a-c, Trophozoites of _Monocystis agilis_.
a and b, Young individuals showing changes of body-form.
c, Older individual, still enveloped in a coat of spermatozoa.
d, e, Trophozoites of _M. magna_ attached to seminal funnel of
_Lumbricus_.
Goblet-shaped epithelial cells, in which the extremity of the parasite
is inserted.]
The effects on the host are confined to the parasitized cells. These
generally undergo at first marked hypertrophy and alteration in
character; this condition is succeeded by one of atrophy, when the
substance of the cell becomes in one way or another practically absorbed
by the growing parasite (cf. also COCCIDIA). Since, however, the
Gregarines never overrun their hosts in the way that many other Sporozoa
do (because of their lack, in general, of the power of endogenous
multiplication), the number of cells of any tissue attacked, even in the
case of a strong infection, is only a very small percentage of the
whole. In short the hosts do not, as a rule, suffer any appreciable
inconvenience from the presence of the parasites.
Morphology.
The body of a Gregarine is always of a definite shape, usually oval
or elongated; in one or two instances (e.g. _Diplodina_) it is
spherical, and, on the other hand, in _Porospora_ (fig. 3) it is
greatly drawn out and vermiform. In many adult Gregarines, the body is
divided into two distinct but unequal regions or halves, the anterior
part being known as the _protomerite_, the hinder, generally the
larger, as the _deutomerite_. This feature is closely associated with
another important morphological character, one which is observable,
however, only during the earlier stages of growth and development,
namely, the presence of a definite organ, the _epimerite_, which
serves for the attachment of the parasite to the host-cell (fig. 6).
[Illustration: After Siedlecki, from Lankester's _Treatise on
Zoology_.
FIG. 5.--Part of a section through the apparatus of fixation of a
_Pterocephalus_, showing root-like processes extending from the
Gregarine between the epithelial cells. g, Head of Gregarine; r,
Root-like processes; ep, Epithelial cells.]
In those Gregarines (most intestinal forms) which become attached to
an epithelial cell, the attachment occurs by means of a minute
projection or beak (rostrum) at the anterior end of the sporozoite,
which pushes its way into the cell, followed by the first part of the
growing germ. This portion of the body increases in size much quicker
at first than the rest (the extracellular part), more or less fills up
the host-cell, and forms the well-developed epimerite or secondary
attaching organella. The extracellular part of the Gregarine next
grows rapidly, and a transverse septum is formed at a short distance
away from (outside) the point where the body penetrates into the cell
(fig. 6); this marks off the large deutomerite posteriorly (distally).
Leger thinks that this partition most likely owes its origin to
trophic considerations, i.e. to the slightly different manner in which
the two halves of the young parasite (the proximal, largely
intracellular part, and the distal, extracellular one) may be supposed
to obtain their nutriment. In the case of the one half, the host-cell
supplies the nutriment, in that of the other, the intestinal liquid;
and the septum is, as it were, the expression of the conflicting limit
between these two methods. Nevertheless, the present writer does not
think that mechanical considerations should be altogether left out of
account. The septum may also be, to some extent, an adaption for
strengthening the body of the fixed parasite against lateral thrusts
or strains, due to the impact of foreign bodies (food, &c.) in the
intestine.
[Illustration: From Wasielewski, after Leger.
FIG. 6.--_Corycella armata_, Leger. a, Cephalont; b, Epimerite in
host-cell; c, Sporont.]
At the point where the body becomes actually intracellular, it is
constricted, and this constriction marks off the epimerite
(internally) from the middle portion (between this point and the
septum), which is the protomerite. Further growth is restricted,
practically, to the extracellular regions, and the epimerite often
comes to appear ultimately as a small appendage at the anterior end of
the protomerite. A Gregarine at this stage is known as a cephalont.
Later on, the parasite breaks loose from the host-cell and becomes
free in the lumen, the separation taking place at the constriction
between the protomerite and the epimerite; the latter is left behind
in the remains of the host-cell, the former becomes the anterior part
of the free trophozoite.
In other Gregarines, however, those, namely, which pass inwards,
ultimately becoming "coelomic," as well as those which become entirely
intracellular, no epimerite is ever developed, and, further, the body
remains single or unseptate. These forms, which include, for instance,
_Monocystis_ (fig. 4), _Lankesteria_, _Diplocystis_, are
distinguished, as _Acephalina_ or _Aseptata_ (_Haplocyta_,
_Monocystida_), according to which character is referred to, from the
others, termed _Cephalina_ or _Septata_ (_Polycystida_).
The two sets of terms are not, however, completely identical or
interchangeable, for there are a few forms which possess an epimerite,
but which lack the division into protomerite and deutomerite, and are
hence known as _Pseudomonocystida_; this condition may be primitive
(_Doliocystis_) or (possibly) secondary, the partition having in
course of time disappeared. Again, _Stenophora_ is a septate form
which has become, secondarily, completely intracellular during the
young stages, and, doubtless correlated with this, shows no sign of an
epimerite.
