swimming, first becomes somewhat flattened and the mouth elongated. A
bilateral symmetry is thus brought about. Two mesenteries now make
their appearance transversely to the long axis of the mouth, which
divide the enteric cavity into two _unequal chambers_. The
mesenteries consist of a fold of hypoblast with a prolongation of the
epiblast between the two limbs of the fold. The larger chamber
next becomes divided by two fresh mesenteries into three, and a
similar division then takes place in the smaller chamber. The stage
with six chambers is almost immediately succeeded by one with eight,
owing to the appearance of two fresh mesenteries in the second-formed
set of chambers. At the stage with eight chambers there is a marked
period of repose. The number of chambers is increased to ten by the
division of the third-formed set of chambers, and to twelve by the
division of the fourth-formed set. It will be observed that the number
of the chambers increases in arithmetical progression by the continual
addition of two, alternately cut off from the primitive large and
small chambers. The freshly formed chambers are always formed
immediately on one side of the primitive mesenteries. The stages with
six and ten are of very short duration. The two primitive chambers are
necessarily at the ends of the long axis of the mouth. After the
division of the enteric cavity into twelve chambers, these chambers
become about equal in size, and the formation of the tentacles
commences. The law regulating the appearance of the tentacles is
nearly the same as that for the mesenteries, but is not quite so
precise. One tentacle makes its appearance for each chamber. The most
remarkable feature in the appearance of the tentacles is due to the
fact that the tentacle surmounting the primitive largest chamber
arises before any of the others, and long retains its supremacy (fig.
80 A). This fact, coupled with the inequality of the two primitive
chambers, supplies some grounds for speculating on a possible descent
of the Coelenterata from bilaterally symmetrical forms with
distinctly differentiated dorsal and ventral surfaces. The supremacy
of the first-formed tentacle is not confined to the Actinozoa, but as
has already been indicated, is also found in the Scyphistoma (p. 166)
of the Acraspeda.
[FIG. 80. TWO STAGES IN THE DEVELOPMENT OF ACTINIA MESEMBRYANTHEMUM.
(After Lacaze Duthiers.)
In the younger ciliated embryo A, viewed from the side, only one
tentacle is developed. _m._ mouth.
The older larva B is viewed from the face when 24 tentacles have
just become established. The letters shew the true order of
succession of the tentacles; but _e_ and _f_ are transposed.]
After the twelve tentacles have become established they become
secondarily divided into two cycles of six respectively larger and
smaller tentacles, which alternate with each other. The two tentacles
pertaining to the two original chambers belong to the cycle of larger
tentacles. The mesenteric filaments appear first of all on the primary
pair of septa. The increase in the number of tentacles and chambers
from 12 to 24 has been found to take place in a very remarkable and
unexpected way. The law is expressed by Lacaze Duthiers as follows.
"The appearance of the new chambers is not, as has been believed, a
consequence of the production of a single chamber between each of the
twelve already existing chambers, but of the birth of two new chambers
in each of the six elements (chambers) of the smaller cycle." The
result of this law is that a pair of tentacles of the third cycle is
placed in every alternate space, between a large and a small tentacle,
of the two already existing cycles, which may conveniently be called
the first and second cycles (fig. 80 B).
The twenty-four tentacles formed in the above manner are obviously at
first very irregularly arranged (fig. 80 B), but they soon acquire a
regular arrangement in three graduated cycles of 6, 6 and 12. The
first cycle of the six largest tentacles is the large cycle of the
previous stage, but the two other cycles are heterogeneous in their
origin, each of them being composed partly of the twelve tentacles
last formed, and partly of the six tentacles of the second cycle of
the previous stage.
The further law of multiplication has been thus expressed by Lacaze
Duthiers: "The number of chambers and still later that of the
corresponding tentacles is carried from 24-48 and from 48-96 by the
birth of a pair of elements in each of the 12 or 24 chambers, above
which are placed the smallest tentacles which together constitute the
fourth or fifth cycle. Since, after the formation of each fresh cycle,
the arrangement of the tentacles again becomes symmetrical, it is
obvious that all the equal sized cycles except the first are formed of
tentacles entirely heterogeneous as to age."
The fixation of the free-swimming larva takes place during the period
when the tentacles are increasing from 12 to 24.
The general formation of the chambers in Bunodes and Sagartia is
nearly the same as in Actinia.
In the two types of Actinozoa with an embolic gastrula stage the laws
as to the formation of the tentacles do not appear to be the same as
those regulating the forms observed by Lacaze Duthiers.
In Cerianthus four tentacles are formed simultaneously at the period
when only four chambers are present. In Arachnitis (Edwarsia) the
succession of the tentacles is stated (A. Agassiz, 166) to resemble
that in Cerianthus. There are originally four tentacles, and at one
extremity of the long axis of the mouth are the oldest tentacles,
while at the other tentacles are constantly added in pairs. An odd
tentacle is always found at the extremity of the mouth opposite the
oldest tentacles.
In the other species with an embolic gastrula eight tentacles would
seem to appear simultaneously at the period when eight chambers are
present; though on this point Kowalevsky's description is not very
clear. The presence of such a stage would seem to indicate a close
affinity to the Alcyonidæ.
Amongst the sclerodermatous Actinozoa, except Caryophyllium, the
embryo closely resembles that of the delaminate Malacodermata. The
first stages occur in the ovary, and the larva is dehisced into the
body cavity as a two-layered ciliated planula.
The laws affecting the formation of the first twelve tentacles and
septa appear to be nearly the same as for the Malacodermata. The hard
parts begin as a rule to be formed when twelve tentacles have
appeared, at which period also the fixation of the larva takes place.
On fixation the larva becomes very much flattened.
The first parts of the corallum to appear are twelve of the septa,
which arise simultaneously in folds of the enteric wall in the
chambers _between the mesenteries_, and correspond therefore with
the tentacles and not, as might be supposed, with the mesenteries.
Each septum is formed by the coalescence of three calcareous plates
which originate in separate centres of calcification. The concrescence
of the three produces a Y-shaped plate with the single limb directed
inwards and the two limbs outwards (fig. 81). The theca does not arise
till after the septa have become formed, and is at first a somewhat
membranous cup quite distinct from the septa. The columella is formed
still later by the coalescence of a series of nodules which are formed
in a central axis enclosed by the inner ends of the septa.
[FIG. 81. LARVA OF ASTROIDES CALYCULARIS SHORTLY AFTER IT HAS BECOME
ATTACHED. (After Lacaze Duthiers.)
The figure shews the development of the Y-shaped septa in the
intervals between the mesenteries. The position of the latter is
indicated by the faint shading. The theca has become developed
externally.]
After the formation of the theca the septa become divided into two
cycles by the predominant growth of six of them. On the coalescence of
the septa with the theca the space between the two limbs of the Y
becomes filled up with calcareous tissue. The law of the formation of
the third cycle of septa (12-24) has not been worked out, so that it
is not possible to state whether it follows the peculiar principles
regulating the growth of the tentacles.
The whole of the skeletal parts occupy a position between the epiblast
and hypoblast, and are exactly homologous in this respect with the
skeleton of the Alcyonidæ. By Lacaze Duthiers they are however
believed to originate in the hypoblast, but from the observations of
Kowalevsky there be little doubt that they arise in the connective
tissue between the two embryonic layers which is probably epiblastic
in origin.
A peculiar larva, probably belonging to the Actinozoa, has been
described by Semper[83]. It has an elongated form and is provided with
a longitudinal ridge of cilia. There is a mouth at one end of the body
and an anus at the opposite extremity. The mouth leads into an
oesophagus, which opens freely into a stomach with six mesenteries. In
the skin are numerous thread-cells. A mesotrochal worm-like larva,
also provided with thread-cells, and found at the same time, was
conjectured by Semper to be a younger form of this larva.
[83] "Ueb. einige tropische Larven-formen." _Zeit. f. wiss.
Zool._, vol. XVII. 1867.
Ctenophora. The ovum of the Ctenophora is formed of an outer
granular protoplasmic layer and an inner spongy mass with fatty
spherules. It is enveloped in a delicate vesicle, the diameter of
which is very much greater than that of the contained ovum. This
vesicle appears to be filled with sea-water, in which the ovum floats.
Fertilized ova may usually be easily obtained by keeping the captured
adults in water from 12-24 hours. The two main authorities on the
development of these forms (Kowalevsky, No. 147 and 178 and Agassiz,
No. 172) are unfortunately at variance on one or two of the most
fundamental points. It seems however that the embryonic layers are
formed by a kind of epibolic gastrula; while the true gastric cavity,
as distinct from the gastrovascular, is formed by an invagination, and
deserves therefore to be regarded as a form of stomodæum.
[FIG. 82. FIVE STAGES IN THE DEVELOPMENT OF IDYIA ROSEOLA. (After
Agassiz.)
The protoplasmic layer of the ovum is represented in black.]
The early stages are very closely similar in all the types so far
observed. Segmentation commences by the outer layer of the ovum, which
throughout behaves as the active layer, forming a protuberance at one
pole, which may be called the formative pole. Close below this
protuberance is placed the nucleus. In the median line of the
protuberance a furrow appears (fig. 82 A), which gradually deepens
till it divides the ovum into two. The granular layer follows the
furrow so that each of the fresh segments, like the original ovum is
completely invested by a layer of granular protoplasm. Each
segment contains a nucleus. A second similar division at right angles
to the first gives rise to four segments (fig. 82 B), and the segments
so formed become again divided into eight (fig. 82 C). In the division
into eight, which takes place in a vertical plane, the segments formed
are of unequal size, four of them being much smaller than the others.
The eight segments are arranged in the form of a slightly curved disc
round a vertical axis--the future long axis of the body;--and there is
a cavity in this axis which, like the segmentation cavity of
_Sycandra raphanus_, is open at both extremities. The disc with
its concavity on the side of the formative pole has the shape
sometimes of an ellipse (fig. 82 C) and sometimes of a rectangle, in
which the four small spheres occupy the poles of the longer axis. A
bilateral symmetry is thus even at this stage clearly indicated.
In the next phase of segmentation the granular layer surrounding each
segment again forms a protuberance at the formative pole, but, instead
of each segment becoming divided into two equal parts, the
protoplasmic protuberance alone is divided off from the main segment.
In this way sixteen spheres become formed, of which eight are large
and are formed mainly of the yolk material of the inner part of the
ovum, and eight are small and entirely composed of the granular
protoplasm. The eight small spheres form a ring on the formative
surface of the large spheres (fig. 82 D).
The small spheres now increase very rapidly (fig. 82 E), partly by
division _and partly by the formation of fresh cells from the large
spheres_; and spread over the large spheres, forming in this way an
epibolic gastrula. They constitute a layer of epiblast. (fig. 83 A.)
The large cells in the meantime remain relatively passive, though
during the process they divide, in some cases more or less
irregularly, while in Eucharis they divide into sixteen. The axial
segmentation cavity would seem during the process to become
obliterated.
There is an important discrepancy between the statements of Kowalevsky
and Agassiz as to the course of the growth of the small cells.
According to Agassiz the small cells grow most rapidly at the
formative pole and cover this before they meet at the opposite pole.
The reverse statement is made by Kowalevsky. It would seem that the
above discrepancy is due to an interchange on the part of the
one or the other of these authors of the two poles of the embryo, in
that according to Agassiz the formation of the mouth takes place _at
the formative pole_, and according to Kowalevsky _at the pole
opposite to this_.
Without attempting to decide between the above views, we shall speak
of the pole at which the mouth is formed as the oral pole.
[FIG 83. FOUR STAGES IN THE DEVELOPMENT OF IDYIA ROSEOLA. (After
Agassiz.)
_s.c._ sense capsule; _st._ stomodæum.]
The formation of the alimentary cavity commences shortly after the
complete investiture of the embryo by the epiblast cells. At the oral
pole an invagination of epiblast cells takes place (fig. 83 B), which
makes its way towards the opposite pole. More especially from the
figures given by Agassiz, and from the explanation of his plates, it
would seem that a large chamber is formed in the hypoblast at the end
of the invaginated tube, into which this tube soon opens (fig. 83 C).
The invaginated tube would seem to give rise to the so-called stomach,
while the chamber at its aboral extremity is no doubt the
infundibulum, which as may be gathered from Kowalevsky's statements,
is lined by a flattened epithelium. At a later period the
gastrovascular canals grow out from the infundibulum as four pouches,
which are surrounded by, and grow at the expense of, the large central
cells, which have in the meantime arranged themselves in four masses,
and appear to serve as a kind of yolk. The nuclei of these large cells
according to Kowalevsky disappear, and the cells themselves break up
into continually smaller masses.
The main difficulty in the above description of Agassiz is the origin
of the infundibulum. In the absence of definite statements on this
head it seems reasonable to conclude that it arises as a space
hollowed out in the central cells, and that its walls are formed of
elements derived from the yolk cells[84]. On this interpretation the
alimentary canal of the Ctenophora would consist, as in the
Acraspedote Medusæ and Actinozoa, of two sections: (1) A true
hypoblastic section consisting of the infundibulum and the
gastrovascular canals derived from it; and (2) an epiblastic
section--the stomodæum--forming the stomach.
[84] Chun (No. 174) gives a short statement of his observations,
which accords with the interpretation in the text.
