage to attack larger Rotifers; thus
I was able to observe under the microscope how it fell upon a _Rattulus_
but little smaller than itself and destroyed it. First it plunged the sharp
prongs of its mastax deep into the tender frontal area of its unhappy
victim; then followed a pumping action of the gizzard, and stroke by stroke
the whole contents of the victim's body passed into the brigand's stomach."
From this it is an easy transition to the ectoparasitism of _Drilophagus_,
_Balatro_, and {213}some species of _Albertia_, which cling to their host
by the exserted trophi.
RENAL ORGANS.—The kidneys consist of a pair of convoluted tubes, formed of
a succession of perforated, so-called "drainpipe" cells (Fig. 106, _k_);
they open directly or indirectly into the cloaca. Their walls are thin in
the straight parts, but thick and glandular in the coils which occur at
intervals. These tubes bear little tag-like appendages, hanging freely into
the body-cavity, often widening towards the free end, and flattened or
circular in section (Fig. 106, _ns_). They show during life a peculiar
flickering motion in their interior, like the equivalent "flame-cells" of
many Platyhelminthes (see p. 25), and are in function the representatives
of the multicellular renal funnels of Annelids. On one side, especially on
the edge of the flattened tags, the appearance is as of a tapering
whip-like lash, attached by its base to the free end of the tag and waving
in its cavity; but the side view of the flattened tags shows an appearance
of successive transverse or oblique waves. In many if not all cases the
free end of the tag is closed by a vacuolated plug of protoplasm, which
sometimes at least bears two flagella waving freely in the body-cavity. The
probable explanation of the two distinct wave appearances within the tag is
that the protoplasmic plug bears on its inner face a row or tuft of long
cilia hanging down into the cavity of the tag. The tags probably keep up a
current of liquid through the kidneys, while the contents of the
body-cavity are constantly replenished by osmosis.
The two renal tubes may end blindly below the disc, or else join by a short
transverse dorsal communication in front of the brain, as in
_Stephanoceros_, _Atrochus_ (Fig. 112, C), and _Apsilus_ among
Flosculariaceae, _Lacinularia_ among Melicertidae, and _Hydatina_ among the
Illoricate Ploima (Fig. 106, _rc_). In some species of _Asplanchna_, if not
all, a recurrent branch occurs opening at either end into the main tube of
its own side.
The kidneys unite to discharge into the cloaca near its orifice, and on its
distal (primitively ventral) side in many Melicertidae. In Bdelloida the
common duct formed by their fusion opens into the ventral side of a dilated
bladder-like section of the cloaca (Fig. 109, A, _bl_), which contracts
rhythmically to discharge the liquid; while in the majority of the class
they open singly or by a common duct into a separate contractile vesicle or
_bladder_, which also discharges at regular intervals into the cloaca on
its ventral or distal side (Figs. 106, _bl_ and 112).
{214}[Illustration: FIG. 112.—Apsilidae: A, _Apsilus lentiformis_, ♀,
dorsal view (after Metschnikoff); the square brain is seen with nerves to
the lateral antennae; B, larva of _A. lentiformis_ (?), showing the paired
eyes and ciliated cupped foot; C, adult of _Atrochus appendiculatus_, ♂
(after Wierzejski). _al_, Lateral antennae; _am_, median antenna (just in
front is seen the renal commissure); _an_, anus; _br_, brain, below which
the paired eyes are seen; _c_, cloaca; _em_, embryo; _em'_, _em'_, _em'''_,
three successive stages of embryos in the uterus of C; _k_, kidney. The
coarser muscles are striated.]
This bladder may reach when expanded one-third the diameter of the whole
animal, and contract as often as three times per minute; so that in a
period of nine minutes a bulk of water equal to that of the animal must
have diffused through the body-wall, to be removed by the kidneys. It is
obvious that while the function of the kidneys is primitively excretory,
the passage of the water through the body must bring in the oxygen
dissolved in the external {215}medium, and carry off the carbonic acid
formed in the tissues, and so fulfil the act of respiration. This mechanism
is physiologically comparable with that of the contractile vacuole of
fresh-water Protozoa. In a few genera (_Conochilus_, _Lacinularia_,
_Pterodina_) the kidneys open separately after a slight dilatation into the
cloaca.
NERVOUS SYSTEM.—The nervous centre of the Rotifera is the _brain_ (Fig.
112, C, _br_), a ganglion lying dorsal to the pharynx; and when this is
short it may be immediately below the surface of the disc (_Microcodon_).
In Bdelloida a second ganglion is present below the pharynx, and is
connected with the former by lateral cords which contain ganglion cells.
From the brain, nerves are given off to the disc, to the muscles, and to
the integument of the body, as well as to the sense organs. The largest
nerves are two given off from the sides of the brain, each of which divides
into a lateral and a ventral trunk, which run nearly the whole length of
the animal.
The brain of several Notommatidae has a curious appendage, white by
reflected light and very opaque; it is a sac full of chalky mineral matter,
which dissolves readily in dilute acids.
SENSE ORGANS.—The most widely diffused sense organs are the antennae or
feelers, which may serve for touch or smell, or possibly both. Each antenna
is a conical or tubular outgrowth of the skin; from its apex projects a
fine pencil of sense hairs borne on a protoplasmic cushion, which receives
a nerve. Often the antenna is elongated, and may then contain a muscle by
which it is retractile (lateral antennae of _Melicerta_); sometimes it is
reduced to a slight prominence bearing the setae (dorsal antenna of this
genus). There are usually three antennae—a _median dorsal_ (Figs. 109, B,
_a_, and 112, C, _am_) and two _lateral_ (Figs. 106, 112, C, and 115, A,
_al_), often approximated towards the ventral surface, and sometimes all
but fused on the middle line, or completely united (_Conochilus
dossuarius_, _Copeus caudatus_).[259]
Most Rotifers possess an organ of sight. This in its simplest form is a
refractive globule seated in a red pigmented cup through which the nerve
passes; in other cases it lies directly on the brain. Very frequently the
eye is paired (Figs. 112, B, and 115, A); and these paired eyes may lie on
the brain, and then {216}are so close together that the pigment-cups have
the shape of an _x_, or else they are seated in the dorsal region of the
head behind the disc. In some cases they lie just under the ciliary wreath,
or even within the region of the disc, and pass towards its ventral side in
_Pedalion_ (Fig. 117, A, _e_). In _Rotifer_ they lie just under the dorsal
side of the proboscis just below its apex. The median and two lateral eyes
often exist together, as in _Eosphora_; and sometimes additional paired
eyes exist. In _Furcularia longiseta_, var. _grandis_ a pair of pigment
spots (eyes?) occurs at the hinder end of the body just in front of the
foot.
The active Ploima show a spontaneity of movement and marked power of
avoiding obstacles, etc. This is still more marked in the very active
_Pedalion_, which, as Rousselet notes, clearly avoids capture by the
dropping tube, aided by its sense of sight, as he suggests, or by the
tactile or olfactory powers of the antennae. They must rank as psychically
high in the scale of creatures of simple organisation.
REPRODUCTIVE ORGANS AND REPRODUCTION.—The most conspicuous organ in the
female is the large _yolk-gland_ or _vitellarium_ (Figs. 106 and 109, A,
_vm_), which was regarded as the ovary by all the older observers. It
consists usually of eight cells, with conspicuous nuclei, lying on the
ventral side of the stomach, and frequently displaced to one side; but in
most Asplanchnidae it forms a broad transverse band of numerous cells. In
_Pterodina_ it is horseshoe-shaped, while in Seisonaceae and Bdelloida it
is paired, either gland containing four or eight cells. The true _ovary_ or
_germarium_ (Fig. 106, _gm_) lies more or less hidden between the
yolk-gland and the stomach; it is composed of numerous minute rounded
cells, of which the hindmost for the time being enlarges by nutrition from
the yolk-gland, and finally receives a membranous shell. This true ovary is
somewhat lateral in most Rotifers, but is median in Asplanchnidae, and
paired in _Pterodina_, Bdelloida, and Seisonaceae. A membranous covering is
common to the ovary and yolk-gland (paired when these are paired); it is
continued into a thin-walled tube or oviduct, which opens into the cloaca
on its ventral side beyond the bladder or common renal duct. In the
viviparous species the mature ovum (Fig. 112, _em_) usually lies in the
oviduct, dilating it into a sort of "uterus" until the birth of the young.
The ordinary eggs or "summer eggs" are formed without any
{217}fertilisation, and develop immediately; they are often hatched within
the tube of the tubicolous species.
Under certain conditions the unfertilised females produce exclusively
smaller eggs, which develop into males. Maupas[260] has demonstrated that a
rise in temperature to a minimum of 26° C. (79° F.) is the efficient
factor. But as Bergendal points out,[261] the critical temperature probably
varies with the antecedent conditions of the race, since males occur in
Greenland at a very much lower temperature; and it would seem probable that
a temperature approaching that at which the pools habitually dry up is what
is necessary for the production of males, as a provision for those
fertilised eggs, which, having a hard shell often adorned with prickly
prominences, and usually remaining for some time before development, are
capable of withstanding drought; such eggs are termed "winter eggs," but a
better term would be "resting eggs" (German, "Dauereier").[262]
[Illustration: FIG. 113.—_Diglena catellina._ (After Weber.) A, Male; B,
the pair _in copula_; C, female, _p_, Penis; _te_, testis.]