[Illustration: From Wasielewski, after Leger.
FIG. 7.--Forms of Epimerites.
1, _Gregarina longa_.
2, _Sycia inopinata_.
3, _Pileocephalus heerii_.
4, _Stylorhynchus longicollis_.
5, _Beloides firmus_.
6, _Cometoides crinitus_.
7, _Geneiorhynchus monnieri_.
8, _Echinomera hispida_.
9, _Pterocephalus nobilis_.]
With regard to the epimerites themselves, they are of all variety of
form and shape and need not be described in detail (fig. 7). In one or
two cases, however, another variety of attaching organella is met
with. Thus in _Pterocephalus_, only the rostrum of the sporozoite
penetrates into the host-cell, and no epimerite is formed. Instead, a
number of fine root-like processes are developed from near the
anterior end, which pass in between the host-cells (fig. 5) and thus
anchor the parasite firmly. Similarly, in the curious
_Schizogregarinae_, the anterior end of the (unseptate) body forms a
number of stiff, irregular processes, which perform the same function
(fig. 8). It is to be noted that these processes are non-motile, and
not in any way comparable to pseudopodia, to which they were formerly
likened.
A very interesting and remarkable morphological peculiarity has been
recently described by Leger (18) in the case of a new Gregarine,
_Taeniocystis_. In this form the body is elongated and metamerically
segmented, recalling that of a segmented worm, the adult trophozoites
possessing numerous partitions or segments (each corresponding to the
septum between the proto- and deuto-merite in an ordinary Polycystid),
which divide up the cytoplasm into roughly equal compartments. Leger
thinks only the deutomerite becomes thus segmented, the protomerite
remaining small and undivided. The nucleus remains single, so that
there is no question as to the unicellular or individual nature of the
entire animal.
[Illustration: After Leger and Hagenmuller, from Lankester's _Treatise
on Zoology_.
FIG. 8.--Three Individuals (G) of _Ophryocystis schneideri_, attached
to wall of Malpighian tubule of _Blaps_ sp. p, Syncytial protoplasm of
the tubule; c, Cilia lining the lumen.]
Minute structure.
The general cytoplasm usually consists of distinct ectoplasm and
endoplasm, and is limited by a membrane or cuticle (epicyte), secreted
by the former. The cuticle varies considerably in thickness, being
well developed in active, intestinal forms, but very thin and delicate
in non-motile coelomic forms (e.g. _Diplodina_). In the former case it
may show longitudinal striations. The cuticle also forms the hooks or
spines of many epimerites. The ectoplasm usually shows (fig. 9A) a
differentiation into two layers, an outer, firmer layer, clear and
hyaline, the sarcocyte, and an inner layer, the myocyte, which is
formed of a network of muscle-fibrillae (mainly longitudinal and
transverse, fig. 9B). The sarcocyte alone constitutes the septum,
traversing the endoplasm, in septate Gregarines. The myonemes are
undoubtedly the agents responsible for the active "gregarinoid"
movements (of flexion and contraction) to be observed in many forms.
The peculiar gliding movements were formerly thought to be produced by
the extrusion of a gelatinous thread posteriorly, but Crawley (8) has
recently ascribed them to a complicated succession of wave-like
contractions of the myocyte layer. This view is supported by the fact
that certain coelomic forms, like _Diplodina_ and others, which either
lack muscle-fibrils or else show no ectoplasmic differentiation at
all, are non-motile. The endoplasm, or nutritive plasm, consists of a
semi-fluid matrix in which are embedded vast numbers of grains and
spherules of various kinds and of all sizes, representing an
accumulation of food-material which is being stored up prior to
reproduction. The largest and most abundant grains are of a substance
termed para-glycogen, a carbohydrate; in addition, flattened
lenticular platelets, of an albuminoid character, and
highly-refringent granules often occur.
[Illustration: After Schewiakoff, from Lankester's _Treatise on
Zoology_.
FIG. 9A.--Longitudinal section of a Gregarine in the region of the
septum between protomerite and deutomerite.
Pr, Protomerite.
De, Deutomerite.
s, Septum.
en, Endoplasm.
sc, Sarcocyte.
c, Cuticle.
m, f, Myocyte fibrils (cut across).
g, Gelatinous layer.]
[Illustration: FIG. 9B.--_Gregarina munieri_, showing the network of
myocyte fibrillae.]
The nucleus is always lodged in the endoplasm, and, in the septate
forms, in the deutomeritic half of the body. It is normally spherical
and always limited by a distinct nuclear membrane, which itself often
contains chromatin. The most characteristic feature of the nucleus is
the deeply-staining, more or less vacuolated spherical karyosome
(consisting of chromatin intimately bound up with a plastinoid basis)
which is invariably present. In one or two instances (e.g.