The observations of Kowalevsky on the alimentary system do not wholly
tally with those of Agassiz. He finds that the oral side of the embryo
becomes hollowed out, and that the hollow, lined by flattened cells,
becomes constricted off as the infundibulum, from which the radial
canals subsequently grow out. To the infundibulum there leads a narrow
canal lined by a columnar epithelium which becomes the gastric cavity.
While the alimentary canal is becoming formed a series of important
changes takes place in other parts of the embryo. The rows of
locomotive paddles first appear as four longitudinal equidistant
linear thickenings of the epiblast near the aboral pole (fig. 83 D).
On the projecting surface of these ridges stiff cilia appear which
coalesce together to form the paddles. While the embryo is still
within the egg the rows of paddles are quite short and also double.
There are in Pleurobrachia about eight or nine pairs of paddles in
each row. Each double row eventually separates into two.
In all the forms except the Eurostomata (Beroe) two tentacles grow out
as thickenings of the epiblast (fig. 84 B, _t._). They are placed
at the opposite poles of the long transverse axis of the embryo.
A process of the contractile gelatinous tissue of the body, the origin
of which is described below, makes its way, according to Kowalevsky,
into the tentacles.
The central apparatus of the nervous system and the otoliths are
formed at the aboral pole from a thickening of the epiblast, but the
full details of their formation have not been elucidated. It may be
well to preface my account of their development with a short statement
of their adult structure.
They consist in the adult of a vesicle with a ciliated lining situated
at the bifurcation of the two anal tubes, and of certain structures
connected with this vesicle. From the floor of the vesicle is
suspended a mass of otoliths by four leaf-like bodies known as
suspenders. The roof is very delicate and has the form of a four-sided
pyramid. Six openings lead into the vesicle. Through four of these,
placed at the four corners, there pass out four ciliated grooves
continuous with the suspenders. These grooves, after leaving the
otolithic vesicle, bifurcate and pass to the eight rows of paddles. At
the two sides the walls of the vesicle are continuous with two
thickened ciliated plates with swollen edges, opposite the centres of
which are two lateral openings into the vesicle, completing the six
openings. Through the lateral openings the sea-water is driven by the
action of the cilia of the plates.
The development of these parts is as follows--In the aboral thickening
of epiblast a cavity makes its appearance, the walls of which
constitute the rudiment of the otolithic vesicle (fig. 83 B and C,
_s.c._). The roof of the cavity is extremely delicate. On each
side of it a thickening of cells becomes established, regarded by
Kowalevsky as the rudiment of the nervous ganglia. These thickenings
appear to give origin to the lateral ciliated plates. The otoliths
arise from cells at four separate points at the corners of the
ciliated plates opposite the rows of paddles (fig. 84 A, _ot._).
[FIG. 84. TWO STAGES IN THE DEVELOPMENT OF PLEUROBRACHIA
RHODODACTYLA. (After Agassiz.)
_ot._ otolith; _t._ tentacle.]
In Pleurobrachia there is at first only one otolith at each corner.
The otoliths are gradually transported towards the centre of the
vesicle (fig. 84 B, _ot._) and are there attached, though the
four leaf-like suspenders do not arise till very late. The otoliths go
on increasing in number throughout life.
The gelatinous tissue of the Ctenophora appears as a homogeneous layer
between the epiblast and the yolk cells, and is probably homologous
with the layer formed in the same situation in all other
coelenterate forms. Into the layer a number of anastomosing cells,
mainly derived from the epiblast, though according to Chun (No. 174)
also in part from the hypoblast, make their way. These cells would
appear to be mainly, if not entirely (Chun), of a contractile nature.
It is probable that the great mass of the gelatinous tissue of the
adult is an intercellular substance derived from these cells.
The whole of the above changes are completed while the embryo is still
enclosed in the egg-capsule. During their accomplishment the oro-anal
axis, which was originally very short, increases greatly in length
(fig. 83), so that the embryo acquires an oval form similar to that of
the adult.
The exact period of leaving the egg does not appear to be very
constant but the hatching never takes place till the embryo has
practically acquired all the organs of the adult.
In the majority of types the differences between the just hatched
larva and the adult are inconsiderable, and in all cases the larva has
a somewhat oval form. In the case of the Tæniatæ (Cestum, etc.), the
larva has the characteristic oval form, and the subsequent changes
amount almost to a metamorphosis.
The larva of the Lobatæ, such as Eucharis, Bolina, etc., can hardly be
distinguished from Pleurobrachia, and undergoes therefore considerable
changes after hatching.
_Eucharis multicornis_ while still in the larval condition is
stated by Chun to become sexually mature.
The new genus Ctenaria recently described by Haeckel, which is
intermediate between the Ctenophora and the Medusæ clearly proves that
the Ctenophora are more closely related to the Medusæ than to the
Actinozoa but their development, especially the presence of a
stomodæum, shews that they have affinities (in spite of the
rudimentary velum of Ctenaria) with the Acraspedote as well as with
the Craspedote Medusæ; and it may be noted that the Acraspeda have
undoubted affinities with the Actinozoa.
_Summary and general considerations._
Even in the adult condition the lower forms of Coelenterata do not
rise in complexity much beyond a typical gastrula. Ontogeny
nevertheless brings clearly to light the existence of a larval
form--the planula--which recurs with fair constancy amongst all the
groups except the Ctenophora.
We are probably justified in assuming that the planula is a repetition
of a free ancestral form of the Coelenterata. The planula, as it
most frequently occurs, is a two-layered ciliated nearly cylindrical
organism, with at most a rudimentary digestive cavity hollowed out in
the inner layer, and as a rule no mouth. In the outer layer are
numerous thread-cells.
How many of these characters did the ancestral planula possess? I
think it is not unreasonable to assume that the only two characters
about which there can be much doubt are the rudimentary condition of
the digestive cavity and the absence of a mouth. Paradoxical as it may
seem, it appears to me not impossible that the Coelenterata may have
had an ancestor in which a digestive tract was physiologically
replaced by a solid mass of amoeboid cells. This ancestor was perhaps
common to the Turbellarians also. The constant presence of
thread-cells in the inner layer of their epiblast fits in with their
derivation from a form similar to the planula. While the solid
parenchymatous digestive canal of Convoluta and Schizoprora and other
forms amongst the Turbellarians, though very probably secondary, may
perhaps be explained by such a view of their origin.
The planula in its primitive condition is not bilaterally symmetrical,
but frequently, as amongst the Actinozoa, it becomes flattened on two
sides before undergoing its conversion into the adult form. Perhaps
the bilateral form of planula is the starting point both for the
Coelenterata and the Turbellaria. In this connection the peculiar
unilateral development of a tentacle in Scyphistoma and Actinia should
be noted.
The planula occurs in the majority of sessile forms of Hydrozoa except
the Tubularidæ and Hydra. It is also characteristic of the
Trachymedusæ and Siphonophora. Amongst the Acraspeda it is also
present, but has an exceptional mode of ontogeny which is discussed in
connection with the germinal layers.
It is characteristic both of the Octocoralla and Hexacoralla, but is
not found in the Ctenophora.
In the Tubularidæ and in Hydra an abbreviated development leads no
doubt to the absence of a _free_ planula stage, and the absence
of a larval form amongst the Ctenophora may, as has already been
stated, be probably explained in the same way.
The Coelenterata of all the Metazoa are characterized by the greatest
simplicity in the arrangement of their germinal layers; and for this
reason very considerable interest attaches to the mode of formation of
the layers amongst them. Two germinal layers are constantly found,
which correspond _in a general way_ to the epiblast and hypoblast. It
might have been anticipated that a certain amount of uniformity would
have existed in the mode of formation of the layers. This however is
not the case. In perhaps the majority of forms they become
differentiated by a process of delamination, but in a not
inconsiderable minority the two layers owe their origin to an
invagination.
Delamination is constant (with the doubtful exception of some
Tubularidæ) amongst the Hydromedusæ and Siphonophora. It is perhaps in
the main characteristic of the Actinozoa.
Invagination by embole takes place, so far as is known, constantly
amongst the Acraspeda and frequently amongst the Actinozoa;
and an epibolic invagination is characteristic of the Ctenophora.
If confidence is to be placed in the recorded observations on which
this summary is founded, and there is no reason why in a general way
it should not be so placed, the conclusion is inevitable that of the
above modes of development the one must be primitive and the other a
derivative from it, for, if this conclusion be not accepted, the
absolutely inadmissible hypothesis of a double origin for the
Coelenterata would have to be adopted.
Two questions arise from these considerations:--
(1) Which is the primitive, delamination or invagination?
(2) How is the one of these to be derived from the other?
There is a great deal to be said in favour of both delamination and
invagination; but it will be convenient to defer all discussion of the
question to the general chapter on the formation of the layers
throughout the animal kingdom.
The hypoblast cells are often filled with yolk material, and secondary
modifications are thus produced in the development. The most important
examples of such modifications are found in the Siphonophora and
Ctenophora.
In the simplest forms amongst the Hydrozoa there is no trace of a
third layer or mesoblast. The epiblast is typically formed, as was
first shewn by Kleinenberg, of an epithelial layer and a subepithelial
interstitial layer of cells. The cells of the former are frequently
produced into muscular or nervous tails, and those of the latter give
rise to the thread-cells and generative organs and in some cases to
muscles[85]. In many cases, amongst all the Coelenterate groups, and
constantly amongst the Ctenophora the epiblast is simplified and
reduced to a single layer. The hypoblast undergoes in most cases no
such differentiation but simply forms a glandular layer lining the
gastric chamber and its prolongations into the tentacles; but in the
Actinozoa it appears to give rise to muscles, and strong evidence has
been brought forward to shew that in some groups it gives rise to the
generative organs.
[85] The questions relating to the generative organs of the
Coelenterata are dealt with in the second part of this work.
Between the epiblast and hypoblast a structureless lamella appears
always to be interposed.
In many Coelenterata further differentiations of the epiblast are
present. In many forms the layer gives rise to a hard external
skeleton. This is most widely spread amongst the Hydrozoa, where in
the majority of cases it takes the form of the horny perisarc, and in
the Hydrocoralla (Millepora and Stylasteridæ) of a hard calcareous
skeleton. The skeleton in these forms, though closely resembling the
mesoblastic skeleton of the Actinozoa, has been shewn by Moseley (164)
to be epiblastic.
In the Actinozoa an epiblastic skeleton is exceptional, and according
to most authorities absent. Quite recently however Koch (167) has
found that the axial branched skeleton of most of the Gorgonidæ, viz.
the Gorgoninæ and Isidinæ, is separated from the coenosarc by an
epithelium, which he believes to be epiblastic, and to which no doubt
the axial skeleton owes its origin. A similar epithelium surrounds the
axis of the Pennatulidæ.
In the Medusæ the epiblast also gives rise to a central nervous
system, which however continues to form a constituent part of the
layer, and to the organs of special sense[86].
[86] The differentiation of the nervous and muscular systems in
the Hydrozoa is treated of in the second part of this work.
A special differentiation of the hypoblast is found in the solid axis
of the tentacles. This axis replaces the gastric prolongation found in
many forms, and the cells composing it differentiate themselves into a
chorda-like tissue, which has a skeletal function, and is no longer
connected with nutrition. This axis is placed by many morphologists
amongst the mesoblastic structures.
In all the higher Coelenterata certain tissues become interposed
between the epiblast and hypoblast, which may be classified together
as the mesoblast.
The most important of these are:
(1) The various distinct muscular layers.
(2) The gelatinous tissue of the Medusæ and Ctenophora.
(3) The skeletogenous tissue of the Actinozoa.
In most cases the muscular fibres are connected with epithelial cells,
but in certain forms amongst the Medusæ and in the majority if not all
the Actinozoa they constitute a distinct layer, sometimes separated
from the epiblast by a structureless membrane, _Æquorea
Mitrocoma_. Such layers when on the outer side of the membrane
separating epiblast and hypoblast are undoubtedly epiblastic in
origin, but in some cases amongst the Actinozoa they adjoin the
hypoblast, and are very probably derived from this layer.
The origin of the gelatinous tissue is still involved in much
obscurity.
It originates as a homogeneous layer between epiblast and hypoblast,
which in the Hydromedusæ never becomes cellular though traversed by
elastic fibres.
In the Acraspeda it contains anastomosing cells in the main apparently
(Claus) derived from the hypoblast, and in the Ctenophora it is richly
supplied with muscular stellate cells for the most part of epiblastic
origin, though some are stated by Chun to come from the hypoblast. On
the whole it seems probable, that the gelatinous tissue may be
regarded as a product _of both layers_; and there are some grounds for
thinking that it is an immense development of the membrane always
interposed between the two primary layers. It must however be borne in
mind that a membrane, regarded by the Hertwigs as the equivalent of
the ordinary membrane between the epiblast and hypoblast, can be
usually demonstrated on both surfaces of the gelatinous tissues in
Medusæ. The skeletogenous layer of the Actinozoa is probably the
morphological homologue of the gelatinous tissue; but the evidence we
have is on the whole in favour of the connective-tissue cells it
contains being epiblastic in origin. It gives rise to the skeleton of
the Hexacoralla, to the spicular skeleton of Alcyonium, the axial
skeleton of Corallium, and the skeleton of the Helioporidæ and
Tubiporidæ.
