Sections through the ovum of LeptoplanaFig. 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.
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. 86B). 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. 86C,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 withinthe 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-œsophageal 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 Cœlenterata, so that if the views put forward in the previous chapter as to the similarity of the turbellarian and cœlenterate larvæ are correct, the cutis corresponds with the deeper layer of the cœlenterate 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 rhabdocœlous 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.
Larva of Eurylepta auriculataFig. 87. Larva of Eurylepta auriculata immediately after hatching. Viewed from the side.(After Hallez.)m. mouth.
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,alobed 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 infig. 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.
Turbellarian larvaFig. 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.
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, asdescribed 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 theventral 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.
Planarian larvaFig. 89. Planarian larva (probably Planaria angulata).(From Agassiz.)
Fig. 89. Planarian larva (probably Planaria angulata).(From Agassiz.)
Planarian larvaFig. 90. Planarian larva (probably Plamaria angulata).(From Agassiz.)
Fig. 90. Planarian larva (probably Plamaria angulata).(From Agassiz.)
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.
A type of Planarian larva (figs. 89and90)—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 Dendrocœla.The development of the fresh-water Dendrocœla 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 amœboid 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 embryogradually 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 (?).
Rhabdocœla.The development of some of the Rhabdocœla 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 Rhabdocœla like those of the fresh-water Dendrocœla 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 (videBarrois,No.192).
The type of Desor.The segmentation is regular and leads to the formation of a blastosphere with a large segmentationcavity. The blastosphere is converted by invagination into a gastrula (fig. 91A). 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. 92A) in which the dorsal and ventral surfaces can be distinguished, the mouth (m.) being situated on the ventral surface.
Three stages in the development of LineusFig. 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. 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.
Three Stages in the development of LineusFig. 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 œsophagus;pr.proboscis;ms.muscular layer (?);ls.larval skin about to be thrown off;me.mesoblast;st.stomach.
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 œsophagus;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. 91B), and four invaginations of the epiblast make their appearance on the ventral surface (fig. 92A),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. 91C). 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 andarthropodan 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. 91C). 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. 91C,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 theœsophagus (fig. 92B,cs.)[92], which soon fuse with the walls of the body at the junction of the prostomial and metastomial plates (fig. 92C,cs.), although they remain for some time attached to the œsophagus by a solid cord.
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. 92C) 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 becomesthe 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 withinthe 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. 93A,a.e.).
Two stages in the development of PilidiumFig. 93. Two stages in the development of Pilidium.(After Metschnikoff.)ae.archenteron;œ.œsophagus;st.stomach;am.amnion;pr.d.prostomial disc;po.d.metastomial disc;c.s.cephalic sack.
Fig. 93. Two stages in the development of Pilidium.(After Metschnikoff.)
ae.archenteron;œ.œsophagus;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 œsophageal and gastric regions (fig. 93B,œ.andst.). 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. 93B,pr.d.andpo.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.
Philidium and NemerteaFig. 94.A. Pilidium with an advanced nemertine worm.B. Ripe embryo of the Nemertea in the position it occupies in Pilidium.(Both after Bütschli.)œ.œsophagus;st.stomach;i.intestine;pr.proboscis;lp.lateral pit;an.amnion;n.nervous system.
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.)
œ.œsophagus;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 growspari passuwith 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 œsophagus (fig. 94) which is eventually placed in communication with the exteriorby a ciliated canal[93]. The proboscis arises as an hollow invagination at the point where the two anterior discs fuse in front.
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 onAmphiporus lactifloreus(one of the Enopla), and those of Dieck onCephalothrix 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 theepiblast, 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 onTetrastemma 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 NemertineMalacobdellahave 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 segmentationcavity. 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 (videChapter 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. 95A), but sometimes naked.
The ciliated forms are generally completely covered with cilia, but inDistomum lanceolatumthe 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 thelarva of Monostomum mutabile (fig. 95A), 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.
Metamorphosis of the DistomeæFig. 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.
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 theirfurther development one of two things may happen. They may either (1) develop fresh Rediæ or Sporocysts by a process of internal budding (fig. 95C); or else (2) there may be formed in them, by an analogous process, larvæ with long tails known as Cercariæ (fig. 95D.) The direct development of Cercariæ is the usual course, though inDistomum globiparumthe 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. 95E), 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.
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 totailed 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 ofDistomum 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.
InPolystomum integerrimum, which lives in the urinary bladder ofRana 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 (videZeller,Nos.216and217).
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 tostrengthen 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]).
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. 96G), 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.