The male organs consist of a testis (Fig. 113, A, _te_) with accessory
glands, a large seminal vesicle, and a protrusible or projecting penis
(_p_). In _Notommata_ and _Diglena_ true intromission at the cloaca (B) has
been seen by many observers; but it {218}appears equally certain that in
many cases the male bores into the body-wall of the female at any point,
and deposits the spermatozoa in the body-cavity, so that they must pass
through the wall of the oviduct to effect fertilisation. Maupas finds that
the process of fertilisation is ineffective except upon such newly-hatched
females as would otherwise be the parents of small male eggs; that
fertilisation is inoperative even for these at a later age when their eggs
have begun to mature; and that it is wholly useless for those that lay
ordinary summer eggs. The parent of male or winter eggs would thus be
comparable to the queen bee, which if not fertilised produces drones. These
sexual relations find a close parallel in the Ostracod and Phyllopod
Crustacea, as well as in many plant-lice (Homoptera).
DEVELOPMENT.—This has only been fully studied in the summer egg; in
_Brachionus_ by Salensky,[263] in _Melicerta_ by Joliet[264]; in _Eosphora
digitata_ and several other species by Tessin[265]; in _Callidina_ and
_Melicerta_ by Zelinka,[266] the last two observers having utilised modern
methods of research.[267] We shall base our account on Zelinka's
observations. As in the case of most "parthenogenetic" eggs, the ovarian
egg begins by a very uneven division to form two cells: the minute "first
polar body" which undergoes no further development; and the definitive egg,
which by its repeated divisions gives rise to the tissues and organs.
Segmentation is very unequal, and recalls that of Molluscs in several
respects. The first division gives rise to a smaller and a larger cell.
Both of these divide again, the latter unequally, so that now there are
three smaller cells and one large one; and after repeated divisions of the
small cells and unequal divisions of the larger one, a stage is reached
where there are a number of small cells and one large one, which sinks in
and is overgrown by the small ones. Just prior to this the large cell
undergoes equal divisions; its cells are the "hypoblast" cells (Fig. 114,
_hyp_), and give rise to the gullet, stomach, and intestine, with their
appendages, and the generative organs; while the smaller cells constitute
the "epiblast" (_ep_), which gives rise to the body-wall and muscles, to
the cement glands, nervous system, pharynx and mastax, and probably to the
kidneys.
{219}[Illustration: FIG. 114.—Development of _Callidina_. (After Zelinka.)
A, Early stage showing involution of granular cells (_g_), to form the
mastax or gizzard. B, Involution complete. C, Second involution of epiblast
cells to form pharynx. D, The embryo bent on itself at ventral fold (_vf_).
E, Showing ingrowth of epiblast to form brain (_br_): _an_, involution of
epiblast to form cloaca; _br_, brain; _ep_, epiblast; _fg_, involution to
form cement glands of foot; _g_, granular cells; _gi_, gizzard; _hyp_,
hypoblast; _m_, mouth; _o_, ovary; _sp_, salivary glands; _vf_, limiting
body from foot.]
Owing to the elongation of the body within the narrow space of the egg the
hinder part is bent up on the ventral surface (D, E); and this part,
narrower than the rest, forms the foot, the centre of which is at first
occupied by a column of hypoblast. The cloaca is now formed by a dorsal
ingrowth of epiblast (the "proctodaeum") at the junction of the foot and
the body (_an_). The hypoblast in the body anterior to the cloacal ingrowth
forms the digestive apparatus; the part immediately behind forms the
reproductive organs (_o_); and the hindmost part apparently disappears. An
ingrowth of epiblast at the extreme tip of the foot gives rise to the
cement glands (_fg_). The muscles arise from the epiblast cells. The disc
arises from the modification of epiblast cells lateral to and behind the
mouth, enclosing a so-called "polar area"; it is completed by the
transformation of cells on the ventral side of the mouth. The brain (_br_)
is formed by the multiplication of epiblast cells; and in Bdelloida a
ventral ingrowth below the mouth forms the sub-oesophageal ganglion. The
ciliated cup in _Melicerta_ is formed as a ventral hollow, only later on
united with the ciliated furrow of the wreath by the lateral grooves.[268]
In _Melicerta_ the two eyes are formed in the polar area. The young as
hatched {220}differs from the adult in the greater simplicity of its
ciliary wreath; and in the tubicolous forms the cupped end of the
foot-gland is ciliated, and two eyes are present on the polar area, which
later sink in, and often disappear more or less completely. It is stated
that the young hatched from winter eggs do not pass through this larval
state.
[Illustration: FIG. 115.—_Stephanoceros eichhornii._ (After Cubitt.) A,
Dorsal view of the upper part in its tube: _al_, lateral antenna; _e_, eye;
_em_, developing embryo in uterus; _g_, gizzard; _s.g_, median salivary
gland. B, Extremity of foot. C, Lateral view of base of disc: am, median
antenna; _of_, oral funnel; _s.g_, median salivary gland; _tf_, in the
crop, indicates the ciliated tube prolonging the funnel; _tr_,
horseshoe-shaped trochus.]
CLASSIFICATION.[269]
ORDER I. FLOSCULARIACEAE.—Females mostly tubicolous, attached by a long
contractile foot. Disc produced into a wide funnel-shaped contractile cup,
produced into lobes with long setae (_Floscularia_) or coarse cilia
(_Stephanoceros_), or entire (Apsilidae); an outer row of fine cilia rarely
present; trochus a horseshoe, open behind. Oral funnel a slender tube
hanging freely into a large pharyngeal crop; trophi uncinate projecting
freely into the crop. Kidneys often united by an anterior cross-piece.
Body-wall often containing a definite system of canals, filled with
refractive granules, and serving by their contraction to {221}dilate the
disc. Males (Fig. 107, 1) and larvae vermiform with a ciliated pedal cup,
and a simple wreath, with two eyes on the disc.
Fam. 1. Flosculariidae: _Floscularia_ Oken, _Stephanoceros_ E. (Fig.
115).
Fam. 2. Apsilidae: _Apsilus_ Metschnikoff (Fig. 112, A), _Acyclus_ Leidy,
_Atrochus_ Wierzejski (Fig. 112, C).
The family Flosculariidae contains some most exquisite forms;
_Stephanoceros_, the "Crown Animalcule," being probably the most lovely of
the Class, and many of the Floscules coming not far behind. The Apsilidae
are mostly mud-dwellers.
ORDER II. MELICERTACEAE.—Females (except in _Trochosphaera_) attached or
tubicolous; tube variable. Disc with a dorsal gap (except _Conochilus_)
often two-lobed or corolla-like; a ventral lip often separating off a
ventral ciliated cup continuous by a pair of gutters with the ciliated
groove; trochus of stronger cilia than the cingulum. Trophi malleoramate in
a distinct mastax. Intestine much curved dorsally, cloaca long eversible
(except _Trochosphaera_). Males and larvae as in Order I.
[Illustration: FIG. 116.—_Melicerta ringens._ (After Joliet). A, Side view;
B, dorsal view. _al_, Lateral antennae; _ci_, cingulum seen by
transparency; _g_, gizzard; _p_, pellet in ciliated cup, about to be
deposited on edge of tube; _tr_, trochus.]
Fam. 3. Melicertidae: _Melicerta_ E. (Fig. 116), _Limnias_ Schrank,
_Cephalosiphon_ E., _Oecistes_ E., _Lacinularia_ E., _Megalotrocha_ E.,
_Conochilus_ E., _Octotrocha_ Thorpe.
Fam. 4. Trochosphaeridae: _Trochosphaera_ Semper (Fig. 118, D).
The Melicertidae embrace a large number of tubicolous forms, many of which
are social. This habit is especially noticeable in _Lacinularia socialis_,
which forms a gelatinous incrustation easily seen by the naked eye; and in
_Conochilus volvox_, which forms free-swimming globular aggregates, the
young attaching themselves when hatched to the centre of the ball, and the
ball splitting up into two as soon as undue pressure is exerted at the
periphery by overcrowding. In this genus the eyes are very conspicuous in
the adult, as they are in the similar free-swimming aggregates of
_Lacinularia racemovata_.
_Trochosphaera_ (Fig. 118, D) is remarkable for its peculiar {222}spherical
shape, the absence of a foot, the limitation of the viscera to the lower
hemisphere, and the dorsal position of the ovary. But a reference to the
figure will show that the outgrowth of a foot in the quadrant between the
mouth and anus and the flattening of the upper hemisphere would bring its
organs on the whole into close correspondence with those of the rest of the
Order. It is recorded from South China, the Philippines, and North-East
Australia, and has only been seen by Semper, the founder of the genus, and
by Thorpe, who saw the male of the first species, and described a
second.[270]
ORDER III. BDELLOIDA.—Females creeping like a leech, as well as swimming
(males unknown), susceptible of desiccation and revival ("anabiotic"). Body
telescopic at both ends. Disc (except in _Adineta_) chiefly composed of two
dorsal lobes like kettle-drums, wholly retractile; a dorsal proboscis or
trunk-like prolongation of the body ends in a ciliated, sensory, and
adhesive cup used in crawling, and overhung by a pair of membranous flaps.
Trophi ramate; brain with a ventral ganglion, forming a complete ring.
Eyes, two on the proboscis or brain, or absent. Bladder a mere dilatation
of the rectum. Foot often possessing blind spurs, as well as two or three
retractile perforated toes, or forming a terminal disc perforated by
numerous pores of the cement glands, rarely ciliated.[271]
Fam. 5. Philodinidae: _Philodina_ E. (Fig. 110), _Rotifer_ Schrank,
_Actinurus_ E., _Callidina_ E. (Fig. 109), _Adineta_ H.