_Alternations of generations._
Alternation of generations is of common occurrence amongst the
Hydrozoa, and something analogous to it has been found to take place
in Fungia amongst the Actinozoa. It is not known to occur in the
Ctenophora.
The chief interest of its occurrence amongst the Hydromedusæ and
Siphonophora is the fact that its origin can be traced to a division
of labour in the colonial systems of zooids so characteristic of these
types.
In the Hydromedusæ an interesting series of relations between
alternation of generations and the division of the zooids into
gonophores and trophosomes can be made out. In Hydra the generative
and nutritive functions are united in the same individual. The
generative swellings in these forms cannot, as has been ably argued by
Kleinenberg, be regarded as rudimentary gonophores, but are to be
compared to the generative bands developed in the Medusæ around parts
of the gastro-vascular system. A condition like that of Hydra, in
which the ovum directly gives rise to a form like its parent, is no
doubt the primitive one, though it is not so certain that Hydra itself
is a primitive form. The relation of Hydra to the Tubularidæ and
Campanularidæ may best be conceived by supposing that in Hydra most
ordinary buds did not become detached, so that a compound Hydra became
formed; but that at certain periods particular buds retained their
primitive capacity of becoming detached and subsequently developed
generative organs, while the ordinary buds lost their generative
function.
It would obviously be advantageous for the species that the detached
buds with generative organs should be locomotive, so as to distribute
the species as widely as possible, and such buds in connection with
their free existence would naturally acquire a higher organization
than the attached trophosomes. It is easy to see how, by a series of
steps such as I have sketched out, a division of labour might take
place, and it is obvious that the embryos produced by the highly
organized gonophores would give rise to a fixed form from which the
fixed colony would be budded. Thus an alternation of generations would
be established as a necessary sequel to such a division of labour. To
test the above explanation it is necessary to review the main facts
with reference to alternations of generations amongst the Hydromedusæ.
Hydromedusæ[87]. In many instances amongst the Tubularidæ,
Sertularidæ and Campanularidæ medusiform buds are produced which
become detached and develop sexual organs.
[87] For a full account of this subject the reader is referred to
the beautiful memoir of Allman (No. 149).
Such Medusæ are divided into two great groups, the Ocellata and
Vesiculata, according to the characters of the marginal sense organs.
In the Ocellata the sense organs have the form of eyes, and in the
Vesiculata of auditory vesicles. The latter seem to be usually budded
off from the Campanularia stocks, and the generative organs extend in
folded bands over the radial canals. These bands have been regarded by
Allman as composed of rudimentary gonophores, and he called the Medusæ
which give rise to them blastochemes. He regards them as representing
a more complicated type of alternation of generations with three
instead of two generations in the series. The Hertwigs have brought
what appear to me conclusive grounds for rejecting this view, and have
demonstrated that the generative organs of these types resemble those
of ordinary Medusæ.
In many forms the medusiform buds though fully developed do not become
detached; whether detached or not they are known as phanerocodonic
gonophores. In other forms again buds which begin as if they were
going to form Medusæ never reach that condition but remain permanently
in an undeveloped state. They have been called by Allman adelocodonic
gonophores.
In all the above cases two generations at the least interpose between
the successive sexual periods, viz.:--
(1) A trophosome produced directly from the ovum.
(2) A gonophore budded from this.
In a very large number of types the gonophores do not develop directly
on the hydroid stem, but arise on specially modified zooids resembling
rudimentary trophosomes which have been named blastostyles by Allman.
On the sides of each blastostyle a series of gonophores usually
becomes developed. The blastostyles either remain exposed as in all
the Gymnoblastic or Tubularian Hydroids, or as in all the
Calyptoblastic Hydroids (Sertularidæ and Campanularidæ) they become
invested by a special case--known as the gonangium--which is formed of
perisarc lined by epiblast. In the forms with blastostyles three
generations interpose between the successive stages of sexual
reproduction, (1) the trophosome developed directly from the ovum, (2)
the blastostyle budded from this, (3) the gonophore budded from the
blastostyle.
Such being the main facts, in order to prove that the existing
condition of polymorphism amongst the Hydromedusæ is to be explained
as hypothetically suggested above, it is still necessary to shew that
(1) the free medusiform gonophores are really only modified
trophosomes, or rather that the trophosomes and gonophores are both
modifications of some common type, and (2) that the fixed so-called
adelocodonic gonophores are retrograde derivatives of the free
medusiform gonophores. Unless these points can be established it might
be maintained that the Medusæ were special zooids, developed _de novo_
and not by a modification of trophosome zooids. To demonstrate these
propositions at length would carry me too far into the region of
simple Comparative Anatomy, and I content myself with referring the
reader to a discussion of the Hertwigs (No. 146, p. 62) where the
first point appears to me fully established. With reference to the
second point I will only say that the structure and development of the
adelocodonic gonophores can only be explained on the assumption that
they are retrograde forms of the phanerocodonic gonophores, and that
the opposite view, that the phanerocodonic gonophores are derived from
the adelocodonic, leads to a series of untenable positions.
The Trachymedusæ, as has been shewn above, develop directly. They are
probably derived from gonophores in which the trophosome has
disappeared from the developmental cycle.
To sum up, three types of development are found amongst the
Hydromedusæ.
(1) No alternations of generations. Permanent form, a sexual
trophosome. _Ex._ Hydra.
(2) Alternations of generations. Trophosome fixed, gonophore free or
attached. _Ex._ Gymnoblastic and Calyptoblastic Hydroids, and
Hydrocoralla.
(3) No alternations of generations. Permanent form, a sexual Medusa.
_Ex._ Trachymedusæ.
Siphonophora. In the Siphonophora alternations of generations
take place in the same way as in the Hydromedusæ, but the starting
point appears to be a Medusa. The gonophores may remain fixed or
become detached.
Acraspeda. With the exception of Pelagia and Lucernaria, in
which the development involves a simple metamorphosis, all the
Acraspeda undergo a form of alternations of generations. The ovum, as
already described, develops into a fixed form--the Scyphistoma--which
increases asexually by normal budding, and can even form a permanent
colony.
[FIG. 85. THREE STAGES IN THE ALTERNATIONS OF GENERATIONS OF AURELIA
AURITA. (From Gegenbaur.)
A. Polype stage. B. Commencing strobilization. C. Completed
strobilization.]
The formation of the sexual Medusa form takes place by a kind of
strobilization of the body of the fixed Scyphistoma. A series of
transverse constrictions becomes formed round the body below the
mouth, dividing it up into corresponding rings, each of which
eventually gives rise to a Medusa known as an Ephyra (fig. 85). In
each of these rings is a dilation of the stomach, and a section of
each of the four rudimentary mesenteries described in connection with
the development of the Scyphistoma. As the constrictions become deeper
the segments of the body between them become disc-like, and their
edges are produced into eight lobes containing prolongations of the
gastric cavity (fig. 85 C). The lower surface of each disc, which
forms the future aboral surface of the Medusa, becomes convex, in part
owing to the development of gelatinous tissue. On the opposite surface
a muscular layer becomes developed. During the above process the body
of the Scyphistoma gradually grows in length and continues to be
segmented, so that a series of Ephyræ are uninterruptedly formed, of
which those near the base are the youngest. The original terminal ring
of tentacles of the Scyphistoma gradually atrophies.
In the further development of the Ephyræ each of their eight lobes
becomes bifid at its extremity.
As the Ephyræ successively reach this condition they become detached,
and by a series of remarkable changes, amounting almost to a
metamorphosis, and accompanied by an enormous growth in size, reach
the adult condition.
The alternation of generations in the Acraspeda cannot be quite so
simply explained as in the Hydromedusæ, though the principle is
probably the same in the two cases.
Actinozoa. Amongst the Actinozoa there occurs in Fungia a
peculiar process which is, as shewn by Semper (171), in many ways
analogous to alternations of generations[88]. From the larva a
nurse-stock is developed, at the end of which a cup-like coral
resembling the adult is formed as a bud. The bud becomes detached and
then gives rise to a permanent sexual Fungia. From the nurse-stock
there is formed however a fresh bud at the centre of the scar left on
the detachment of the old one. The fresh bud eventually becomes
separated from the nurse-stock leaving a small portion of its stem
behind; each succeeding bud similarly leaves a small portion of its
stem, so that the nurse-stock eventually acquires a jointed
appearance. In the above process we clearly have, as in the
Hydromedusæ, a non-sexual form--the nurse-stock--produced directly
from the larva, giving rise by budding to a sexual form; all the
conditions of an alternation of generations are therefore fulfilled.
It seems however possible that the nurse-stock itself may eventually
become sexual.
[88] Vide also Moseley. _Notes by a Naturalist of the
Challenger_, pp. 524 and 525.
BIBLIOGRAPHY.
_Coelenterata. General._
(145) Alex. Agassiz. _Illustrated Catalogue of the Museum of
Comparative Anatomy at Harvard College_, No. II. American Acalephæ.
Cambridge, U. S., 1865.
(146) O. and R. Hertwig. _Der Organismus d. Medusæ u. seine Stellung
z. Keimblättertheorie._ Jena, 1878.
(147) A. Kowalevsky. "Untersuchungen üb. d. Entwicklung d.
Coelenteraten." _Nachrichten d. kaiser. Gesell. d. Freunde d. Naturer
kenntniss d. Anthropologie u. Ethnographie._ Moskau, 1873. (Russian.)
For abstract vide _Jahresberichte d. Anat. u. Phys._ (Hoffman u.
Schwalbe), 1873.
_Hydrozoa._
(148) L. Agassiz. _Contributions to the Natural History of the United
States of America._ Boston, 1862. Vol. IV.
(149) G. J. Allman. _A Monograph of the Gymnoblastic or Tubularian
Hydroids._ Ray Society, 1871-2.
(150) G. J. Allman. "On the structure and development of Myriothela."
_Phil. Trans._, Vol. CLXV. p. 2.
(151) P. J. van Beneden. "Mém. sur les Campanulaires de la Côte
d'Ostende considérés sous le rapport physiologique, embryogénique, et
zoologique." _Nouv. Mém. de l'Acad. de Brux._, Tom. XVII. 1844.
(152) P. J. van Beneden. "Recherches sur l'Embryogénie des Tubulaires
et l'histoire naturelle des différents genres de cette famille qui
habitent la Côte d'Ostende." _Nouv. Mém. de l'Acad. de Brux._, Tom.
XVII. 1844.
(153) C. Claus. "Polypen u. Quallen d. Adria." _Denk. d.
math.-naturwiss. Classe d. k. k. Akad. d. Wiss. Wien_, Vol. XXXVIII.
1877.
(154) J. G. Dalyell. _Rare and Remarkable Animals of Scotland._
London, 1847.
(155) H. Fol. "Die erste Entwicklung d. Geryonideneies." _Jenaische
Zeitschrift_, Vol. VII. 1873.
(156) Carl Gegenbaur. _Zur Lehre vom Generationswechsel und der
Fortpflanzung bei Medusen und Polypen._ Würzburg, 1854.
(157) Thomas Hincks. "On the development of the Hydroid Polypes,
Clavatella and Stauridia; with remarks on the relation between the
Polype and the Medusoid, and between the Polype and the Medusa."
_Brit. Assoc. Rep._, 1861.
(158) E. Haeckel. _Zur Entwicklungsgeschichte d. Siphonophoren._
Utrecht, 1869.
(159) Th. H. Huxley. _Oceanic Hydrozoa._ Ray Society, 1858.
(160) Geo. Johnston. _A History of British Zoophytes._ Edin. 1838. 2nd
Edition, 1847.
(161) N. Kleinenberg. _Hydra, eine anatomisch-entwicklungsgeschichtliche
Untersuchung._ Leipzig, 1872.
(162) El. Metschnikoff. "Ueber die Entwicklung einiger Coelenteraten."
_Bull. de l'Acad. de St Pétersbourg_, XV. 1870.
(163) El. Metschnikoff. "Studien über Entwicklungsgeschichte d.
Medusen u. Siphonophoren." _Zeit. f. wiss. Zool._, Bd. XXIV. 1874.
(164) H. N. Moseley. "On the structure of the Stylasteridæ." _Phil.
Trans._ 1878.
(165) F. E. Schulze. _Ueber den Bau und die Entwicklung von
Cordylophora lacustris._ Leipzig, 1871.
_Actinozoa._
(166) Al. Agassiz. "Arachnitis (Edwarsia) brachiolata." _Proc. Boston
Nat. Hist. Society_, 1860.
(167) Koch. "Das Skelet d. Alcyonarien." _Morpholog. Jahrbuch_, Bd.
IV. 1878.
(168) A. Kowalevsky. "Z. Entwicklung d. Alcyoniden, Sympodium
coralloides und Clavularia crassa." _Zoologischer Anzeiger_, No. 38,
1879.
(169) H. Lacaze Duthiers. _Histoire nat. du Corail._ Paris, 1864.
(170) H. Lacaze Duthiers. "Développement des Coralliaires." _Archives
de Zoologie expérimentale et générale_, Vol. I. 1872 and Vol. II.
1873.
(171) C. Semper. "Ueber Generationswechsel bei Steinkorallen etc."
_Zeit. f. wiss. Zool._, Bd. XXII. 1872.
_Ctenophora._
(172) Alex. Agassiz. "Embryology of the Ctenophoræ." _Mem. of the
Amer. Acad. of Arts and Sciences_, Vol. X. No. III. 1874.