_Diplocystis schneideri_) the nucleus has more than one karyosome. All
the chromatin of the nucleus is not, however, confined to the
karyosome, some being in the form of grains in the nuclear sap; and in
some cases at any rate (e.g. _Diplodina, Lankesteria_) there is a
well-marked nuclear reticulum which is impregnated with granules and
dots of chromatin.
[Illustration: From Wasielewski, after A. Schneider.
FIG. 10.--Schizogony in _Ophryocystis francisci_. a, Rosette of small
individuals, produced from a schizont which has just divided; b, A
later stage, the daughter-individuals about to separate and assuming
the characters of the adult.]
Life-history.
A sexual multiplication (schizogony) is only known certainly to occur
in a few cases, one being in a Monocystid form, a species of
_Gonospora_, which is for a long time intracellular (Caullery and
Mesnil [4]), the rest among the _Schizogregarinae_, so named for this
reason, in which schizogonous fission takes place regularly during the
free, trophic condition. Usually, the body divides up, by a process of
multiple fission (fig. 10), into a few (up to eight)
daughter-individuals; but in a new genus (_Eleutheroschizon_), Brasil
(3) finds that a great number of little merozoites are formed, and a
large amount of vacuolated cytoplasm is left over unused.
In the vast majority of Gregarines, however, the life-cycle is limited
to gametogony and sporogony. A very general, if not indeed universal,
prelude to gametogony is the characteristic and important feature of
the order, known as association, the biological significance of which
has only lately been fully brought out (see H. M. Woodcock [31]). In
normal association, two individuals which are to be regarded as of
opposite sex, come into close contact with each other and remain thus
attached. The manner in which the parasites join varies in different
forms; the association may be end-to-end (terminal), either by like or
by unlike poles, or it may be side-to-side (lateral) (fig. 12). The
couple (syzygy) thus formed may proceed forthwith to encystment and
sporoblast-formation (_Lankesteria, Monocystis_), or may continue in
the trophic phase for some time longer (_Gregarina_). In one or two
instances (_Zygocystis_), association occurs as soon as the
trophozoites become adult. This leads on to the interesting
phenomenon of precocious association (neogamy), found in non-motile,
coelomic Gregarines (e.g. _Cystobia_, _Diplodina_ and _Diplocystis_),
in which the parasitism is most advanced. Woodcock (_loc. cit._) has
described and compared the different methods adopted to ensure a
permanent union, and the degree of neogamy attained, in these forms.
Here it must suffice to say that, in the extreme condition (seen, for
instance, in _Diplodina minchinii_) the union takes place very early
in the life-history, between individuals which are little more than
sporozoites, and is of a most intimate character, the actual cytoplasm
of the two associates joining. In such cases, there is absolutely
nothing to indicate the "double" nature of the growing trophozoite,
but the presence of the two nuclei which remain quite distinct.
[Illustration: From Wasielewski, after Leger.
FIG. 11.--_Eirmocystis spp._ a, b, Associations of two and three
Gregarines; c, Chain of five parasites; p, Primite; s, Satellites.]
There can be little doubt that, in the great majority, if not in all
Gregarines, association is necessary for subsequent sporulation to
take place; i.e. that the cytotactic attraction imparts a
developmental stimulus to both partners, which is requisite for the
formation of primary sporoblasts (gametes). This association is
usually permanent; but in one or two cases (perhaps _Gonospora sp._)
temporary association may suffice. While association has fundamentally
a reproductive (sexual) significance, in some cases, this function may
be delayed or, as it were, temporarily suspended, the cytotactic
attraction serving meanwhile a subsidiary purpose in trophic life.
Thus, probably, are to be explained the curious multiple associations
and long chains of Gregarines (fig. 11) sometimes met with (e.g.
_Eirmocystis_, _Clepsydrina_).
Encystment is nearly always double, i.e. of an associated couple.
Solitary encystment has been described, but whether successful
independent sporulation results, is uncertain; if it does, the
encystment in such cases is, in all probability, only after prior
(temporary) association. In the case of free parasites, a
well-developed cyst is secreted by the syzygy, which rotates and
gradually becomes spherical. A thick, at first gelatinous, outer
cyst-membrane (_ectocyst_) is laid down, and then a thin, but firm
internal one (endocyst). The cyst once formed, further development is
quite independent of the host, and, in fact, often proceeds outside
it. In certain coelomic Gregarines, on the other hand, which remain in
very close relation with the host's tissues, little or nothing of an
encystment-process on the part of the parasites is recognizable, the
cyst-wall being formed by an enclosing layer of the host
(_Diplodina_).
[Illustration: From Wasielewski, after Leger.