This group is remarkable for the great resisting powers of its members to
drought and to heat and cold when dried, a fact which may explain the
absence of males, though Janson records the occurrence of winter eggs in
four species of _Callidina_ and in _Adineta vaga_. The body is often
strongly pigmented; red in _Philodina roseola_, _Callidina scarlatina_, and
_C. russeola_, yellow in _P. citrina_, _Rotifer citrinus_, and _Discopus
synaptae_. Most of the species are dust- or moss-dwellers; some, such as
_Rotifer vulgaris_, are equally common in organic débris in infusions,
pools, and ditches. _Discopus_ adheres to the skin of the Holothurian
_Synapta_.
ORDER IV. ASPLANCHNACEAE.—Females ovoid, footless except in
_Asplanchnopus_. Disc often bearing a pair of antennae; circular, often
prolonged at the margin into two rounded lobes, interrupted {223}dorsally,
depressed at the ventral side into a deep ventral funnel. Trophi incudate
(virgate in _Ascomorpha_), mastax enlarged dorsally into a wide crop;
stomach large, blind. Kidneys large, with a "recurrent duct" and numerous
tags; bladder large. Brain large, with a median eye, and frequently paired
smaller eyes at the base of the marginal processes of the disc; anterior
antennae paired, relatively far back on dorsal surface. Males (Fig. 107, 5)
relatively large, frequently found.
Fam. 6. Asplanchnidae: _Asplanchna_ G., _Asplanchnopus_ De Guerne, (?)
_Ascomorpha_, Perty, (?) _Dinops_ Western.
ORDER V. SCIRTOPODA.—Females of conical shape, with the body prolonged into
hollow limb-like expansions (see p. 201) moved by strong muscles, and
ending in branched setose fins like the limbs of Crustacea. Disc as in
Bdelloids, but not retractile. Foot represented by two subventral toes,
ciliated, inconstant or absent. Trophi malleoramate. Eyes two,
latero-ventral, on the disc. Male (Fig. 107, 8) conical, with simple setae.
Fam. 7. Pedalionidae: _Pedalion_ H. (Fig. 117),[272] _Hexarthra_
Schmarda.[273]
ORDER VI. PLOIMA.—Free-swimming forms, more rarely parasites, often
adherent by their trophi to a host. Disc variable, often bearing within the
cingulum a number of lobes fringed with coarse compound cilia. Foot rarely
absent, marked off by a sharp constriction. Mastax variable, rarely
malleoramate, never incudate or uncinate. Intestine not blind. Males
small.[274]
SUB-ORDER A. ILLORICATA.—Ploima with a soft flexible integument; disc
variable; ciliated auricles sometimes present (Synchaetidae, Notommatidae);
foot rarely absent; trophi usually malleate.
{224}Fam. 8. Microcodonidae: _Microcodon_ E., _Microcodides_ Bergendal.
Fam. 9. Rhinopidae: _Rhinops_ H.
Fam. 10. Hydatinidae: _Hydatina_ E. (Fig. 106), _Notops_ H., _Hudsonella_
Zach., _Cyrtonia_ Rouss.
Fam. 11. Synchaetidae: _Synchaeta_ E.
Fam. 12. Notommatidae: _Notommata_ E., _Pleurotrocha_ E., _Copeus_ G.,
_Proales_ G., _Furcularia_ G., _Eosphora_ G., _Triophthalmus_ E.,
_Diglena_ E. (Fig. 113), _Distemma_ E., _Triphylus_ E., _Taphrocampa_ G.,
_Albertia_ Duj., _Balatro_ Clap.
Fam. 13. Drilophagidae: _Drilophagus_ Vejdovsky.
Fam. 14. Triarthridae: _Triarthra_ E., _Polyarthra_ E., _Pteroessa_ G.,
_Pedetes_ G.
[Illustration: FIG. 117.—_Pedalion mirum_, female. (After Hudson.) A,
Ventral view; B, side view. _a_, Median antenna; _al_, antero-lateral limb;
_an_, anus; _ci_, cingulum; _dl_, dorso-median limb; _e_, eye; _f_,
ciliated pedal processes; _l_, lip; _m_, mouth; _pl_, postero-lateral limb;
_tr_, trochus; _vl_, ventro-median limb.]
To this group belongs the eyeless _Hydatina_, a classical object of study,
common in greenish pools, whose male was the first male Rotifer to be
figured by Ehrenberg (1838), though he did not recognise its nature, and
gave it the name of _Enteroploea hydatina_. _Rhinops_ has the back of the
corona curiously prolonged forwards into a sort of proboscis bearing two
eyes. Some species of _Notommata_ and _Proales_ are distinctly annulated;
in _Taphrocampa_ the segmentation is so marked as to give the appearance of
mesenteric septa extending inwards from the body-wall to the intestine.
_Microcodon_ has a wreath which is very peculiar in its extreme simplicity,
with the mouth nearly central, and the eye lying just dorsal to the mouth.
The Triarthridae, which resemble the Scirtopoda in having strong leaping
spines fringed by fine bristles, should perhaps be placed in the next
sub-Order.
SUB-ORDER B. LORICATA.—Ploima with a firm elastic cuticle {225}of definite
form, persistent after death, continuous, or divided by thinner strips into
plates or shields, which again may be areolated. The cuticle may also be
shagreened or embossed in various ways.
Fam. 15. Rattulidae: _Rattulus_ E., _Mastigocerca_ E., _Coelopus_ G.,
_Diurella_ (?) Eyfurth.
Fam. 16. Dinocharididae: _Dinocharis_ E., _Scaridium_ E., _Stephanops_ E.
Fam. 17. Salpinidae: _Salpina_ E., _Diaschiza_ G., _Ploesoma_ Herrick,
_Diplax_ G., _Diplois_ G.
Fam. 18. Euchlanididae: _Euchlanis_ E., _Dapidia_ G., _Apodoides_ Joseph.
Fam. 19. Cathypnidae: _Cathypna_ G., _Distyla_ Eckstein, _Monostyla_ E.
Fam. 20. Coluridae: _Colurus_ E., _Metopidia_ E., _Monura_ E., _Mytilia_
G., _Cochleare_ G., _Dispinthera_ G.
Fam. 21. Pterodinidae: _Pterodina_ E., _Pompholyx_ G.
Fam. 22. Brachionidae: _Brachionus_ E., _Noteus_ E., _Schizocerca_ Daday.
Fam. 23. Anuraeidae: _Anuraea_ E., _Notholca_ G., _Eretmia_ G.
The group includes a number of very minute forms, besides others
conspicuous both for size and beauty. A soft dorsal flap above the head
occurs in _Stephanops_; also in Coluridae, a large family of minute
species, where the flap is movable, and looks in profile like a hook
overhanging the forehead. The genus _Pterodina_, like _Pedalion_ and
_Triarthra_, combines a Bdelloid disc with malleoramate trophi, while its
exsertile wrinkled foot ends in a ciliated cup like that of a larval
tubicolous species.
_Brachionus_, a large, often flat, transparent form, with a long wrinkled
foot, is a very common genus, known to the earlier observers, and
repeatedly figured by them. _Pompholyx_ has a sack-like lorica, no foot,
and carries its immense egg suspended by an elastic thread from the cloaca.
The Anuraeidae lack the foot, and often have great spines or bristles
projecting from the lorica, which no doubt facilitate floating. They are
abundant in the "plankton" or floating fauna of large lakes far from the
shore. Many marine species belong to this family.
ORDER VII. SEISONACEAE.—Marine Rotifers parasitic on the Crustacean
_Nebalia_; males resembling the females. Body elongated, with a slender
retractile neck, a much reduced disc, an elongated foot with a terminal
perforated disc as in _Callidina_. Trophi virgate exsertile. Genito-urinary
cloaca opening at the base of the neck in the male, at the hinder end of
the body in the female. Intestine complete (_Seison_) or blind
(_Paraseison_).[275]
{226}Fam. 24. Seisonidae: _Seison_ Grube; _Paraseison_ Plate;
_Saccobdella_ Van Beneden and Hesse.
HABITS.—The habitat of Rotifers is well known to the student of pond life.
Every dip from a greenish pool will give us a supply, if there be not an
excessive contamination by manure; and such pools give us some of the
largest and most beautiful forms, such as _Hydatina_ and _Brachionus_,
swimming about among the fibrous Algae and feeding on the organic débris
among them. Almost any organic infusions freely exposed to the open air
will yield Ploima shortly after the active putrefaction is completed. The
finer water-weeds yield most of the beautiful tubicolous forms. A whole
group of species and genera are quasi-pelagic in fresh and salt water,
constituting a large proportion of the "plankton" or floating life near the
surface; and some of these are found in deep water or in the depths of the
lakes. Among them are the Asplanchnidae, Triarthridae, and Anuraeidae. A
number of Loricates, such as _Notholca_ and _Eretmia_, are armed with long
spines, which doubtless render floating easier.
Among tubicolous forms _Conochilus volvox_ and _Lacinularia racemovata_
have this pelagic habit, forming floating globular or ovoid colonies, and
two species of _Floscularia_ also float freely in their tubes.
The following forms occur in salt or brackish water,[276] those marked with
an asterisk (*) also occurring in fresh water:—
_Floscularia campanulata.* Melicerta tubicolaria.* Rotifer citrinus.*
Discopus synaptae. Synchaeta baltica, S. monopus, S. apus, S. tremula,*
S. longipes, S. tavina. Asplanchna girodi.* Asplanchnopus syringoides.
Hexarthra polyptera. Notommata naias, N. reinhardti. Proales decipiens.