(173) G. J. Allman. "Contributions to our knowledge of the structure
and development of the Beroidæ." _Proc. Roy. Soc. Edinburgh_, Vol. IV.
1862.
(174) C. Chun. "Das Nervensystem u. die Musculatur d. Rippenquallen."
_Abhand. d. Senkenberg. Gesellsch._, B. XI. 1879.
(175) C. Claus. "Bemerkungen u. Ctenophoren u. Medusen." _Zeit. f.
wiss. Zool._, XIV. 1864.
(176) H. Fol. _Ein Beitrag z. Anat. u. Entwickl. einiger
Rippenquallen._ 1869.
(177) C. Gegenbaur. "Studien ü. Organis. u. System d. Ctenophoren."
_Archiv. f. Naturgesch._, XXII. 1856.
(178) A. Kowalevsky. "Entwicklungsgeschichte d. Rippenquallen." _Mém.
Acad. St Pétersbourg_, VII. série, Tom. X. No. 4. 1866.
(179) J. Price. "Embryology of Ciliogrades." _Proceed. of British
Assoc._, 1846.
(180) C. Semper. "Entwicklung d. Eucharis multicornis." _Zeit. f.
wiss. Zool._, Vol. IX. 1858.
CHAPTER VII.
PLATYELMINTHES[89].
[89] I. Turbellaria.
1. Dendrocoela.
2. Rhabdocoela.
II. Nemertea.
1. Anopla.
2. Enopla.
III. Trematoda.
1. Distomeæ.
2. Polystomeæ.
IV. Cestoda.
TURBELLARIA.
Although there is perhaps no group in the animal kingdom the ontogeny
of which would better repay a thorough investigation than the
Turbellarians, yet the difficulties to be overcome have hitherto
proved too great.
The fresh-water Rhabdocoela and Dendrocoela do not undergo any
metamorphosis, and leave the ovum in a condition in which they cannot
easily be distinguished in their general appearance from Infusoria.
Many marine Dendrocoela also develop directly, while, as was first
shewn by Joh. Müller, other marine Dendrocoela undergo a more or
less complicated metamorphosis.
Marine Dendrocoela. Of the marine Dendrocoela which do not undergo a
metamorphosis the form most fully worked out is Leptoplana
tremellaris--(vide Keferstein, No. 187, and Hallez, No. 185).
The ova are surrounded by large albuminous capsules secreted by a
special gland. They are laid a great number at a time, and
adhere together so as to form masses not unlike the spawn of
nudibranchiate Molluscs.
Within the egg-capsule the ovum floats freely and undergoes a
segmentation similar in many respects to the characteristic molluscan
type. The ovum divides into two, and then into four parts, from each
of which a small segment is then separated off. The four small
segments, which appear to give rise to the epiblast, increase in
number by division and gradually envelop the large segments[90]; so
that an epibolic invagination clearly takes place. Between the small
and the large cells is a small segmentation cavity, fig. 86 A and B.
At the time when twelve epiblast cells are present, each of the four
large cells divides into two unequal parts (Hallez), fig. 86 A. In
this way four large (_hy_) and four small cells (_m_) are formed. The
latter are placed at the opposite pole of the ovum to the epiblast
cells, and give rise to the mesoblast, while the four large cells
remain as the hypoblast.
[90] It is probable, though it has not been observed, that the
growth of the layer of small cells is assisted by the formation
of fresh cells from the hypoblast spheres.
[FIG. 86. SECTIONS THROUGH THE OVUM OF LEPTOPLANA TREMELLARIS IN
THREE STAGES OF DEVELOPMENT. (After Hallez.)
_ep._ epiblast; _m._ mesoblast; _hy._ yolk cells (hypoblast); _bl._
blastopore.]
In the course of the enclosure of the hypoblast cells by the epiblast,
the mesoblast cells gradually travel towards the formative pole (fig.
86 B). In the process they become first of all divided so as to form
four linear streaks, and finally unite into a continuous layer between
the epiblast and hypoblast, which obliterates the segmentation cavity
(fig. 86 C, _m_).
Before the completion of the epibole a closely packed layer of fine
cilia appears, which causes a rotation of the embryo within the
egg-capsule. During the above changes a fifth hypoblast cell is formed
by the division of one of those already present; and at a later period
four of the hypoblast cells give rise within the nearly closed
blastoporic area to four small cells. In connection with these cells a
complete hyploblastic wall becomes subsequently established, which
encloses the original large hypoblast cells. The latter then become
resolved into a vitelline mass.
From a comparison with other types it may be regarded as probable that
the enteric wall originates by a process of continuous budding off of
small cells from the large cells, which commences with the formation
of the four cells above mentioned.
The blastopore becomes nearly obliterated, but whether it gives rise
to the mouth, which is formed in the same place, has not been
determined. In front of the mouth a small and very transitory rudiment
of an upper lip makes its appearance. The protrusible pharynx is
stated by Hallez to arise as an hypoblastic bud, while its sheath has
an epiblastic origin. Two pairs of eyes and the supra-oesophageal
ganglia also become early developed.
The peripheral ciliated layer of small cells becomes divided into two
strata, of which the outer remains ciliated and forms the true
epiblast: the inner probably forms the cutis. In it are developed
rod-like bodies, which seem to be homologous with the thread-cells of
the Coelenterata, so that if the views put forward in the previous
chapter as to the similarity of the turbellarian and coelenterate
larvæ are correct, the cutis corresponds with the deeper layer of the
coelenterate epiblast. The mesoblast, like the epiblast, becomes
divided into two strata. The outer one is stated to form the circular
and longitudinal muscles; the inner one to give rise to a muscular
reticulum, the spaces within which constitute the parenchymatous body
cavity.
The later changes are not of great importance. At a period slightly
after the formation of the mouth and ganglia two pairs of stiff hairs
become formed at the sides of the body. The embryo has by this time
grown so as to fill up its capsule, in which however it continues
rapidly to rotate, and also commences to exhibit active contractions.
It next becomes hatched, and passes from a spherical to a flattened
elongated form. The ventral oral opening is at first central, but
soon, by a process of unequal growth, becomes carried towards the
posterior end of the body. The pairs of stiff hairs in the meantime
considerably increase in number. The remains of the yolk cells now
disappear, and the enteric walls become more distinct. The alimentary
canal, which is at first simple in outline like that of a
rhabdocoelous Turbellarian, soon assumes a dendritic form.
The young animal after these changes resembles its parent, except in
the possession of only two pairs of eyes and in the absence of
generative organs.
Of the types with a complete metamorphosis the free larvæ of various
species of Thysanozoon have been observed by Joh. Müller (190) and
Moseley (189), and the complete development of Eurylepta auriculata
has been studied by Hallez.
[FIG. 87. LARVA OF EURYLEPTA AURICULATA IMMEDIATELY AFTER HATCHING.
VIEWED FROM THE SIDE. (After Hallez.)
_m_. mouth.]
The stages within the egg of this latter type agree precisely with
those already described in Leptoplana. After the formation of the
mouth the body elongates, remaining however cylindrical. A fold forms
on the anterior side of the mouth, giving rise to a large upper lip.
Two posterior processes are next formed, and other processes soon
arise, constituting the whole of those found in the free larva. The
embryo next shakes off its egg membranes by a series of vigorous
contractions. When free it has the form represented in the annexed
figure (fig. 87).
It is so similar to Müller's (fig. 88) and Moseley's larvæ that all
three may be dealt with together.
The body is somewhat oval, and slightly pointed behind. At the
anterior end are placed the eyes, two in the youngest larva of Müller,
and twelve in the older larva (fig. 88), and in the middle of the
ventral surface is the mouth. It is surrounded by a strong fold, and
leads into an alimentary canal, which is at first simple, but in the
older larvæ is much branched. A bilobed ganglion connected with two
nerve cords is placed anteriorly. The superficial epithelium is
ciliated, and below it is a layer of cells (cutis) derived from the
primitive epiblast, in which are formed the usual rods (Hallez). The
chief peculiarity of the larva consists in the presence of elongated
processes covered with long cilia, and so connected together by a
ciliated band that the whole together forms, in Müller's larva at any
rate, _a_ _lobed præoral ciliated band_ (fig. 88). This band is not
quite so clear in Hallez' figures. Müller's youngest larva was
provided with eight very long lobes; three were dorsal, viz. a median
anterior, and two lateral placed far back; three ventral, viz. a
median in the front of the mouth forming a large upper lip, and two
processes at the sides of the mouth. The number was completed by two
lateral processes of the body. All the processes except the dorsal
median one are shewn in fig. 88. In Hallez' larva, fig. 87, the six
posterior processes form a rather definite ring, while one flagellum
projects from the front end of the body immediately below the eyes,
and a second flagellum behind. In Moseley's youngest larva six
processes only were present, though subsequently eight became formed
as in Müller's larvæ.
[FIG. 88. MÜLLER'S TURBELLARIAN LARVA (PROBABLY THYSANOZOON). VIEWED
FROM THE VENTRAL SURFACE. (After Müller.)
The ciliated band is represented by the black line.
_m_. mouth; _u.l._ upper lip.]
The metamorphosis consists in the whole animal growing longer and
flatter, and in the arms becoming gradually shorter and shorter till
they finally disappear altogether, and the larva acquires the ordinary
adult form.
The lobed larval form of the Turbellaria has some points of
resemblance to the Pilidium form of nemertine larva described below,
yet its resemblance to this interesting larva is less close than would
appear to be the case with certain turbellarian larval forms recently
described by Götte and Metschnikoff, which are in some respects
intermediate in character between the larva of Leptoplana and those
just described.
The observations of Götte (No. 184) were made on Planaria Neapolitana
and Thysanozoon Diesingi, and those of Metschnikoff (No. 188) on
Stylochopsis ponticus. The larvæ of all these forms undergo more or
less of a metamorphosis, but the accounts of their development are not
easily reconciled.[91] The early stages of Planaria are like those of
Leptoplana, as described by Keferstein. Four large hypoblast
cells become surrounded by small epiblast cells, which commence to be
formed on the dorsal side. The hypoblast cells divide and arrange
themselves in two bilaterally symmetrical rows. A small blastopore is
left by the small cells on the _ventral surface_, which communicates
with an otherwise closed and ciliated cavity which is formed between
the two rows of hypoblast cells. The blastopore would seem to remain
permanently open, and to be placed at the base of a deep pit, lined by
epiblast cells, which constitutes the stomodæum.
[91] The account of Metschnikoff's observations on Stylochopsis
ponticus given in the German abstract is too obscure to be placed
in the text, but the following are the more important points
which can be gleaned from it.
The ovum becomes first divided into eight segments. By further
division along the equatorial zone, a ring of small cells is
formed which becomes the epiblast. The two poles are at this time
formed of large cells. At one pole four small cells appear, which
are compared by Metschnikoff to the pole cells of the Diptera
(vide Chapter on the development of the Insecta). At the opposite
pole a blastopore is formed leading into a small segmentation
cavity. The epiblast also now gradually grows over the large
cells. At the blastopore pole the large cells give rise to the
hypoblast and the small cells at the opposite pole assist in
forming the epiblast. The blastopore disappears, and with it the
segmentation cavity, while the hypoblast, forming a solid mass,
becomes divided into two halves (Cf. Planaria Neapolitana). The
embryo becomes ciliated and begins to rotate; and the eyes, and
somewhat later (?) the nervous ganglion make their appearance.
In the interior a wide cavity develops between the hypoblast
cells, which becomes ciliated and is placed in communication with
the exterior by an invaginated stomadæum which forms the pharynx.
The larva now, as in Planaria Neapolitana, takes on a
Pilidium-like form. Lateral lobes and an anterior lip grow out
from the under surface, and become covered with long cilia, while
at the upper pole a long flagellum makes its appearance.
The embryo now becomes dorsally convex, while the ventral surface
becomes marked with a median furrow and grows out laterally into two
lobes, and anteriorly into a ventrally-directed upper lip. The whole
surface becomes ciliated, and the cilia are especially prominent on
the ventral processes and the summit of the dorsal dome. A bunch of
strong cilia becomes formed in front of the dome, and a less marked
bunch behind. The larva is now stated by Götte closely to resemble a
Pilidium. It soon, however, extends itself, and the two bunches of
cilia become situated at the anterior and posterior extremities of the
body. The ventral processes become inconspicuous prominences of the
side of the body. Götte believes that the larva undergoes no further
metamorphosis.
[FIG. 89. PLANARIAN LARVA (PROBABLY PLANARIA ANGULATA). (From
Agassiz.)]
[FIG. 90. PLANARIAN LARVA (PROBABLY PLAMARIA ANGULATA). (From
Agassiz.)]
A type of Planarian larva (figs. 89 and 90)--possibly Plan. angulata,
observed by Alex. Agassiz (No. 181),--is very different from any other
so far described, and is remarkable for being divided into a series of
segments corresponding in number with the diverticula of the digestive
cavity. In the youngest specimen (fig. 89) the body was nearly
cylindrical, and divided into eleven rings, corresponding with as many
digestive diverticula. Two eye-spots were present. In a later stage
(fig. 90) the body was considerably flattened and had approached more
to the planarian form.