Furcularia forficula,* F. gracilis, F. reinhardti, F. marina, F.
neapolitana. Diglena catellina,* D. suilla, D. putrida. Pleurotrocha
leptura. Distemma raptor, D. marinum, D. platyceps.* Bothriocerca
longicauda. Polyarthra platyptera.* Triarthra longiseta.* Rattulus
calyptus. Diurella marina, D. brevidactylus, D. brevis. Diaschiza
fretalis. Euchlanis luna. Monostyla quadridentata, M. lunaris. Colurus
amblytelus, C. uncinatus,* C. dactylotus, C. coelopinus, C. pedatus, C.
rotundatus, C. truncatus, C. caudatus.* Mytilia tavina. Pterodina
clypeata. Brachionus bakeri,* B. mülleri. Anuraea valga,* A. biremis,* A.
aculeata,* A. tecta,* A. cochlearis.* Notholca striata,* N. scapha,* N.
thalassia, N. spinifera, N. inermis, N. jugosa, N. rhomboidea. Seison
grubei, S. annulatus. Paraseison asplanchnus, P. nudus, P. proboscideus,
P. ciliatus. Discobdella nebaliae._
Thus about seventy species are recorded as marine. _Synchaeta baltica_ is
truly pelagic, and contributes to the phosphorescence of the ocean.
{227}Other forms again are parasitic. _Proales werneckii_ is found in
_Vaucheria_, a coarse, dark green, thread-like Alga found in fresh water;
and the closely allied _P. parasita_ is not uncommon in the beautiful
floating green spheres of _Volvox_.[277] _Albertia_, _Drilophagus_, and
_Balatro_ are parasitic on or in fresh-water Oligochaetes; the curious
Seisonaceae are parasitic on _Nebalia_, a small Crustacean easily obtained
in masses of whelk's eggs; the aberrant Bdelloid _Discopus_ attaches itself
to the surface of the Holothurian _Synapta_. Similarly among this last
Order _Callidina parasitica_ attaches itself to the limbs of the
fresh-water Crustacea _Gammarus_ and _Asellus_. These are rather commensals
than true parasites. The species of _Brachionus_ often attach themselves
temporarily to the common water-flea _Daphnia_.
Besides a few Ploima, the vast majority of the Bdelloids live in or among
mosses and their roots. Many _Callidina_ inhabit cup-like hollows in the
leaves of the scale mosses (Jungermanniaceae), especially of the genus
_Frullania_. Almost all the members of this Order are susceptible of
desiccation and revival; certain species, such as _Rotifer vulgaris_,
_Philodina roseola_, _Adineta vaga_, etc., can be readily obtained by
moistening gutter dust. The mechanism of the process is as follows: when
desiccation is gradual the animals close up their telescopic bodies and
excrete gelatinous plugs at either end, which effectually seal them against
further drying; if, however, they be dried on a slide without any débris,
the process is too rapid for them to protect themselves, and they therefore
die. This was dimly seen by others, and clearly demonstrated by H.
Davis,[278] who records the following experiment:—The Rev. E. J. Holloway,
having found _Philodina roseola_ in gutters, placed strips of paper there
in the rainy season, and succeeded in obtaining clean gatherings, taking
dry groups of a hundred together, having a varnish-like covering all over;
and being glued to one another, mostly in one plane, and to the paper,
forming a pavement. In the dry condition they resist extremes of
temperature; thus Zelinka found {228}_Callidina_ revive after an exposure
of -20° C. (-4° F. or 36° of frost), and immersion in hot water at 70° C.
(158° F.). They will also resist deprivation of air in a vacuum of an
ordinary air-pump, but not the all but perfect exhaustion of the Sprengel
pump.
A very curious fact in relation to this Class is that often when a new form
is once described from a single locality, fresh and widely distant stations
for it rapidly become known.[279] Thus _Pedalion mirum_, first found at
Clifton in 1872 by Hudson, was a few years after captured in a small pool
above tide-marks on a rocky islet in Torres Straits. Since then it has been
recorded from many different European stations, and a second closely allied
species has been found in Finland. So a species of Ehrenberg's[280] was not
seen again till within the last decade or so; but since then it has been
independently found and described by six observers, who have given it as
many distinct generic names. In the case of _Pedalion_ it may well be that,
as Hudson suggests, the species is of southern origin and has followed the
flag, the winter egg being conveyed in dust by ships or travellers.
The above account of the habits gives the key to the collection of the
various forms. The weed-loving species are collected with the weeds, and
will keep with these in vessels if screened from direct sunlight and
protected against dust. The free-swimming forms may be collected by
sweeping with a net of fine gauze, with a bottle fixed in the bottom.
Except for their power of resisting desiccation, Rotifera are not very
long-lived, and the males are especially short-lived; the most exact
observations are those of Maupas on _Hydatina_. He found that the greatest
age of the unfertilised female was thirteen days, during which it could
produce some fifty eggs; the fertilised female lives for seven or eight
days, producing about sixteen eggs; while the male dies in two or three
days.
The preservation of Rotifers has been recently reduced to a fine art by
Rousselet, who uses a solution consisting of cocaine hydrochlorate, 1
gramme; water, 50 cc.; and methylated spirit, 12 cc. This will keep without
deterioration. When in use it {229}must be diluted in the proportion of two
volumes to three of water. This solution is added cautiously to the capsule
in which the Rotifers lie, and they are watched till their ciliary motions
slacken; when this happens a drop or two of osmic acid solution (½ to 1 per
cent) is added; the Rotifers are then sucked up by a capillary pipette, and
transferred to fresh water; and then into a solution of "Formaline" diluted
to contain 2½ per cent of formic aldehyde. In this solution they are
transferred to shallow cells, ground out of the centre of an ordinary glass
slide, covered with thin glass, and sealed.[281] Other methods of preparing
Rotifers for minute study will be found in the papers of Plate, Tessin, and
Zelinka.
THE ZOOLOGICAL AFFINITIES of the Rotifers have long been a subject of keen
interest. As early as 1851 Huxley[282] suggested that they represent a
primitive form, preserved, with modifications, in the larva of Molluscs,
Annelids and other worms, and Echinoderms. Similar views were later
maintained by Lankester,[283] who termed the larva of Polychaets, etc., a
"trochosphere," for which "trochophore" has been substituted in order to
avoid confusion with the Rotifer _Trochosphaera_; Balfour,[284]
Hatschek,[285] Kleinenberg,[286] and others have developed these views.
Serious difficulties, however, arise in the detailed comparison of Rotifers
with this type; and the special students of this Class have found it
practically impossible to agree in the identification of the various parts,
a difficulty especially felt in the case of the Rotiferan genus
_Trochosphaera_, though this is just the one which presents the closest
superficial resemblance to the Trochophore larva. I have been induced to
take a view of the structure of Rotifers that brings it into close
relationship with the lower Platyhelminthes, and with the more primitive
larva of the Nemertines termed _Pilidium_ (Fig. 60, p. 113). This is
hemispherical, ciliated all over, with the mouth in a ventral funnel lined
by fine cilia; while the edge is fringed with two rows of strong cilia,
separated by a finely ciliated groove, like those of the ciliary wreath of
a Rotifer.
{230}[Illustration: FIG. 118.—Diagram explaining the possible relations of
Rotifers. A, _Pilidium_; B, hypothetical Rotifer modified from
_Asplanchnopus_; C, a Ploimal Rotifer; D, _Trochosphaera aequatorialis_
(modified from Semper, the extension of the ovary into the posterior
ventral quadrant being omitted); E, Mollusc larva (Veliger); F, Trochophore
larva of Annelid. _a_, Anus; _ap_, apical organ; _at_, median antenna (near
which, in B, is a black spot, the brain); _bl_, bladder (receiving the
ramified kidney in B, C, and D); _br_, brain; _f_, foot; _fg_,
cement-glands, replacing apical organ; _g_, ovary; _k_, kidney; _m_, mouth;
_n_, supra-oesophageal ganglion; _nap_, nerve of apical organ; _nr_,
nerve-ring in section; _pot_, praeoral portion of trochus; _s.g_,
shell-gland; _s.n_, sub-oesophageal ganglion; _t_, trochus or ciliary
wreath; _tt_, posterior ciliated ring; _v_, velum, or expanded praeoral
part of trochus.]
The sides are produced on either side into lappets, which we do not take
into account. A cup-shaped depression at the apical pole is lined by
sense-cells, bearing long cilia which are probably sensory. A ring of
nerve-cells passes within the ciliated rim of the hemisphere, and the
stomach is a blind sac. If we compare this organism with a Rotifer, we find
that the wreath corresponds in both, the funnel of the disc in such forms
as Flosculariidae and _Microcodon_ leading to the mouth of _Pilidium_,
while the gut is blind in Asplanchnidae and in some of the highly developed
Seisonidae. The circular nerve-ring of _Pilidium_ is in many Rotifers only
represented by its anterior part, the brain; though in Bdelloids a
sub-oesophageal ganglion completes the ring. This leaves a difficulty with
regard to the apical sense organ; but it is easy to understand that an
organ of sensation should become an organ of fixation. In this case the
foot with its glands would correspond to the sense organ of the Trochophore
larva; and it retains its primitive ciliated character in the larvae and
males of many Rotifera, and the adult female of _Pterodina_ and _Callidina
tetraodon_. Embryology tells {231}us that the anus of Rotifers cannot be
homologous with that of Annelids, etc., for it is formed outside the area
of the blastopore: it is an independent formation, probably due to the
coalescence of the originally blind intestine at its extremity with the
earlier genito-urinary cloaca. On this view we must change the orientation
of the Rotifer, and place it, like a Cuttlefish, mouth downwards: for
"anterior and posterior" we must substitute _oral_ (or _basal_) and
_apical_; for "dorsal" and "ventral" we must use _anterior_ and
_posterior_; while "right" and "left" are unchanged. And this correctly
expresses the actual space-relations in those Ploima like _Rattulus_ that
swim with their disc in contact with the organic débris on which they feed,
with the foot turned outwards and backwards. As these views are now
published for the first time, I have thought it wiser to keep to the
accepted relations in the general description, a course which has the
advantage of avoiding difficulties in the study of the literature of the
Class.