If Agassiz' interesting observations can be trusted we have in this
larva indications of a distinct segmentation, which are of some
morphological importance, especially when taken in connection with the
traces of segmentation found amongst the Nemertines.
A further type, with an incomplete metamorphosis, has been observed by
Girard (183). It is remarkable for having an uniform segmentation, and
for presenting a quiescent stage after passing through a free larval
condition with a large upper lip.
Fresh-water Dendrocoela. The development of the fresh-water
Dendrocoela has been especially investigated by Knappert (No. 186)
and Metschnikoff (No. 188).
The ova are very delicate minute naked cells, which to the number of
4-6 or more become enveloped in a capsule or cocoon together with a
large mass of yolk cells derived from the vitellarium. The yolk cells
exhibit peristaltic movements and send out amoeboid processes. Each
ovum when laid becomes surrounded by an extremely delicate membrane,
which disappears during the course of development. The capsules
consist of a spherical case and a stalk. The latter is first emitted
from the female opening as a thread-like body. Its free end becomes
attached, and then the remainder of the capsule is ejected.
Impregnation takes place before the formation of the capsule. The
segmentation is complete. The ovum first divides into two segments.
One of these next divides, forming three segments. There are
subsequently stages with four, eight, sixteen, and thirty-two
segments.
Metschnikoff's results on the stages subsequent to the segmentation
are not in complete harmony with those of Knappert; but no doubt
represent an advance in our knowledge, and I shall follow them here.
His observations were made on Planaria polychroa.
In the earliest stage observed by him the segmentation was already far
advanced, but no membrane was present round the ovum. At a later stage
the ovum becomes more or less bell-shaped or hemispherical, and
encloses within its concavity a mass of yolk elements. It is now
formed of three concentric layers. An outer layer of flattened
cells--the epiblast, a middle layer of fused cells--the mesoblast, and
an inner solid mass of yolk cells--the hypoblast.
At the upper pole is formed the protrusible pharynx (cf. Knappert),
which is provided with a provisional musculature and a lumen. By its
contractions it takes up the yolk elements which surround the embryo,
and the rapid growth of the embryo no doubt takes place at their
expense. The embryo gradually loses its hemispherical form, and
assumes an elongated and flattened shape. It acquires a coating of
cilia by means of which it rotates. On the fifth day it is hatched.
The alimentary tract long remains solid, even after it has acquired
its branched form. The pharynx becomes withdrawn as soon as the larva
is hatched. It loses its provisional muscles, and subsequently
acquires a permanent musculature. The young after hatching attach
themselves to the body of their parent, on which they feed (?).
Rhabdocoela. The development of some of the Rhabdocoela has
recently been studied by Hallez. The ova are mostly laid in capsules,
one in each capsule. Sometimes the development commences before the
capsules are laid, at other times not till afterwards. In certain
forms (Mesostomum) there are summer eggs with thin capsules which
develop within the parent, while hard capsules, forming what are known
as winter eggs, are laid in the autumn, and the embryo hatched in the
spring.
The ova of the Rhabdocoela like those of the fresh-water
Dendrocoela are enveloped in yolk elements derived from the
vitellarium.
The segmentation probably takes place in the same way as in
Leptoplana. A stage with four equal cells has been observed by Hallez,
and there is subsequently an epibolic gastrula. The embryo becomes
ciliated while still within the capsule and, according to Hallez, the
pharynx arises as a bud of the hypoblast. The proboscis in Prostomum
originates as an epiblastic invagination.
NEMERTEA.
Some Nemertea develop without and some with a metamorphosis.
The most remarkable type of Nemertine development with a metamorphosis
is that in which the ovum develops into a peculiar larval form known
as Pilidium, within which the perfect worm is subsequently evolved.
Closely allied to this type is one in which the sexual worm is
developed within a larval form as in Pilidium, but in which the larva
has no free-swimming stage, and is therefore without the
characteristic appendages of the Pilidium. This is known as the type
of Desor and is confined (?) to the genus Lineus. The Pilidium and the
Desor type may be first considered (vide Barrois, No. 192).
The type of Desor. The segmentation is regular and leads to the
formation of a blastosphere with a large segmentation cavity.
The blastosphere is converted by invagination into a gastrula (fig. 91
A). The blastopore is soon carried relatively forwards by the
elongation backwards of the archenteron, and, according to Barrois,
actually forms the mouth. Owing to the elongation of the archenteric
cavity the embryo assumes a bilateral form (fig. 92 A) in which the
dorsal and ventral surfaces can be distinguished, the mouth
(_m._) being situated on the ventral surface.
[FIG. 91. THREE STAGES IN THE DEVELOPMENT OF LINEUS. (After
Barrois.)
A is a side view in optical section.
B and C are two later stages from the ventral (oral) surface.
_ae._ archenteron; _sc._ segmentation cavity; _hy._ hypoblast; _me._
mesoblast; _ep._ epiblast; _m._ mouth; _st._ stomach; _pr.d._
prostomial disc; _po.d._ metastomial disc; _pr._ proboscis.]
[FIG. 92. THREE STAGES IN THE DEVELOPMENT OF LINEUS.] (After
Barrois.)
A. Side view of an embryo at a very early stage as an opaque object.
B and C. Two late stages, seen as transparent objects from the
ventral surface.
_ae._ archenteron; _m._ mouth; _pr. d._ prostomial disc; _po.d._
metastomial disc; _cs._ lateral pit developing in B as a
diverticulum from the oesophagus; _pr_. proboscis; _ms._ muscular
layer (?); _ls._ larval skin about to be thrown off; _me._
mesoblast; _st._ stomach.]
Immediately after the completion of the gastrula a remarkable series
of phenomena takes place. The embryo when viewed from the ventral
surface assumes a pentagonal form (fig. 91 B), and four invaginations
of the epiblast make their appearance on the ventral surface (fig. 92
A), _two in front of (pr.d.) and two behind (po.d.) the mouth_;
they result in the formation of four thickened discs. These discs soon
become separated from the external skin, which closes in forming an
unbroken layer over them (fig. 91 C). The discs grow rapidly, and
first the prostomial pair and subsequently the metastomial fuse
together, and finally the whole four unite into a continuous ventral
plate; analogous it would seem to the ventral plate of chætopodan and
arthropodan embryos. The plate so formed gradually extends itself so
as to close over the dorsal surface, and to form a complete skin
within the primitive larval skin, which at this period is richly
ciliated, though the embryo is not yet hatched (fig. 91 C). While
these changes are taking place, there are budded off from the
invaginated discs a number of fatty cells, which fill up the space
between the discs and the archenteron, and eventually form the
mesoblastic reticulum. During this stage the rudiment of the proboscis
also makes its appearance as a solid process of epiblast, which grows
backwards from the point of fusion of the two prostomial discs at the
front end of the embryo (fig. 91 C, _pr._). A lumen is excavated in it
at a later period. The lateral organs or cephalic pits arise in a
somewhat unexpected fashion as a pair of diverticula from the
oesophagus (fig. 92 B, _cs._)[92], which soon fuse with the walls of
the body at the junction of the prostomial and metastomial plates
(fig. 92 C, _cs._), although they remain for some time attached to the
oesophagus by a solid cord.
[92] Bütschli for Pilidium regards these pits as formed by
invaginations of the epiblast, but Metschnikoff's statements are
in accordance with those in the text.
During these changes the original larval skin separates itself from
the subjacent layer formed by the discs (fig. 92, B and C), and is
soon thrown off completely, leaving the already ciliated (fig. 92 C)
external layer of the invaginated discs as the external skin of the
young Nemertine. During, and subsequently to, the casting off of the
embryonic skin, important changes take place in the constitution of
the various layers of the body, resulting in the formation of the
vascular system and other mesoblastic organs, the nervous system, and
the permanent alimentary tract. These changes appear to me to stand in
need of further elucidation; and the account below must be received
with a certain amount of caution.
It has been already stated that the two discs give rise to fatty
cells, which occupy the space between the walls of the body and the
archenteron. At the period of the casting off of the embryonic skin
fresh changes take place. The discs become very much thickened, and
then divide into two layers, which become the epidermis and subjacent
muscular layers. The muscular layers arise in two masses, separated by
the two cephalic sacks. The anterior mass is formed as an unpaired
anterior thickening, followed by two lateral thickenings. The
posterior mass is much thinner, in correspondence with the rapid
elongation of the metastomial portion of the embryo.
The cells originally split off from the discs undergo considerable
changes, some of them arrange themselves around the proboscis as a
definite membrane, which becomes _the proboscidean sheath_, some
also form a true splanchnic layer of mesoblast, and the remainder,
which are especially concentrated during early embryonic life in the
anterior parts of the body, form the general interstitial connective
tissue. The cephalic ganglia are stated to become gradually
differentiated in the prostomial mesoblast, and the two cords
connected with them in the metastomial mesoblast.
At the time when the larval skin is cast off the original mouth
becomes closed, and it is not till some time afterwards that a
permanent mouth is formed in the same situation. During the early part
of embryonic life the intestine is lined with columnar cells, but,
before the loss of the larval skin, the walls of the intestine undergo
a peculiar metamorphosis. Their cells either fuse or become
indistinguishable, and their protoplasm appears to become converted
into yolk-spherules, which fill up the whole space within the
walls of the body, and are only prevented from extending forwards by a
membrane of connective tissue. This mass gradually forms itself into a
distinct canal, lined by columnar cells.
Pilidium. In the case of the true Pilidium type, the larva is
hatched very early and leads the usual existence of surface larvæ. A
regular segmentation is followed by an invagination which does not
however cause the complete obliteration of the segmentation cavity
(fig. 93 A, _a.e._).
[FIG. 93. TWO STAGES IN THE DEVELOPMENT OF PILIDIUM. (After
Metschnikoff.)
_ae._ archenteron; _oe._ oesophagus; _st._ stomach; _am._ amnion;
_pr.d._ prostomial disc; _po.d._ metastomial disc; _c.s._
cephalic sack.]
The primitive alimentary tract so formed becomes divided into
oesophageal and gastric regions (fig. 93 B, _oe._ and _st._). Even
while the invagination of the archenteron is proceeding, the larva
becomes ciliated throughout, and assumes a somewhat conical form, the
apex of the cone being opposite the flat ventral surface on which the
mouth is situated (fig. 93, A and B). From the apex a flagellum
projects in many forms, giving the larva a helmet-like appearance. In
other forms a bunch of long cilia takes the place of the flagellum
(fig. 94), and in others again the flagellum is not represented. After
the completion of the invagination a lobe grows out on each side of
the mouth, and less well-developed lobes may appear anteriorly and
posteriorly. Round the edge of the ventral surface a ciliated band
makes its appearance.
Two pairs of invaginations of the skin, just as in the type of Desor,
now make their appearance, one pair in front of and the other behind
the mouth (fig. 93 B, _pr.d._ and _po.d._), and each of them by the
closure of the opening of invagination forms a sack, the outer wall of
which becomes very thin and the inner wall (corresponding with the
whole invagination of the type of Desor) very thick. The inner walls
of the four thickenings, which I may speak of as discs, now fuse
together, each disc first uniting with its fellow, and finally the two
pairs uniting.
[FIG. 94.
A. PILIDIUM WITH AN ADVANCED NEMERTINE WORM.
B. RIPE EMBRYO OF THE NEMERTEA IN THE POSITION IT OCCUPIES IN
PILIDIUM. (Both after Bütschli.)
_oe._ oesophagus; _st._ stomach; _i._ intestine; _pr._ proboscis;
_lp._ lateral pit; _an._ amnion; _n._ nervous system.]
A ventral germinal plate is thus established, which gradually grows
round the intestine of the Pilidium to form the skin of the future
Nemertine. The outer thin layer of each of the discs grows _pari
passu_ with the inner layer, and furnishes an amnion-like covering
for the embryo which is forming within the Pilidium (fig. 94,
_an_).
In connection with the young vermiform Nemertine there is formed on
each side an outgrowth from the oesophagus (fig. 94) which is
eventually placed in communication with the exterior by a
ciliated canal[93]. The proboscis arises as an hollow invagination at
the point where the two anterior discs fuse in front.
[93] This is the view of both Metschnikoff (No. 202) and Leuckart
and Pagenstecher (No. 201), and is further confirmed by Barrois,
but Bütschli (No. 193), though he has not observed the earliest
stages of their outgrowth, believes them to be invaginations of
the Nemertine skin.
When the young Nemertine has become pretty well formed within the
Pilidium it becomes ciliated, begins to move, and eventually frees
itself and leads an independent existence, leaving its amnion in the
Pilidium which continues to live for some time.
The central nervous system (fig. 94) is developed either before or
after the detachment of the young Nemertine, according to Metschnikoff
as a thickening of the epiblast. The young Nemertine is at first
without an anus.
The development of the Nemertine within the Pilidium is clearly
identical with that of the Lineus embryo within the larval skin; the
formation of an amnion in the Pilidium constituting the only important
difference which can be pointed out between the modes of origin of the
young Nemertine in the two types.
So far as is known the forms which develop in a Pilidium, or according
to the type of Desor, all belong to the division of the Nemertines
without stylets in the proboscis, known as the Anopla.
Development without Metamorphosis. The majority of the Nemertea,
including the whole (?) of the Enopla, develop without a
metamorphosis. The observations which have been made on this type are
not very satisfactory, but appear to indicate that the formation of
the hypoblast may take place either by invagination or by delamination.