The supposed resemblance of _Pedalion_ to the Crustacea is probably the
result of convergence, not of consanguinity. The Polyzoa are a group of
freely-budding organisms whose structure otherwise recalls in many respects
that of the attached Rotifers; but a close investigation reveals so many
differences in structure, orientation, and development, that we cannot
regard the two groups as at all closely allied.
Thus the Rotifers may be regarded as a group apart, but probably
representing an early offshoot from a free-swimming Platyhelminth, probably
a Rhabdocoele; the modifications being the loss of the general ciliation of
the surface, the arching of the back into an elongated vault, the
conversion of the inner half of the pharynx into a gizzard, the change of
position of the genital and urinary apertures to the antero-dorsal surface,
and the opening of the intestine into the genito-urinary cloaca.
GASTROTRICHA.
This small and very homogeneous group consists of minute fresh-water
organisms, closely resembling many Ciliate Infusoria in their movements,
habit and habitat. They were first described in detail by Ehrenberg, and
placed by him and Dujardin in the neighbourhood of Rotifers. In recent
years A. C. Stokes[287] {232}in America and C. Zelinka[288] in Germany have
contributed, the former a careful description of a number of new species
and their habits, the latter a complete monograph of everything that is
known of the Order.
The Gastrotricha dwell among filamentous Algae and organic débris, and are
of frequent occurrence with Protozoa and Rotifera of similar habit. The
largest known measures only 400 µ (1/60 in.) in length, and the smallest
run as low as 74 µ (1/300 in.).
[Illustration: FIG. 119.—Gastrotricha. (From Zelinka.) A, _Chaetonotus
bogdanovii_, side view (after Schimkewitsch); B, _Gossea antenniger_ (after
Gosse); C, _Dasydetes goniathrix_ (after Gosse); D, _Dasydetes saltitans_
(after Stokes); E, _D. longisetosum_ (after Metschnikoff); F, _Chaetonotus
spinulosus_ (after Stokes); G, _Chaetonotus schultzei_ (after Gosse and
Bütschli). (Magnified.) B-F, × about 390; G, × about 125.]
We shall follow Zelinka in his description of the common species
_Chaetonotus larus_ as a type. The body is nearly circular in section,
flattened a little on the ventral side. The apertures are the terminal
mouth; the anus, nearly terminal and slightly dorsal; the two kidney
openings, ventral, nearly half-way down the trunk; besides the pore of a
cement-gland on either terminal process. The short ventral and post-anal
portion of the trunk with its processes therefore corresponds to the foot
of a Rotifer. The integument of the body is a thin nucleated hypoderm, not
{233}distinctly divided into cells, covered by a chitinised cuticle; it
bears cilia, sensory hairs, and peculiar scale-like processes, sometimes
produced into long bristles.
The cilia are chiefly arranged in two ventral bands, each extending nearly
the whole length of the body, and composed of a series of transverse rows
of single cilia; along these bands the hypoderm is thickened and more
richly nucleated. The sides of the head also bear numerous long cilia.
The scales are hollow processes of the cuticle overlapping from before
backwards. A ventral row lies between the ciliary bands; two series of
alternating dorsal rows lie on the back and sides of the animal, and in the
hirsute species it is these that are produced backward into bristles. A
single large scale, the "frontal shield," protects the head above and
behind, but does not extend down to the ventral surface. On either side of
the head is a pair of flattened oval areas, the "lateral fields." From
between these on either side springs a tuft of motile sensory hairs. Two
pairs of similar tufts arise dorsally on the front margin of the frontal
shield, and a fourth pair spring from the ventral surface a little behind
the mouth. These hairs are distinguished from ordinary cilia by their
length, and their insertion on large nucleated cells receiving nerves; two
pairs of similar hairs lie farther back on the dorsal surface, one in the
front of the neck, one near the base of the pedal processes.
The MUSCLES lie some in the body-wall, and some traverse the body-cavity;
only six pairs occur, simple, unstriated, and longitudinal. There are
neither transverse nor circular muscles.
The ALIMENTARY CANAL is very simple and nearly straight from mouth to anus;
it may be divided into pharynx, gullet, stomach, and rectum. The mouth is
circular, and looks forwards and a little downwards. From the mouth opens
the pharynx, a short chitinous tube, capable of eversion by being pushed
forwards by the gullet; it bears half-way down a circlet of curved hooks,
which open out when it is everted; within these are tooth-like thickenings.
The oesophagus or gullet is thick and muscular, extending through the whole
of the neck of the animal; its cavity, as well as the opening from the
pharynx, is triradiate like a leech-bite, but can be dilated by the action
of the muscular walls, inserted into a firm external cuticle; the internal
wall is also cuticulised, {234}not ciliated as in Rotifers. The hinder end
of the gullet is produced into a short, wide, membranous funnel projecting
freely into the midgut or stomach. The latter is elongated and oval,
composed of four rows of hexagonal cells, with large nuclei. This is
separated by a distinct constriction or sphincter from the short
pear-shaped rectum, which opens by a minute anus on the back just in front
of the pedal processes.
The food is chiefly organic débris; but Gastrotricha have been seen to
attack large Infusoria by nibbling, and to swallow the protoplasm as it
exudes from the wound in their prey.
The NERVOUS SYSTEM is chiefly composed of the large brain, a ganglion lying
like a saddle above and on the sides of the gullet, and in direct
continuity with the nerve-cells of the cephalic sense-hairs. A pair of
dorsal nerve-trunks extend along the whole length of the gullet. The
sense-hairs described with the general integument may be organs of external
taste ("smell") or of touch. Eyes have been described in several species;
and though Zelinka has failed to verify this, I have myself seen a pair of
minute red eyes in the back of the head of an animal (probably a
_Chaetonotus_), whose hasty escape into a mass of débris prevented my
determining its species.
The KIDNEYS are paired tubes lying at the sides of the front of the
stomach, and sending a simple loop into the neck. Each tube is much
convoluted, and ends at the one extremity in a long "flame-cell," like that
of a Rotifer much drawn out, and at the other by a minute pore on the outer
side of the ventral row of scales.
REPRODUCTIVE ORGANS.—Only the female is with certainty known to occur; and
the eggs, though recalling in their thick ornamented shell the fertilised
winter eggs of Rotifers, are probably unfertilised and parthenogenetic like
the summer eggs. The ovaries are two minute patches of cells lying at the
junction of the stomach and rectum. The eggs, as they mature and enlarge,
press against the side and back of the stomach, where they attain a length
of one-third to one-half that of the mother. The extrusion of the egg has
not been observed; but it is laid in the angles of weeds, the moulted
shells of Entomostraca, etc., where its development may be studied. The
sculpture of the shell serves to anchor it if laid among weeds. When
hatched the head, trunk, and pedal processes are of the full adult size,
all subsequent growth being limited to the neck.
{235}The function of testis has been ascribed by Ludwig to a minute
granular organ between the ovaries above the rectum; if this view be
correct the Gastrotricha are hermaphrodite.
The movements of the Gastrotricha are very elegant, recalling those of the
long-necked Ciliate Infusoria, like _Amphileptus, Lacrymaria_, etc., with
the characteristic exception that they always swim forwards; the grace of
their movements being due to the bending of the head and neck on the body.
Those which are provided with long motile bristles like _Dasydetes_,
alternate their gliding with leaps, like the springing Rotifers.
The Gastrotricha are divided into two sub-Orders—EUICHTHYDINA, with two
pedal appendages, containing the genera _Ichthydium_ Ehr., _Lepidoderma_
Zel., _Chaetonotus_ Ehr., and _Chaetura_ Metsch.; and the APODINA, with no
pedal appendages, comprising _Dasydetes_ G. and _Gossea_ Zel.
Their geographical distribution, like that of most microscopic fresh-water
organisms, is cosmopolitan. Few observers have enumerated the members of
this group; of their extra-temperate occurrence we have only the single
observations of Ehrenberg, Schmarda, and Voeltzkow for Nubia, Ceylon, and
Madagascar respectively.
Of the thirty-two species described, twelve are recorded by A. C. Stokes
from Maine and New Jersey only, besides five others that occur also in
Europe. In Europe nineteen species are recorded, one of which, _Ichthydium
podura_, has also been found in Nubia and Ceylon. One species, _Chaetonotus
tabulatus_ Schmarda, has been recorded by its author from Colombia (in
South America). As of the nineteen European species only seven have been
recorded as British, we may expect to find that careful study will well
repay the student in these islands.
The AFFINITIES of this group are probably with the Turbellarians and the
Nematodes; they differ from the former in the highly developed alimentary
canal, and from the latter in the possession of the ciliated ventral bands
and wreath. The general chitinisation of the skin, the primitive
body-cavity, the character of the alimentary canal, the ventral opening of
the renal canals far in front of the anus are characters shared by the
Nematodes, many of which possess bristles like this group. But their
affinity must be rather to some hypothetical ancestral group than to any
living Nematodes, which are destitute {236}of cilia. To the Rotifers the
affinity, dwelt on by Zelinka, is less close.
KINORHYNCHA.
This Class and Order comprises but one genus, _Echinoderes_ (Fig. 120),
founded in 1851 by Dujardin.[289] Reinhard's monograph[290] is the
generally accepted authority on this subject, and contains a full
bibliography, with diagnoses of the individual species, eighteen in number.
[Illustration: FIG. 120.—_Echinoderes dujardinii_ (?), drawn from a
preserved specimen taken at Worthing. × about 210. _b_, Bristle; _c.s_,
caudal spine; _ph_, pharynx; _s_ and _s'_, the spines on the two segments
of the proboscis; _s.g_, salivary glands; _st_, stomach.]
The animals of this group are found in shallow seas with muddy bottom,
below low-water mark, and feed on organic débris. They have been taken in
the Black Sea, Mediterranean, British Channel, and North Sea, and off the
Canary Islands (Lanzarote, Porto Pi, Palma di Mallorca). Their size varies
from 0.86 mm. × 0.22 mm. in _Echinoderes spinosus_, to 0.14 mm. × 0.03 mm.
in _E. kowalevskii_.[291]
The BODY is protected by a strong chitinous cuticle distinctly annulated,
forming eleven rings, besides a retractile proboscis obscurely divided into
two segments at the apex of which the mouth opens. The anus opens on the
extreme end of the last segment, which is frequently retracted; the genital
pores open right and left of the anus; and the renal pores lie on either
side of the back of the ninth segment. The first ring may be undivided, or
else distinctly divided into four plates, one dorsal, {237}two
latero-ventral, and one ventral. In the remaining segments each ring has
only three plates, one dorsal and two ventral, the two latter being
sometimes more or less fused in the last or ventral segment. These plates
all overlap from before backwards.
As the name _Echinoderes_ implies (Thorn-skin), the cuticle is produced
into points, bristles, or spines. The last segment frequently bears a large
pair of these, which have been compared, on the flimsiest grounds, with the
furcal processes of Crustacea and the perforated toes of Rotifers and
Gastrotricha.
The proboscis when extruded has the form of a truncated cone, obscurely
divided into two segments, a ring of strong spines marking the boundary
between them, and a second double ring of spines surrounding the apex. The
eversion is of the type termed by Lankester pleurembolic or acrecbolic, the
sides being first withdrawn, the apex first extruded.
As in so many Invertebrata, the epidermis is not separated by boundaries
into distinct cells. This layer sends out processes each of which lies in a
hollow in the thick cuticle, and perforates it to end in a fine bristle.
Minute orange pigment-granules occur at irregular intervals in this
hypoderm.
The MUSCLES of _Echinoderes_ are simple striated bands. Numerous bands lie
within and attached to the body-wall, extending its whole length; paired
dorsi-ventral muscles separate the intestine from the reproductive organ on
either side, and a complex system effect the movements of the proboscis.
ALIMENTARY CANAL.—The pore at the tip of the proboscis leads into a short
thin-walled tube, which is rarely evaginated; into the base of this tube
projects the short bluntly conical apex of the large ovoid muscular pharynx
(or gullet?); this is lined by an epithelial layer of nucleated protoplasm,
which secretes a strong cuticle. The stomach is a wide tube, somewhat
dilated in each segment between the paired dorsi-ventral muscles, and
tapering behind to end in the terminal anus. Four minute glands open at the
junction of the pharynx and stomach.
KIDNEYS.—These are a pair of blind pear-shaped sacs, ciliated within (the
only case of ciliation in _Echinoderes_), lying in the eighth segment, and
opening by the taper ends right and left on the back of the ninth segment.
NERVOUS SYSTEM.—All that has been clearly defined of this is a small brain
or ganglion lying dorsally at the junction of the {238}pharynx and stomach.
From two to eight eye-spots have been described by earlier writers, but
Reinhard was unable to find them in the (distinct) species which he
principally worked at, though he noted their existence in the solitary
specimen of the original species, _E. dujardini_, which he obtained.
REPRODUCTIVE ORGANS.—The sexes are distinct. The reproductive glands form a
pair of tubular sacs, opening ventrally on either side of the anus, and
extending forwards beside the gut as far forwards as the fifth to the
second segment in the male, but only to the fourth at furthest in the
female. The ova are large nucleated cells embedded in the protoplasmic
lining of the ovarian sac, and acquiring a distinct shell as they approach
its opening. Three-quarters of the testis sac is occupied with granular
protoplasm containing a quantity of small nuclei; the lower part alone
contains mature spermatozoa. Adjoining each external opening in the male
are a pair of short hollowed spines, which may perhaps serve as organs of
copulation; but nothing is really known of this process or of the
development of the egg. It is almost certain, from the absence of
developing eggs within _Echinoderes_, that the genus is not viviparous.
From the foregoing description it is obvious that _Echinoderes_ approaches
the Nematoda very closely: the two main points of difference are its
ciliated kidneys and its bilaterally paired sexual organs. Possibly the
study of such forms as _Desmoscolex_ (Fig. 81, p. 159) may reveal closer
affinities.
----
[Zelinka (_Verh. D. Zool. Ges._, 1894 and 1898), has given a preliminary
account of a new research on this group. The principal addition is the
discovery of a ventral nerve-cord, with a ganglionic dilatation in each
segment, lying in the ectoderm of the body-wall, as indeed do the brain
and nerve-collar. He divides the genus into two Orders according as the
orifice of the retracted fore-part of the body is slit-like or circular.
The former (Homalorhagae) retract the first two segments with the
proboscis; they are mud-dwellers, sluggish, eyeless: the latter group
(Cyclorhagae) only retract the first segment with the proboscis; they
crawl among algae, and mostly have paired pigmented eye-spots, each with
a lens, imbedded in the brain.—M. H., _Jan._ 1901.]
ARCHIANNELIDA, POLYCHAETA, AND MYZOSTOMARIA
BY
W. BLAXLAND BENHAM, D.Sc. (Lond.), Hon. M.A. (Oxon.)
{241}CHAPTER IX
THE CHAETOPODOUS WORMS—THE ARCHIANNELIDA—ANATOMY OF _NEREIS_, AS TYPICAL OF
THE POLYCHAETA
Those animals which possess lateral bundles of bristles (technically termed
"chaetae") for use in locomotion constitute the group of "Bristle-worms,"
or CHAETOPODA. The body of these animals is made up of a preoral lobe or
prostomium, and a number of more or less distinct segments following one
another in a line, and repeating one another in their internal and external
structure. The Chaetopoda embrace the following smaller groups or
Orders:—I. Archiannelida, II. Polychaeta, III. Myzostomaria, IV.
Oligochaeta. The Archiannelida, although without the characteristic
chaetae, are yet anatomically so similar to the true Chaetopoda that they
must be included in the group, just as certain fishes are classed as
"Vertebrata," although they do not possess vertebrae. The old term Annelida
is sometimes used to include the above-mentioned groups, together with the
Gephyrea[292] and the Hirudinea or leeches.
ORDER I. ARCHIANNELIDA.
The Archiannelida are very simple worms, but simplicity may be, and very
frequently is, the result of degeneration; and it is not always possible to
determine whether a simple animal is primitively, _i.e._ ancestrally
simple, or whether it is secondarily simplified. Hence the term
_Haplodrili_ has been employed by Professor Lankester as the name of the
group; a term which does not prejudge the question as to whether or not the
worms are {242}"primitive." It is quite possible, and even probable, that
_Dinophilus_ is ancestrally simple; whilst many features in _Polygordius_
appear to be the result of simplification. For this reason it would be well
to separate _Dinophilus_ from the other two genera, on account of its much
less elaborate and more generalised structure,—so generalised, in fact,
that the worm is by some authorities placed amongst the Planarians; for the
present, however, the group Archiannelida may be regarded as containing
three genera: _Dinophilus_, _Protodrilus_, and _Polygordius_.[293]
[Illustration: FIG. 121.—_Dinophilus taeniatus._ (From Harmer.) The left
figure represents the dorsal surface of a young individual, × 76; the mouth
and alimentary tract are seen by transparency: _p_, prostomium, with two
bands of cilia and a pair of eyes; _a_, anus; _t_, tail; 1 to 5, the
segments with ciliated bands. The right figure shows the anatomy of the
male, × 38: _b_, rectum; _c_, body-cavity; _d_, vas deferens; _m_, muscular
organ (pharynx); _n'_, the first nephridium; _oe_, entrance to oesophagus;
_p_, penis; _st_, intestine (stomach); _s.x_, seminal vesicle (5th
nephridium).]
_Dinophilus_ is represented on our coasts by at least two species: _D.
gigas_ Weldon[294] and _D. taeniatus_ Harmer.[295] The latter is about
one-twelfth of an inch in length, bright orange in colour, and more or less
abundant, at springtime, in the rock pools around Plymouth, where it may be
found amongst green algae, or on the mud at the bottom of the pools.
{243}The animal consists of a broad prostomium, with a pair of eyes; and of
a body, distinctly constricted in immature specimens into five or six
segments, followed by a short conical tail. There are neither chaetae nor
tentacles; locomotion is chiefly effected by means of the bands of cilia
which encircle the body in a regular fashion, two bands round the head, and
two round each segment in _D. taeniatus_; in some species there is only a
single band on each segment. The whole of the ventral surface is covered
with cilia, by the aid of which the animal probably "creeps" along the
weeds.
The alimentary canal is straight, and divisible into the regions shown in
Fig. 121; a muscular protrusible organ, which is a ventral outgrowth of the
foregut, is employed as a "sucker." The coelom is more or less obliterated
(or ill developed). The excretory system in the genus is varied: in some
species, as in _D. gigas_, it is stated to be constructed on the Planarian
plan; in others, as in _D. taeniatus_, the organs are definite nephridia.