Invaginate types have been observed by Barrois (No. 192), Dieck (No.
196) and Hubrecht.
Barrois' fullest observations were made on _Amphiporus lactifloreus_
(one of the Enopla), and those of Dieck on _Cephalothrix galatheæ_
(one of the Anopla).
A regular segmentation is followed by a blastosphere stage with a
small segmentation cavity. In Barrois' type the inner ends of the
cells of the blastosphere are stated to fuse into a kind of syncytium.
A small invagination takes place, and the cells which take part in it
separate from the epiblast, and then fuse with the syncytium
within the blastosphere. Dieck finds that in Cephalothrix the
invaginated mass simply vanishes.
Barrois' statements about the fusion of the syncytium derived from the
epiblast cells with the invaginated cells must be regarded as very
doubtful. The formation of the germinal layers takes place, according
to Barrois, by the separation of the internal mass of cells into
mesoblast and hypoblast. The proboscis is formed, according to this
author, from the mesoblastic tissues. Dieck, on the other hand, with
greater probability, states that the proboscis is formed by an
invagination. In Cephalothrix a further point deserves notice, in that
the whole of the primitive epiblast becomes shed. In this fact there
may perhaps be recognised the last trace of a metamorphosis like that
in the type of Desor.
Delaminate types have been studied by Barrois (No. 192) and Hoffman
(No. 198), both of whom give circumstantial accounts of their
development.
Hoffman's account is especially deserving of attention, since his
observations were, to a great extent, made by means of artificial
sections. The following account is taken from him. His observations
were made on _Tetrastemma varicolor_, and Tetrastemma appears to
be the genus in which this type of development has been most
completely made out. After a regular segmentation the embryo forms a
solid mass of cells, the outermost of which soon become distinguished
as a separate epiblastic layer. At the same time the larva leaves the
egg, and the epiblast cells become coated by an uniform covering of
cilia. At the anterior extremity of the body is a bunch of long cilia;
and at the hinder end two stiff bristles are formed, but soon
disappear.
The internal mass of cells is still quite uniform, but as the larva
grows in length the outermost of them arrange themselves as a columnar
layer, constituting the mesoblast. Of the cells internal to the
mesoblast the outer become columnar, and are converted into the walls
of the alimentary tract, while the inner ones undergo fatty
degeneration, and form a kind of food-yolk. In the later development
the characters of the adult are gradually acquired without
metamorphosis, and the larval skin passes directly into that of the
adult. Both mouth and anus are formed nearly simultaneously by a
rupture of the enteric wall from within. The nervous system arises as
a thickening of the epiblast, which Hoffman states he has been able to
see in sections. Hoffman also states that the epithelium of the
proboscis is formed as a diverticulum of the alimentary tract, and
that its sheath is formed by a special mesoblastic growth.
Barrois is less precise than Hoffman, from whom he differs in certain
particulars. Hoffman's statements about the proboscis are important if
accurate, but require further confirmation.
Malacobdella. The early stages in development of the peculiar
ectoparasitic Nemertine _Malacobdella_ have been worked by Hoffman
(No. 199) by means of sections, and there appears to be a close
agreement between the development of Malacobdella and that of
Tetrastemma.
The segmentation is uniform, and there is no trace of a segmentation
cavity. On the third day after impregnation the outermost
cells of the embryo become flattened and ciliated, and distinguished
from the remaining spherical cells of the embryo as the epiblast. With
the appearance of cilia a rotation of the embryo commences. On the
fourth day the embryo becomes oval, and at one of the poles--the
future anal pole--a separation takes place between the epiblast and
the inner cells, giving rise to the body cavity. In it are a number of
loose oval cells, which soon become stellate, and form a mesoblastic
reticulum connecting the body wall and central cells of the embryo,
which may now be spoken of as hypoblast. The body cavity increases in
size, leaving at last the hypoblast and epiblast united only at one
point--the oral pole--at which, on the fifth day, a crown of long
cilia appears. The solid mass of hypoblast in the interior becomes
differentiated into an outer layer of cells--the true glandular
epithelium of the alimentary tract--and an inner core, the cells of
which soon undergo fatty degeneration, and serve as food-yolk.
The later stages of development, and the formation of the proboscis,
etc., have not been worked out.
General considerations. Of the types of larvæ hitherto found
amongst the Nemertea, those with a metamorphosis, viz. the Pilidium
type and that of Desor, are to be regarded as the primitive. But even
in Pilidium there are evidences of a great abbreviation in
development. Pilidium itself is probably a more or less modified
ancestral form, while the peculiar development of the Nemertine within
it is to be explained as a very much shortened record of a long series
of changes by which the Pilidium became gradually converted into a
Nemertine. The formation of the body wall of the Nemertine by four
epiblastic invaginations is a remarkable embryological phenomenon, for
which it is not easy to assign a satisfactory meaning; and it is
probable that it is merely a secondary process of growth similar to
the formation of imaginal discs in the larvæ of Diptera (vide Chapter
on Tracheata), which has had its origin in the abbreviation of the
development just alluded to. The development of the type of Desor is
clearly a simplification of the Pilidium type, and its peculiarities
are to be explained by the fact that the first larval form has no free
existence. The types without metamorphosis have no doubt a development
of a still more simplified character; they are remarkable however in
presenting us, if the existing descriptions are to be trusted, with
examples of delamination and invagination coexisting in closely allied
forms.
TREMATODA.
The eggs of the Trematoda consist of a germ or true ovum enclosed in a
mass of yolk cells, which undergo disintegration and subsequent
absorption at varying periods of the development. From the
observations of E. van Beneden (No. 218), Zeller (No. 217), etc. it is
known that the segmentation is usually complete, but generally
somewhat irregular.
Unfortunately we are still completely in the dark as to the mode of
formation of the germinal layers. The embryos of the entoparasitic
forms or Distomeæ become free in a very imperfect condition, and the
ova are small while in the Polystomeæ the development is as a rule
nearly completed before hatching, and the ova are large. It will be
convenient to treat separately the development of the two groups.
Distomeæ. The embryos of the Distomeæ are hatched either in
some moist place or more usually in water. In the majority of genera
the larvæ pass through a complicated metamorphosis, accompanied by
alternations of generations. But for some genera, _e.g._
Holostomum, etc., the life history has not yet been made out. The
whole life history of comparatively few forms has been followed, but
sufficient fragments are known to justify us in making certain general
statements, which no doubt hold true for a large proportion of the
Distomeæ.
The larvæ are usually ciliated (fig. 95 A), but sometimes naked.
The ciliated forms are generally completely covered with cilia, but in
_Distomum lanceolatum_ the cilia are confined to an area at the front
end of the body, in the centre of which a median spine is placed. An x
shaped pigment spot, sometimes provided with a rudimentary lens
(_Monostomum mutabile_), is also generally situated on the dorsal
surface.
In some instances a more or less completely developed alimentary tract
is present (_Monostomum capitellum_, _Amphistomum subclavatum_), but
usually there can only be distinguished in the interior of the larva a
transparent mass of cells bounded by a more or less distinctly marked
body wall with ciliated excretory channels.
Ed. van Beneden has shewn that the ciliated covering is developed
while the embryo is still in the egg, and long before the yolk cells
are completely absorbed. It would seem that even before hatching this
ciliated covering is to a great extent independent of the mass within.
In the larva of Monostomum mutabile (fig. 95 A), which offers an
example of an extreme case of the kind, there is present within the
ciliated epidermis a fully developed independent worm.
The non-ciliated larvæ are less highly organized than the ciliated
forms, and are covered by a cuticle: their anterior extremity is
sometimes provided with a circular plate armed with radiate ridges and
spines.
The free-swimming or creeping embryos make their way into or on to the
body of some invertebrate (occasionally vertebrate) form, usually a
Mollusc, to undergo the first stage in their metamorphosis. They may
either do this on the gills of their host, or very frequently they
bore their way into the interior of the body. Soon after the larvæ
have reached a satisfactory position the epidermis becomes stripped
off, and there emerges a second larval form developed in the interior
of the first larva, much as a Nemertine is developed within the larva
of Desor. In the case of Monostomum mutabile the new worm is, as
stated above, fully formed within the ciliated larva at the time of
hatching.
The worm which proceeds from the above metamorphosis has different
characters corresponding with those of the larva from which it
proceeded. If the original larva had an alimentary canal it has one
also, and then grows into the form known as a Redia (Fig. 95, B and
C).
The Redia has anteriorly a mouth leading into a muscular pharynx and
thence into a cæcal stomach. Posteriorly the body is prolonged into a
kind of blunt caudal process, at the commencement of which are a pair
of lateral papillæ. There is a perivisceral cavity, and the body walls
are traversed by excretory tubes.
If the original larva is without an alimentary tract, the second form
becomes what is known as a Sporocyst. The Sporocyst is a simple
elongated sack with a central body cavity; when derived from the
metamorphosis of a ciliated embryo its walls are provided with
excretory tubes, but such tubes are absent in Sporocysts developed
from non-ciliated larvæ. Some Sporocysts send out numerous branches
amongst the viscera of their hosts.
[FIG. 95. VARIOUS STAGES IN THE METAMORPHOSIS OF THE DISTOMEÆ (from
Huxley.)
_A._ Ciliated larva of Monostomum mutabile. _a._ larval skin. _b._
Redia developed within it. _B._ Redia of Monostomum mutabile. _C._
Redia of Distomum pacificum, with germs of a second brood of Rediæ.
_D._ Redia containing Cercariæ. _E._ Cercaria. _F._ Full-grown
Distomum.]
The Rediæ and Sporocysts rapidly grow in size and sometimes increase
by transverse division. In the course of their further development one
of two things may happen. They may either (1) develop fresh Rediæ or
Sporocysts by a process of internal budding (fig. 95 C); or else (2)
there may be formed in them, by an analogous process, larvæ with long
tails known as Cercariæ (fig. 95 D.) The direct development of
Cercariæ is the usual course, though in _Distomum globiparum_ the
reverse is true; but where this does not take place the Rediæ or
Sporocysts of the second generation give rise to Cercariæ.
The Cercariæ are developed from spherical masses of cells found in the
body cavity of the Sporocyst or Redia. The exact origin of these
masses is still somewhat obscure, but they are stated by Wagener (No.
212) to be derived from the body wall. They are probably to be
regarded as internal buds.
The spherical bodies grow rapidly in size, their posterior extremity
is prolonged into a process which forms the tail, while the anterior
part forms the trunk. When fully formed (fig. 95 E), the trunk has
very much the organization of an adult Distomum. There is an anterior
and a ventral sucker, the former of which contains the opening of the
mouth, and is often provided with a special chitinous armature. The
mouth leads into a muscular pharynx, and this into a bilobed cæcal
alimentary tract. An excretory system of the ordinary type is present,
consisting of longitudinal contractile trunks continuous anteriorly
with branched ciliated canals, which, as has recently been shewn by
Bütschli, may be provided with funnel-shaped ciliated internal
openings[94]. The contractile trunks unite posteriorly, but instead of
opening directly to the exterior are prolonged into a vessel which
traverses the substance of the tail, and after a longer or shorter
course bifurcates into two branches which open laterally.
[94] O. Bütschli, "Bemerkungen üb. d. excretorischen
Gefässapparat d. Trematoden." _Zoologischer Anzeiger_, 1879,
No. 42.
The tail is provided with an axial rod of hyaline connective tissue,
like the notochord of the tail of a larval Ascidian, and is frequently
provided with membranous expansions. It is used as a swimming organ.
Beneath the epidermis are layers of circular and longitudinal muscular
fibres, the latter arranged in the tail as two bands.
The Cercariæ when fully developed leave the Sporocyst or Redia, and
then their host, and become free. In most Rediæ there is a special
opening, not far from the mouth, by which they pass out. There is no
such opening in Sporocysts, but the Cercariæ bore their way through
the walls.
After leaving their parent the Cercariæ pass into the external medium,
and for a short period have a free existence. They soon however enter
a new host, making their way into its body by a process of boring,
which is effected by the head (especially when armed with chitinous
processes) assisted by movements of the tail.
The second host is usually some Invertebrate (Mollusc, Worm,
Crustacean, Insect larva, &c.), but occasionally a Fish or Amphibian
or even a vegetable. The tail is very often lost as the Cercaria bores
its way into its host, but whether it is so or not, the Cercaria,
after it has once reached a suitable post in its new host, assumes a
quiescent condition, and surrounds itself with a many-layered capsule.
The cephalic armature and tail (if still present) are then exuviated,
and the generative organs gradually become apparent though very small.
In other respects the organization is not much altered.
Though an encysted Cercaria may remain some months without further
change, it eventually dies unless it be introduced into its permanent
vertebrate host, an act which is usually effected by the host in which
it is encysted being devoured. It then becomes freed from its capsule
as a fully formed Trematode, in which the generative organs rapidly
complete their development.
In some cases the Rediæ or Sporocysts do not give rise to tailed
Cercariæ, but to tailless forms. In such cases, as a rule, the
encystment takes place in the host of the Redia or Sporocyst, but the
tailless larvæ sometimes pass through a free stage like the Cercariæ.
In the case of _Distomum cygnoides_, parasitic in the bladder of the
Frog, the Cercaria passes directly into the adult host without the
intervention of an intermediate host.