Of these tubes there are five pairs, the last pair in the male serving as a
seminal vesicle. Each nephridium is a ciliated tube, the internal end of
which lies in the body-cavity and appears to be blocked by a ciliated
tongue-shaped appendage. The first pair corresponds to the "larval
nephridia" of Trochosphere larvae.
The nervous system, which is in contact with the epidermis, consists of a
brain in the prostomium, and, on each side of the body, a ventral cord with
five ganglia, connected by transverse commissures in as many segments.
The sexes are separate, and are usually similar; the male of _D.
gyrociliatus_ is, however, much smaller than the female. The generative
organs occupy the greater part of the body-cavity; in the male the testes
communicate, by means of the pair of seminal vesicles, with a median
eversible apparatus. In the female the paired ovaries communicate with a
median sac which serves as a spermatheca.
The development is simple:[296] the worm itself is more like a larval
Polychaete than a full-grown worm. _Dinophilus_ is an extremely interesting
form, and it has been suggested that, while still possessing certain
Planarian characteristics, it may be looked upon as closely resembling the
ancestor from which the Chaetopoda have arisen.
{244}_Protodrilus_ and _Polygordius_ are distinctly Annelidan in character.
_Protodrilus_[297] is found in the mud of the "Pantano," an inlet of the
sea near Messina; whilst of _Polygordius_[298] one species at least occurs
on our shores, and several others in the Mediterranean and elsewhere. The
worms are cylindrical, with many segments, but these segments are only
indistinctly marked externally—by girdles of cilia in _Protodrilus_, or by
faint grooves in _Polygordius_; but there are none of the characteristic
Chaetopod bristles or chaetae. The small prostomium which overhangs the
mouth is provided with a pair of ciliated pits, and carries a pair of
tentacles, serving as sensory organs, which, in _Protodrilus_, are also
respiratory. The anus is surrounded by glandular papillae in _Polygordius_,
by means of which the animal can fix itself; these are represented in
_Protodrilus_ by a couple of processes.
The nervous system lies entirely in the epidermis. The body-cavity is
regularly segmented by transverse septa passing from the body-wall to the
intestinal wall. The foregut presents a slight eversible portion in
_Polygordius_, whilst in _Protodrilus_ it has a peculiar U-shaped muscular
diverticulum on its ventral surface, corresponding with the similar
apparatus in _Dinophilus_; it is capable of eversion, and aids the worm in
burrowing, as well as in seizing and swallowing the mud. The vascular
system is represented by a dorsal and a ventral vessel, neither of which,
however, is contractile. In _Protodrilus_ the dorsal vessel divides into
two branches in the first segment, each of which passes to the tip of the
tentacle, and returning, joins its fellow to form the ventral vessel. In
some species of _Polygordius_ there is a pair of vessels connecting the
dorsal and ventral vessels in every segment, but no vessel to the tentacle.
The blood is colourless in some species of _Polygordius_, but may be yellow
(_P. neapolitanus_), red (_P. lacteus_), or green (_P. erythrophthalmus_).
Paired nephridia, with distinct funnels, occur regularly throughout the
body.
The sexes are separate in _Polygordius_, whilst _Protodrilus_ is
hermaphrodite, bearing ova in the first seven segments and testes in the
remaining segments. The genital cells are produced from the body-wall in
every segment; their mode of discharge is unknown in the male
_Polygordius_, though probably the nephridia {245}convey the spermatozoa to
the exterior; but in the female the body-wall ruptures to allow the ova to
escape, and then the animal dies. The development of _Polygordius_ has been
made the subject of very careful study; the larva has long been known, and
is a typical "trochosphere" of rather a depressed form. This "trochosphere"
larva is of considerable importance, as it makes its appearance in sundry
groups of animals in some form or another. Here, in _Polygordius_, it has
the appearance of a couple of wide but low cones united together by their
bases, which form the equator of the larva. This equator carries a double
girdle of cilia, dividing the animal into a preoral and postoral region;
for the mouth is placed on one side of the animal between the two girdles,
while the anus lies at the apex of the postoral cone, and is surrounded by
another girdle of cilia. The alimentary canal is divisible into three
regions; it is separated from the body-wall by an extensive space, which
contains cells destined to give rise to muscles and nephridia. A nervous
system (apical plate) is present at the apex of the preoral cone. This
little larva swims freely on the surface of the sea, moving, balancing, and
feeding by means of the girdle of cilia. It soon increases in length by the
active growth of the apex of the postoral cone, which becomes cylindrical
and then segmented externally and internally. The greater part of the
original larva remains of the same shape as before, and forms the head
(prostomium and peristomium): small tentacles grow out of the preoral lobe,
and after a gradual reduction in the relative size of the "head" by the
growth of the segmented "body," the animal becomes worm-like and develops
into a _Polygordius_.[299]
ORDER II. POLYCHAETA.
ANATOMY OF NEREIS.—In order to obtain a general idea of a Polychaete worm,
it is well to study a concrete example, and for this purpose the common
_Nereis_ serves excellently. Several species (see p. 315) occur more or
less commonly on our coasts, and the general remarks will apply to one as
well as to another.
_Nereis pelagica_ Linnaeus reaches a length of 5 to 6 inches, and is about
¼ inch across. It is convex above, nearly flat below. Its {246}colour is
brown or bronze. The worm, which is to be found in shallow water, is made
up of a considerable number of rings or segments, constituting the "trunk"
or "body," terminated at each end by modified segments known as "head" and
"tail" (Fig. 122). The segments composing the trunk are all alike, except
for small proportional differences, and it will be convenient to describe a
"typical segment" before referring to the head or tail.
A TYPICAL BODY SEGMENT carries on each side a muscular lobed outgrowth,
bearing bundles of bristles or "chaetae," and filamentous sensory organs
known as "cirri." To this lateral locomotor organ Huxley gave the name
"parapodium" (Fig. 124). Each parapodium or foot consists of a basal
portion, supporting a dorsal and a ventral process, the "notopodium"
(_ntp_) and "neuropodium" (_nrp_) respectively, each of which is bilobed.
The lobes are very vascular and glandular, and probably serve as
respiratory organs or "gills."
[Illustration: FIG. 122.—_Nereis pelagica_ L., natural size. (From
Ehlers.)]
[Illustration: FIG. 123.—Chaetae of _Nereis_; enlarged. A, from
neuropodium; B, from notopodium of _N. diversicolor_; C, swimming chaeta of
Heteronereid stage of _N. dumerilii_.]
The chaetae, or bristles, of each bundle project from the mouth of a great
sac, the lips of which are particularly prominent in _Nereis_. Each chaeta
arises from a single cell situated at the bottom of the sac. The chaetae of
_Nereis_, as of many other Polychaetes, are of a kind usually termed
compound or "jointed," each being composed of a long stalk and a small
"appendix" articulated in a cup at its {247}extremity (Fig. 123). The shape
of the cup varies; it is in some cases of equal height all round, or it is
higher on one side than on the other. Further, the appendix may be short
and curved, or more elongate and spear-like; it is generally notched or
finely toothed on one side.
[Illustration: FIG. 124.—_Nereis._ Somewhat diagrammatic transverse section
through the body. On the left the chief constituents of the vascular system
are represented; on the right side the chaetae and their muscles, as well
as the distribution of the lateral nerve, etc., are shown. _ac_, aciculum;
_bv_, network of blood-vessels; _ch_, chaetae (only a few are shown) in two
bundles; _ch.l_, lips of the chaetigerous sac; _cil.org_, dorsal ciliated
organ; _cir_, circular muscular coat; _coe_, coelom; _d.cir_, dorsal
cirrus; _d.long_, dorsal bundle of longitudinal muscles; _d.ve_, dorsal
blood trunk; _ep_, epidermis; _INT_, intestine; _int.cap_, blood
capillaries in its wall; _m.ch_, muscles which move the chaetae; _N.c_,
ventral nerve cord; _ner_, lateral sensory nerve, dividing into a ventral
branch entering the ventral cirrus, and a dorsal branch (_n.cir_) for the
dorsal cirrus; _neph_, nephridium, seen through the oblique muscle through
which its funnel passes; _nrp_, neuropodium; _nrp.lig_, neuropodial lobe or
ligule; _ntp_, notopodium; _ntp.lig_, notopodial ligule; _obl_, oblique
transverse muscle (muscle of the parapodium); _pv_, peripheral
blood-vessel; _v.cir_, ventral cirrus; _v.long_, ventral bundle of
longitudinal muscles; _v.ve_, ventral blood trunk.]
In addition to these projecting locomotor chaetae, there is embedded in
each of the two chaetigerous lobes a much stouter and dark-coloured,
needle-shaped bristle known as an "aciculum," whose point only just
projects beyond the surface. This aciculum extends into the interior of the
body much farther than do the locomotor chaetae, and it is to it that the
muscles serving to move the whole bundle of chaetae are attached. The
acicula thus serve as an internal skeleton to the parapodium. The shape of
the parapodium, the relative lengths of cirri and lobes, the shape and
arrangement of the chaetae, are all employed as specific characters.
{248}The HEAD consists of a preoral portion above the mouth, the
"prostomium," and a postoral region surrounding the mouth, the
"peristomium" (Fig. 125). The prostomium varies in shape in different
species of _Nereis_; but it always carries on its dorsal surface two pairs
of eyes. From its narrower anterior end there arises a pair of short,
somewhat conical, sensory processes known as the "prostomial tentacles." A
second pair of processes springs from the under surface, and rather to the
side of the prostomium; these are known as the "palps," and in _Nereis_ are
much more conspicuous than the tentacles; each is composed of two parts, a
large basal piece and a smaller terminal joint, capable of being withdrawn
into the former. The palps are highly muscular, and though they are sensory
organs, act also as great lateral lips.