The life history of a typical entoparasitic Trematode is shortly as
follows:
(1) It leaves the egg as a ciliated or non-ciliated free larva.
(2) This larva makes its way on to the gills or into the body of some
Mollusc or other host, throws off its epidermis and becomes a Redia or
Sporocyst.
(3) In the body cavity of the Redia or Sporocyst numerous tailed
larvæ, known as Cercariæ, are developed by a process of internal
gemmation.
(4) The Cercariæ pass out of the body of their parent, and out of
their host, and become for a short time free. They then pass into a
second, usually invertebrate host, and encyst.
(5) If their second host is swallowed by the vertebrate host of the
adult of the species, the encysted forms become free, and attain to
sexual maturity.
The majority of these stages are simply parts of a complicated
metamorphosis, but in the coexistence of larval budding (giving rise
to Cercariæ or fresh Rediæ) with true sexual reproduction there is in
addition a true alternation of generations.
Polystomeæ. The ova of the Polystomeæ are usually large and not
very numerous, and they are in most cases provided with some process
for attachment. Some species of Polystomeæ, _e.g._ Gyrodactylus,
are however viviparous. The young leave the egg in a nearly perfect
state, and at the utmost undergo a slight metamorphosis and no
alternations of generations. Some however (Polystomum, Diplozoon) are
provided with temporary cilia, but the number investigated is too
small to determine whether ciliation is the rule or the exception. The
ciliated larvæ have a short free existence. The cilia are developed on
special cells which may be arranged in transverse bands in the same
way as in the larvæ of many Chætopods, but are not, in the larvæ at
present known, distributed uniformly. When the free larvæ become
parasitic the cells with cilia shrink up.
In _Polystomum integerrimum_, which lives in the urinary bladder
of _Rana temporaria_, the eggs when laid in the spring pass out
into the water. The segmentation is complete, and the embryo when
hatched is provided with most of the adult organs, but presents
certain striking larval characters. It has five rings of ciliated
cells. Three of these are placed anteriorly, and are especially
developed on the ventral surface, the posterior one being incomplete
dorsally; two are placed posteriorly, and are especially developed on
the dorsal surface. Anteriorly there is a tuft of cilia.
The larva itself resembles somewhat an adult Gyrodactylus, and is
provided (1) with a large posterior disc armed with hooks, and (2)
with two pairs of eyes which persist in the adult state. After a
certain period of free existence the larva attaches itself to the
gills of a tadpole. The rings of ciliated cells shrink up, and some of
the six pairs of suckers found in the adult commence to be formed on
the posterior disc. When the bladder of the tadpole is developed, the
young Polystomum passes down the alimentary tract to the cloaca, and
thence to the urinary bladder, where it slowly attains to sexual
maturity. When the larva becomes attached to the gills of a very young
tadpole, its development is somewhat more rapid in consequence of
better nutrition from the more delicate gills. It then reaches its
full development in the gill cavity, and, though smaller and provided
with differently organised generative organs to the normal form,
produces generative products and dies without being transported to the
bladder (vide Zeller, Nos. 216 and 217).
The ova of Diplozoon, a form parasitic on the gills of freshwater fish
(Phoxinus, etc.), are provided with a long spiral filament (Zeller,
No. 215). The embryo has five ciliated areas, four lateral and one
posterior. The young form is known as Diporpa. Sexual maturity is not
attained till two individuals unite permanently together. They unite
by the ventral sucker of each of them becoming attached to the dorsal
papilla of the other. Subsequently these parts coalesce, and the
ventral suckers disappear in the process. Gyrodactylus, parasitic,
like Diplozoon, on the gills of freshwater fishes (Gasterosteus,
etc.), is remarkable for its mode of reproduction. It is viviparous,
producing a single young one at a time, and, what is still more
remarkable, the young while still within its parent produces a young
one, and this again a young one, so that three generations may be
present within the parent. It seems probable that the second and third
generations are produced asexually, the generative organs not being
developed; while the young Gyrodactylus of the first generation
springs from a fertilized ovum (Wagener, No. 214).
CESTODA.
On anatomical grounds the affinity of the Cestoda to the Trematoda has
been insisted on by the majority of anatomists. The existence of such
intermediate forms as Amphilina tends to strengthen this view;
and the striking resemblances between the two groups in the structure
of the egg and characters of the metamorphosis appear to me to remove
all doubt about the matter.
The ripe egg is formed of a minute germ enveloped in yolk cells, the
whole being surrounded by a membrane, which is very delicate in most
forms, but in certain types has a firmer consistency, and is provided
with an aperture, covered by an operculum, by which the larva escapes.
The early development, up to the formation of a six-hooked larva,
generally takes place in the uterus, but in the types with a firmer
egg-shell it takes place after the egg has been deposited in water.
The segmentation (E. van Beneden, No. 218, Metschnikoff, No. 228) is
complete, and during its occurrence the yolk cells surrounding the
germ are gradually absorbed, so that the mass of segmentation spheres
grows in size, till at the close of segmentation it fills up nearly
the whole egg-shell.
As was first shewn by Kölliker for Bothriocephalus salmonis, the
embryonic cells separate themselves at the close of segmentation into
a superficial layer and a central mass.
The further development takes place on two types. In the cases where
the egg-shell is strong, and the egg is laid prior to the formation of
the embryo, a ciliated larva is developed (Bothriocephalus latus,
ditremus, Schistocephalus dimorphus, Ligula simplicissima, etc.[95]).
[95] Vide for list of such forms at present known Willemoes Suhm,
No. 231.
Of these forms Bothriocephalus latus may be taken as type.
The development of the embryo requires many months for its completion.
The outer layer becomes ciliated while the central mass has already
become developed into a six-hooked embryo. The embryo leaves its shell
by the opercular aperture, and for some time swims rapidly about by
means of its long cilia. The ciliated coating is eventually stripped
off, and the six-hooked larva emerges.
In the second type of embryo the external cellular layer does not
become ciliated. This is the most usual arrangement, and is even found
in many species of Bothriocephalus.
The central mass of cells becomes developed, as in the other type,
into a six-hooked (rarely four-hooked) embryo (fig. 96 G), but the
superficial layer separates from the central, and either disappears or
becomes (_Bothriocephalus proboscideus_) a cuticular layer. Between
the six-hooked embryo and the outer layer of cells one or more thick
membranes become deposited (E. van Beneden). The eggs are carried out
of the alimentary canal in the proglottis and transported to various
situations on land or in water. They usually remain within the
proglottis, invested by their thick shell, till taken up into the
alimentary canal of a suitable host, or they may be swallowed after
the death and decay of the proglottis. They are subsequently hatched
after their shell has become softened by the action of the digestive
fluids.
[FIG. 96. DIAGRAMS OF VARIOUS STAGES IN THE DEVELOPMENT OF THE
CESTODA. (From Huxley.)
A. Cysticercus. B. and C. Cysticerci in the everted (B) and inverted
(C) condition. D. Coenurus. E. and F. Diagrams of Echinococcus. It
is most probable that Tænia heads are not developed directly from
the wall of the cyst as represented in the diagram. G. Six-hooked
embryo.]
Before proceeding to describe their further history, the close
resemblance between the first developmental stages of Cestoda,
especially in the case of the ciliated larvæ, and those of Trematoda,
may be pointed out.
In both there is a ciliated larva, and in both there is developed
within the ciliated skin a second larva, which becomes freed by the
stripping off of the ciliated skin.
The type of development has moreover many analogies with that of the
Nemertine larva of Desor, p. 163 (cf. Metschnikoff), and is probably
like that an abbreviated record of a long history.
The suitable host for the six-hooked embryo to enter is rarely
the same as the host for the sexual form. The embryos having become
transported into the alimentary canal of such a host, and become free,
if previously invested by the egg-shell, soon make their way,
apparently by the help of their hooks, through the wall of the
alimentary tract, and are transported in the blood or otherwise into
some suitable place for them to undergo their next transformation.
This place may be the liver, lungs, muscles, connective tissue, or
even the brain (_e.g._ _Coenurus cerebralis_ in the brain
of sheep).
Here they become enclosed in a granular deposit from the surrounding
tissues, which becomes in its turn enclosed in a connective-tissue
coat. Within lies the solid embryo, the hooks of which in many cases
disappear or become impossible to make out. In other forms,
_e.g._ _Cysticercus limacis_, they remain visible, and then
mark the anterior pole of the worm (fig. 98, _c._). The central
part of the body next becomes transformed into a material composed of
clear non-nucleated vesicles. Accompanying these changes the embryo
grows rapidly in size; a cuticle is deposited by its outer layer, in
which also an external layer of circular muscular fibres and an
internal layer of longitudinal fibres become differentiated, and
internal to both there is formed a layer of granular cells.
With the rapid growth of the body a central cavity is formed, which
becomes filled with fluid, and the embryo assumes the form of a
vesicle. At the same time a system of excretory vessels, sometimes
opening by a posterior pore, becomes visible in the wall of the
vesicle.
The embryo has now reached a condition in which it is known as a
cystic- or bladder-worm, and may be compared in almost every respect
with the sporocyst of a Trematode (Huxley).
[FIG. 97. CYSTICERCUS CELLULOSÆ. (From Gegenbaur, after von
Siebold.)
_a._ Caudal vesicle. _c._ Anterior part of body. _d._ head.]
[FIG. 98. CYSTICERCUS WITH SMALL CAUDAL VESICLE.
A. Head involuted. B. Head everted.
_a._ Scolex. _b._ caudal vesicle. _c._ (in A) six embryonic hooks.]
The next important change consists in the development of a head, which
becomes the head of the adult Tænia. This is formed in an involution
of the outer wall of the anterior extremity of the cystic worm. This
involution forms a papilliform projection on the inner surface of the
wall of the cystic worm, with an axial cavity opening by a pore on the
outer surface. The layer of cells forming the papilla soon becomes
divided into two laminæ, of which the outer forms a kind of investing
membrane for the papilla. The papilla itself now becomes moulded into
a Cestode head, which however is developed in an inverted position.
The suckers and hooks (when present) of the head are developed on a
surface bounding the axial lumen of the papilla, which is the true
morphological outer surface, while the apparent outer surface of the
papilla is that which eventually forms the interior of the (at first)
hollow head. Before the external armature of the head has become
established, four longitudinal excretory vessels, continuous with
those in the body of the cystic worm, make their appearance. They are
united by a circular vessel at the apex of the head. The development
is by no means completed with the simple growth of the head, but the
whole inverted papilla continues to grow in length, and gives rise to
what afterwards becomes part of the trunk. The whole papilla
eventually becomes everted, and then the cystic worm takes the form
(fig. 97) of a head and unsegmented trunk with a vesicle--the body of
the cystic worm--attached behind. The whole larva is known as a
Cysticercus. The term scolex, which is also sometimes employed, may be
conveniently retained for the head and trunk only. The head differs
mainly from that of the adult in being hollow.
There are great variations in the relative size of the head and the
vesicle of Cysticerci. In some forms the vesicle is very small (fig.
98), _e.g._ _Cysticercus limacis_; it is medium-sized in _Cysticercus
cellulosæ_ (fig. 97), and in some forms is much larger. The embryonic
hooks, when they persist, are found at the junction of the trunk and
the vesicle (fig. 98 A, _c_). Though the majority of cystic worms only
develop one head, this is not invariably the case. There is a cystic
worm found in the brain of the sheep known as _Coenurus
cerebralis_--the larva of _Tænia coenurus_, parasitic in the intestine
of the dog--which forms an exception to this rule. There appears, to
start with, a tuft of three or four heads, and finally many hundred
heads are developed (fig. 96 D). They are arranged in groups at one
(the anterior?) pole of the cystic worm.
A still more complicated form of cystic worm is that known as
Echinococcus, parasitic in the liver, lungs, etc. of man and various
domestic Ungulata. In the adult state it is known as _Tænia
echinococcus_ and infests the intestine of the dog. The cystic worm
developed from the six-hooked embryo has usually a spherical form, and
is invested in a very thick cuticle (fig. 96 E and F, and fig. 99). It
does not itself directly give rise to Tænia heads, but after it
reaches a certain size there are formed on the inner side of its walls
small protuberances, which soon grow out into vesicles connected with
the walls of the cyst by narrow stalks (figs. 96 F and 99 C). In the
interior of these vesicles a cuticle is developed. It is in these
secondary vesicles that the heads originate. According to Leuckart,
they either arise as outgrowths of the wall of the vesicle on the
inner face of which the armature is developed, which subsequently
become involuted and remain attached to the wall of the vesicle by a
narrow stalk, or they arise from the first as papilliform projections
into the lumen of the vesicle, on the outer side of which the armature
is formed. Recent observers only admit the second of these modes of
development. The Echinococcus larva, in addition to giving rise to the
above head-producing vesicles, also gives rise by budding to fresh
cysts, which resemble in all respects the parent cyst. These cysts may
either be detached in the interior (fig. 96 F) of the parent or
externally. They appear to spring in most cases from the walls of the
parent cyst, but there are some discrepancies between the various
accounts of the process. In the cysts of the second generation
vesicles are produced in which new heads are formed. As the primitive
cyst grows, it naturally becomes more and more complicated, and the
number of heads to which one larva may give rise becomes in this way
almost unlimited.
Cysticerci may remain a long time without further development, and
human beings have been known to be infested with an Echinococcus cyst
for over thirty years. When however the Cysticercus with its head is
fully developed, it is in a condition to be carried into its final
host. This takes place by the part of one animal infested with
cysticerci becoming eaten by the host in question. In the alimentary
canal of the final host the connective-tissue capsule is digested, and
then the vesicular caudal appendage undergoes the same fate, while the
head, with its suckers and hooks, attaches itself to the walls of the
intestine. The head and rudimentary trunk, which have been up to this
time hollow, now become solid by the deposition of an axial tissue;
and the trunk very soon becomes divided into segments, known as
proglottides (fig. 99 A). These segments are not formed in
the same succession as those of Chætopods; the youngest of
them is that nearest to the head, and the oldest that furthest removed
from it. Each segment appears in fact to be a sexual individual, and
is capable of becoming detached and leading for some time an
independent existence. In some cases, _e.g._ _Cysticercus
fasciolaris_, the segmentation of the trunk may take place while
the larva is still in its intermediate host.
[FIG. 99. ECHINOCOCCUS VETERINORUM. (From Huxley.)
A. Tænia head or scolex. _a._ hooks. _b._ suckers. _c._ cilia in
water vessel. _d._ refracting particles in body wall.
B. single hooks.
C. portion of cyst. _a._ cuticle. _b._ membranous wall of primary
cyst. _c._ and _e._ scolex heads. _d._ secondary cyst.]
The stages in the evolution of the Cestoda are shortly as follows:
1. Stage with embryonic epidermis either ciliated (Bothriocephalus,
etc.) or still enclosed in the egg-shell. This stage corresponds to
the ciliated larval stage of the Trematoda.
2. Six-hooked embryonic stage after the embryonic epidermis has been
thrown off. During this stage the embryo is transported into the
alimentary tract of its intermediate host, and boring its way into the
tissues, becomes encapsuled.
3. It develops during the encapsuled state into a cystic worm,
equivalent to the sporocyst of Trematoda.
4. The cystic worm while still encapsuled develops a head with suckers
and hooks, becoming a Cysticercus. In some forms (_Coenurus_,
_Echinococcus_) reproduction by budding takes place at this
stage. The head and trunk are known as the scolex.
5. The Cysticercus is transported into the second and permanent host
by the infested tissue being eaten. The bladder-like remains of the
cystic worm are then digested, and by a process of successive budding
a chain of sexual proglottides are formed from the head, which remains
asexual.
[FIG. 99 A. TETRARHYNCUS. (From Gegenbaur; after Van Beneden.)
A. Asexual state.
B. Sexual stage with ripe proglottides.]
The above development is to be regarded as a case of complicated
metamorphosis secondarily produced by the necessities of a parasitic
condition, to which an alternation of sexual and gemmiparous
generations has been added. The alternation of generations only occurs
at the last stage of the development, when the so-called head, without
generative organs, produces by budding a chain of sexual forms, the
embryos of which, after passing through a complicated metamorphosis,
again become Cestode heads.
In the case of Coenurus and Echinococcus two or more asexual
generations are interpolated between the sexual ones. It is not quite
clear whether the production of the Tænia head from the cystic worm
may not be regarded as a case of budding. There are some grounds for
comparing the scolex to the Cercaria of Trematodes, cf. Archigetes.
As might be anticipated from the character of the Cestode
metamorphosis, the two hosts required for the development are usually
forms so related that the final host feeds upon the intermediate host.
As familiar examples of this may be cited the pig, the muscles of
which may be infested by _Cysticercus cellulosæ_, which becomes the
_Tænia solium_ of man. Similarly a Cysticercus infesting the muscles
of the ox becomes the _Tænia mediocanellata_ of man. The _Cysticercus
pisciformis_ of the rabbit becomes the _Tænia serrata_ of the dog. The
_Coenurus cerebralis_ of the sheep's brain becomes the _Tænia
coenurus_ of the dog. The Echinococcus of man and the domestic
herbivores becomes the _Tænia echinococcus_ of the dog.
Cystic worms infest not only Mammalian forms, but lower Vertebrates,
various fishes which form the food of other fishes, and Invertebrates
liable to be preyed on by vertebrate hosts. So far the Cestodes
(except Archigetes) are only known to attain sexual maturity in the
alimentary tracts of Vertebrata.
The rule that the intermediate host is not the same as the final host
does not appear to be without exception. Redon[96] has shewn by
experiments on himself that a _Cysticercus_ (_cellulosæ_) taken from a
human subject develops into _Tænia solium_ in the intestines of a man.
Redon took four cysts of a Cysticercus from a human subject, and after
three months passed some proglottides, and subsequently the head of
_Tænia solium_.
[96] _Annal. d. Scien. Nat._, 6th Series, Vol. VI. 1877.
Some important variations of the typical development are known.
The so-called head or scolex may be formed without the intervention of
a cystic stage. In Archigetes (Leuckart, No. 227), which infests, in
the Cysticercus condition, the body cavity of various invertebrate
forms (Tubifex, etc.), the six-hooked embryo becomes elongated and
divided into two sections, one forming the head, while the other, with
the six embryonic hooks, forms an appendage, homologous with the
caudal vesicle of other Cysticerci.
The embryo of _Tænia elliptica_ similarly gives rise to a Cysticercus
infesting the dog-louse (_Trichodectes canis_), without passing
through a vesicular condition; but the caudal vesicle disappears, so
that it forms simply a scolex. These cases may, it appears to me, be
probably regarded as more primitive than the ordinary ones, where the
cystic condition has become exaggerated as an effect of a parasitic
life.
In some cases the larva of a Tænia has a free existence in the scolex
condition. Such a form, the larva of Phyllobothrium, has been observed
by Claparède[97]. It was not ciliated, and was without a caudal
vesicle; and was no doubt actively migrating from an intermediate host
to its permanent host.
[97] _Beobachtungen üb. Anat. u. Entwick. Wirbell. Thiere._
Leipzig, 1863.
Scolex forms, without a caudal vesicle, are found in the mantle cavity
of Cephalopoda, and appear to be occupying an intermediate host in
their passage from the host of the cystic worm to that of the sexual
form.
Archigetes, already mentioned, has been shewn by Leuckart (No. 227) to
become sexually mature in the Cysticercus state, and thus affords an
interesting example of pædogenesis. It is not known for certain
whether under normal circumstances it reaches the mature state in
another host.
_Amphilina._ The early stages of this interesting form have been
investigated by Salensky (No. 229), and exhibit clear affinities to
those of the true Cestoda. An embryonic provisional skin is formed as
in Cestodes; and pole cells also appear. Within the provisional skin
is formed an embryo with ten hooks. After hatching the provisional
skin is at once thrown off, and the larva, which is then covered by a
layer of very fine cilia, becomes free. The further metamorphosis is
not known.
BIBLIOGRAPHY.
_Turbellaria._
(181) Alex. Agassiz. "On the young stages of a few Annelids"
(_Planaria angulata_). _Annals Lyceum Nat. Hist. of New York_, Vol.
VIII. 1866.
(182) Dalyell. "Powers of the Creator."
(183) C. Girard. "Embryonic development of Planocera elliptica."
_Jour. of Acad. of Nat. Sci._ Philadelphia. New Series, Vol. II. 1854.
(184) Alex. Götte. "Zur Entwicklungsgeschichte d. Seeplanarien."
_Zoologischer Anzeiger_, No. 4, 1878.
(185) P. Hallez. _Contributions à l'histoire naturelle des
Turbellariés. Thésis à la faculté des Sciences p. le grade d. Docteur
ès-sciences naturelles_, Lille, 1879.
(186) Knappert. "Bijdragen tot de Ontwikkelings-Geschiedenis der
Zoetwater-Planarien." _Provinciaal Utrechtsch Genootschap van Kunsten
en Wetenschappen_. Utrecht, 1865.
(187) W. Keferstein. "Beiträge z. Anat. u. Entwick. ein. Seeplanarien
von St. Malo." _Abh. d. könig. Gesell. d. Wiss. zu Göttingen._ Bd.
XIV. 1868.
(188) El. Metschnikoff. "Untersuchungen üb. d. Entwicklung d.
Planarien." _Notizen d. neurussischen Gesellschaft d. Naturforscher._
Odessa, Bd. V. 1877. Vide Hoffman and Schwalbe's _Bericht_ for 1878.
(189) H. N. Moseley. "On Stylochus pelagicus and a new species of
pelagic Planarian, with notes on other pelagic species, on the larval
forms of Thysanozoon, etc." _Quart. Journ. of Micr. Science._ Vol.
XVII. 1877.
(190) J. Müller. "Ueber eine eigenthümliche Wurmlarva a. d. Classe d.
Turbellarien, etc." Müller's _Archiv f. Anat. u. Phys._ 1850.
(191) ---- "Ueber verschiedene Formen von Seethieren." Müller's
_Archiv f. Anat. und Phys._ 1854.
_Nemertea._
(192) J. Barrois. "L'Embryologie des Némertes." _An. Sci. Nat._ Vol.
VI. 1877.
(193) O. Bütschli. _Archiv f. Naturgeschichte_, 1873.
(194) A. Krohn. "Ueb. Pilidium u. Actinotrocha." Müller's _Archiv_,
1858.
(195) E. Desor. "Embryology of Nemertes." _Proceedings of the Boston
Nat. History Society_, Vol. VI. 1848.
(196) G. Dieck. "Entwicklungsgeschichte d. Nemertinen." _Jenaische
Zeitschrift_, Vol. VIII. 1874.
(197) C. Gegenbaur. "Bemerkungen üb. Pilidium gyrans, etc."
_Zeitschrift für wiss. Zool._, Bd. V. 1854.
(198) C. K. Hoffmann. "Entwicklungsgeschichte von Tetrastemma
tricolor." _Niederländisches Archiv_, Vol. III. 1876, 1877.
(199) ---- "Zur Anatomie und Ontogenie von Malacobdella."
_Niederländisches Archiv_, Vol. IV. 1877.
(200) W. C. Mc Intosh. British Annelids. _The Nemerteans._ Ray
Society, 1873-4.
(201) Leuckart u. Pagenstecher. "Untersuchungen üb. niedere
Seethiere." Müller's _Archiv_, 1858.
(202) E. Metschnikoff. "Studien üb. die Entwicklung d. Echinodermen u.
Nemertinen." _Mém. Acad. imp. Pétersbourg_, VII. Ser., Tom. XIV. No. 8,
1869.
_Trematoda._
(203) T. S. Cobbold. _Entozoa._ Groombridge and Son, 1864.
(204) ---- _Parasites; a Treatise on the Entozoa_, etc. Churchill,
1879.
(205) Filippi. _Mém. p. servir à l'histoire génétique des Trématodes.
Ann. Scien. Nat._ 4th Series, Vol. II. 1854, and _Mem. Acad. Torino_,
1855-1859.
(206) R. Leuckart. _Die menschlichen Parasiten_, Vol. I. 1863, p. 485,
et seq.
(207) H. A. Pagenstecher. _Trematoden u. Trematodenlarven._
Heidelberg, 1857.
(208) C. Th. von Siebold. _Lehrbuch d. vergleich. Anat. wirbelloser
Thiere._ Berlin, 1848.
(209) J. J. S. Steenstrup. _Generationswechsel._ 1842.
(210) R. v. Willemoes-Suhm. "Zur Naturgeschichte d. Polystomum
integerrimum, etc." _Zeit. f. wiss. Zool._ Vol. XXII. 1872.
(211) ---- "Helminthologische Notizen III." _Zeit. f. wiss. Zool._
Vol. XXIII. 1873. Vide this paper for a summary of known observations
and literature.
(212) G. R. Wagener. _Beiträge zur Entwicklungsgeschichte d.
Eingeweidewürmer._ Haarlem, 1855.
(213) G. R. Wagener. "Helminthologische Bemerkungen, etc." _Zeit. f.
wiss. Zool._ Vol. IX. 1850.
(214) G. R. Wagener. "Ueb. Gyrodactylus elegans." _Archiv f. Anat. u.
Phys._ 1860.
(215) E. Zeller. "Untersuchungen üb. d. Entwicklung d. Diplozoon
paradoxum." _Zeit. f. wiss. Zool._ Vol. XXII. 1872.
(216) E. Zeller. "Untersuchungen ü. d. Entwick. u. Bau d. Polystomum
integerrimum." _Zeit. f. wiss. Zool._ Vol. XXII. 1872.
(217) E. Zeller. "Weitere Beiträge z. Kenntniss d. Polystomen." _Zeit.
f. wiss. Zool._ Vol. XXVII. 1876.
_Cestoda._
(218) Ed. van Beneden. "Recherches sur la composition et la
signification d. l'oeuf." _Mém. cour. Acad. roy. Belgique._ Vol.
XXXIV. 1868.
(219) P. J. van Beneden. "Les vers Cestoïdes considérés sous le
rapport physiologique embryogénique, etc." _Bul. Acad. Scien.
Bruxelles._ Vol. XVII. 1850.
(220) T. S. Cobbold. Entozoa. Groombridge and Son, 1864.
(221) ---- _Parasites; a treatise on the Entozoa, etc._ Churchill,
1879.
(222) Th. H. Huxley. "On the Anatomy and Development of Echinococcus
veterinorum." _Proc. Zool. Soc. Vol._ XX. 1852.