[Illustration: FIG. 125.—_Nereis diversicolor_ Müll. × 4. Head, with buccal
region everted. A, Dorsal view; B, ventral view. _a_, Prostomium; _B_,
everted buccal region; _c_, _c'_, peristomial cirri, 1, 2, 3, 4; _d_,
denticles or paragnaths; _e_, eyes; _E_, lower lip; _P_, palp in A,
entrance to pharynx in B; _J_, jaw; T, tentacle; I, peristomium; II, foot
of apparent second segment.]
The peristomium is in many species of _Nereis_ (as in _N. pelagica_)
considerably larger than the trunk segments; it carries at its anterior
edge four filiform cirri on each side, which are directed forwards and used
as feelers. They are arranged in couples; a more anterior couple of dorsal
and ventral cirri, and a more posterior couple of dorsal and ventral cirri.
The TAIL.—As the most anterior segment is perforated by the mouth, and is
modified as described above, so the last or anal segment, which carries the
anus, differs from the rest. It is more or less elongated, cylindrical, and
without parapodia or chaetae. It retains, however, its pair of ventral
cirri, which are very long.
INTERNAL ANATOMY.—In correspondence with the external {249}metamerism there
is an internal repetition of parts. For, except in the anterior segments,
where the powerful protrusible pharynx is situated, the body-cavity or
"coelom" is divided into a series of chambers, by means of muscular septa
inserted, on the one hand, into the body-wall at the level of the grooves
between the external segments, and, on the other, into the wall of the
alimentary canal. Each of these coelomic chambers contains a pair of
nephridia, a portion of the intestine, of the vascular system, and of the
nervous system, as will be seen in Fig. 124.
The epidermis, which forms the outer part of the _body-wall_, consists of a
single layer of cells, covered externally by a thin, tough cuticle. The
latter is usually stated to consist of the chemical substance known as
chitin, but since the cuticle differs from true chitin by dissolving in
caustic potash after a time, Eisig[300] has suggested that its substance is
merely a stage in the formation of chitin. The epidermis contains
gland-cells, which are especially abundant on the lobes of the parapodia.
Below the epidermis lies the circular coat of muscles by whose contraction
the worm diminishes its diameter: it is interrupted on each side at the
junction of the parapodium with the body. Deeper still lie the longitudinal
muscles, which form four great bundles, two dorsal, separated by the
insertion of a small mesentery and dorsal blood-vessel, and two ventral
bundles separated in the middle line by the nerve-cords. These longitudinal
muscles, by their contraction, bend the worm from side to side, and are
continuous from segment to segment. A very characteristic muscle, present
in all the Polychaeta, is an obliquely transverse sheet of fibres passing
from the body-wall at the side of the nerve-cord to the parapodium, where
it spreads out and serves to move the parapodium (Fig. 124). All these
muscles consist of smooth fibres, as in the earthworm.
The ALIMENTARY CANAL may be divided into the following four regions:—(1)
buccal or eversible region, (2) pharynx, carrying the great jaws, (3)
oesophagus, (4) intestine.
The first two regions constitute an "introvert" (Lankester[301]). When
fully everted the whole of the buccal region is turned inside out, and the
terminal aperture leads directly into the pharynx, which is not everted but
merely protruded. {250}Throughout the following chapters the word "buccal"
region is used for that part—if any—which is thus everted (Figs. 125, 126).
Both the buccal and pharyngeal regions are wrapped round by several coats
of muscle, to form apparently a single muscular organ (Fig. 127, _sh_),
which occupies about eight segments in a condition of complete
introversion. The septa are absent from the anterior part of the body.
[Illustration: FIG. 126.—Diagrams to illustrate the action of the
Chaetopodan "introvert." (From Lang.) A shows the apparatus at rest; the
mouth leads into the buccal cavity (_B_), with paragnaths in its wall; _P_,
the pharynx, with jaws at its anterior end; _c_, brain; _p_, protractor
muscles; _r_, retractors. B, the pharynx has been brought forward or
protruded by the eversion, or turning inside out, of the buccal region, so
that the jaws (_j_) now lie some way in front of the head, which is
represented by the brain.]
The buccal region is lined with chitin, which is specially thickened at
certain definite spots, forming small "denticles" or "paragnaths" (Fig.
125), which have a different arrangement in the various species.
The cavity of the pharynx is narrow and the walls thick and muscular; each
side wall carries a large, dark, chitinous "jaw" (Fig. 127, J), which is
hollow at the base, into which the muscles serving to move it are inserted,
whilst the apex is solid, curved, and more or less notched. These two great
jaws are used not only for tearing prey, but for seizing it; for when the
pharynx is entirely protruded the two jaws are wide apart, and when
retraction takes place they come together and grasp the prey.
Eversion of the apparatus is partly effected by protractor muscles (Fig.
126, A, _p_) and partly by the pressure of the coelomic fluid, compressed
by the muscles of the body-wall; the eversion is stopped at a certain stage
by a sheet of muscular tissue or "diaphragm" (Fig. 127, _diaph_) inserted
round the buccal region and attached to the body-wall in the second
segment. The introversion is effected partly by the contraction of this
diaphragm and partly by the action of powerful retractor muscles (Fig. 126,
_r_) inserted into the hinder end of the pharynx and passing to the
body-wall (these are removed in Fig. 127). The {251}movement of the jaws
themselves and of the wall of the apparatus is due to other muscles.
The _oesophagus_ is quite short; into it opens a pair of sacculated
diverticula or glands. Then follows the _intestine_, which extends through
the rest of the body as a thin-walled tube, slightly dilated at the
insertion of the septa.
[Illustration: FIG. 127.—_Nereis_, laid open by removal of the dorsal
body-wall. _br_, Cerebral ganglion, from which three pairs of nerves are
represented as arising; a pair to the tentacles, a pair to the palps, and a
pair (_con_) passing one on each side of the buccal region, to join the
ventral nerve-cord; _mo_, mouth, exposed by removal of the dorsal wall of
the buccal region (_Buc_); _pgn_, the paragnaths in its wall; _Ph_,
pharynx; _J_, the large "jaws" embedded in its wall; _J.mus_, the muscles
which work the jaws; _sh_, the muscular sheath; _diaph_, the "diaphragm";
_oes_, oesophagus; _gl_, its glands; _st_, stomach; _int_, intestine;
_Sept_, septum; _d.v_, dorsal blood trunk; _p.v_, perivisceral branches,
one pair in each segment; _P_, palp; _t_, tentacle; _per_, peristomium;
_per.ci_, two of the four peristomial cirri; _ppIII_, the first parapodium
which belongs to the third true, but second apparent, segment; _cir.m_,
circular muscular coat; _lg.m_, longitudinal muscular coat of the
body-wall.]
The VASCULAR SYSTEM consists of a contractile dorsal vessel and of a
non-contractile ventral vessel extending along the whole length of the
body, from each of which paired and segmentally-arranged {252}vessels pass
to the intestinal wall and to the body-wall, and here form extensive
capillary networks (Fig. 124, p. 247). This type of vascular system is
pretty generally adhered to throughout the Order, but in the
Terebelliformia, Scoleciformia, and Cryptocephala the dorsal vessel and
capillary plexus on the intestine are replaced by a continuous blood sinus,
situated in the substance of the gut-wall. This "perienteric sinus" has the
same relation to the segmental vessels as the dorsal vessel has in the
Nereidiformia, and from it a tubular dorsal vessel arises anteriorly. In
_Arenicola_ the sinus is preceded in the young stage by a network the
branches of which gradually enlarge, meet, and fuse to form the sinus.[302]
Whether it is in all cases secondary is a moot point.
This system of vessels in the majority of Chaetopoda contains a respiratory
fluid coloured red[303] by haemoglobin in solution; in it float a very few
small oval nucleated non-amoeboid corpuscles. But the place of this red
pigment is taken by a green one, named "chlorocruorin," in the
Chlorhaemidae and many Sabelliformia;[304] whilst in _Magelona_[305] the
blood is tinted madder-pink by a number of globules of "haemerythrin." The
blood (or "haemal fluid") is driven forwards in the dorsal vessel, and
passes backwards in the ventral vessel. Respiration in _Nereis_ is carried
on by the whole surface of the body, but naturally with greater activity in
the surface of the parapodia, the lobes of which, with their extensive
vascular plexus, may be termed "gills"; but it must be borne in mind that
these organs have other functions as well.
The _coelomic fluid_, which fills the general body-cavity, is colourless,
and contains amoeboid corpuscles or "leucocytes." It corresponds to the
lymph of Vertebrates, being nutritive in function, in that it conveys
absorbed material from the wall of the intestine to the organs of the body,
and at the same time removes any waste substances from these organs; these
waste substances contain nitrogen, and are ultimately removed by the
nephridia. In _Ophelia_ many of the corpuscles contain a curious
dumb-bell-shaped rod of chitin, and it has been shown[306] that this
substance {253}is a highly complex form of excretory material,—more complex
than guanin, for instance, which exists in the corpuscles of the
Capitelliformia.
In Glyceridae, Capitelliformia, and _Polycirrus haematodes_ (a Terebellid),
the vascular system is absent, and the coelomic corpuscles become coloured
by haemoglobin, and in order that the coelomic fluid may be distributed to
the organs of the body, the peritoneum is ciliated along certain definite
tracts. The fluid in these "anangian" worms thus combines originally
separate functions, and behaves like the "blood" of Vertebrates.