Development of Phoronis from ActinotrochaFig. 163. A series of stages in the development of Phoronis from Actinotrocha.(After Metschnikoff.)A. Young larva.B. Larva after the formation of post-oral ring of tentacles.C. Larva with commencing invagination to form the body of Phoronis.D. Invagination partially everted.E. Invagination completely everted.m.mouth;an.anus;iv.invagination to form the body of Phoronis.
Fig. 163. A series of stages in the development of Phoronis from Actinotrocha.(After Metschnikoff.)
A. Young larva.B. Larva after the formation of post-oral ring of tentacles.C. Larva with commencing invagination to form the body of Phoronis.D. Invagination partially everted.E. Invagination completely everted.
m.mouth;an.anus;iv.invagination to form the body of Phoronis.
The youngest free larva observed by Metschnikoff (No.373) was less developed than the oldest larva found by Kowalevsky.It probably belongs to a different species. The body is uniformly ciliated (fig. 163A). There is a large contractile præ-oral lobe, and the body ends behind in two processes. The mouth (m) is ventral, and the anus (an) dorsal, and not terminal as in Kowalevsky’s larva.
The alimentary tract is divided into stomodæum, stomach and intestine. The two processes at the hind end of the body are the rudiments of the first-formed pair of the arms which are so characteristic of the fully developed Actinotrocha. A second pair of arms next become established on the dorsal side of the previously existing pair, and the region where the anus is placed grows out as a special process. New pairs of arms continue to be formed in succession dorsalwards and forwards, and soon constitutea complete oblique post-oral ring(fig. 163B). They are covered by long cilia. Round the anal process a very conspicuous ciliated ring also becomes established.
At the period when five pairs of arms are present a delicate membrane becomes visible on the ventral side of the intestine which joins the somatic mesoblast anteriorly. This membrane is the rudiment of the future ventral vessel. The somatic mesoblast is present even before this period as a delicate layer of circular muscular fibres.
When six pairs of arms have become formed an involution (fig. 163C,iv) appears on the ventral side, immediately behind the ring of arms. This involution consists both of the epiblast and somatic mesoblast. It grows inwards towards the intestine, and, increasing greatly in length, becomes at the same time much folded.
When it has reached its full development the critical period of the metamorphosis of Actinotrocha into Phoronis is reached, and is completed in about a quarter of an hour. The ventral involution becomes evoluted (fig. 163D), just as one might turn out the finger of a glove which had been pulled inwards. When the involution has been to a certain extent everted, the alimentary canal passes into it, and at the same time the body of the larva becomes violently contracted. By the time the evagination is completed it forms (fig. 162E) a long conical body, containing the greater part of the alimentary tract, andconstituting the body of the young Phoronis. The original anal process remains on the dorsal side as a small papilla (fig. 162E,an).
While these changes have been taking place the præ-oral lobe has become much contracted, and partly withdrawn into the stomodæum. At the same time the arms have become bent forward, so as to form a ring round the mouth. Their bases become much thickened. The metamorphosis is completed by the entire withdrawal of the præ-oral lobe within the œsophagus, and by the casting off of the ends of the arms, their bases remaining as the circumoral ring of tentacles, which form however a lophophore rather than a complete ring. The peri-anal ring of cilia is also thrown off, and the anal process withdrawn into the body of the young Phoronis. There are now three longitudinal vascular trunks, united anteriorly by a circular vessel which is prolonged into the tentacles.
General Considerations.
The development of Phoronis is so different from that of the other Gephyrea that further investigations are required to shew whether Phoronis is a true Gephyrean. Apart from its peculiar metamorphosis Actinotrocha is a very interesting larval form, in that it is without a præ-oral ciliated ring, and that the tentacles of the adult are derived from a true post-oral ring, prolonged into arm-like processes.
The other Gephyrea present in their development an obvious similarity to the normal Chætopoda, but their development stops short of that of the Chætopoda, in that they are clearly without any indications of a true segmentation. In the face of what is known of their development it is hardly credible that they can represent adegenerateChætopod phylum in which segmentation has become lost. Further than this the Gephyrea armata seem in one respect to be a very primitive type in that they retain through life a well-developed præ-oral lobe, which constitutes their proboscis. In almost all other forms, except Balanoglossus, the larval præ-oral lobe becomes reduced to a relatively insignificant anterior part of the head.
Bibliography.
Gephyrea nuda.
(366)A. Kowalevsky.Sitz. d. zool. Abth. d. III. Versam. russ. Naturj.(Thalassema).Zeit. f. wiss. Zool.Vol.XXII.1872, p.284.(367)A. Krohn.“Ueb. d. Larve d. Sipunculus nudus nebst Bemerkungen,”etc. Müller’sArchiv, 1857.(368)M. Salensky.“Ueber die Metamorphose d. Echiurus.”Morphologisches Jahrbuch,Bd.II.(369)E. Selenka.“Eifurchung u. Larvenbildung von Phascolosoma elongatum.”Zeit. f. wiss. Zool.1875,Bd.XXV.p.1.(370)J. W. Spengel.“Beiträge z. Kenntniss d. Gephyreen (Bonellia).”Mittheil. a. d. zool. Station z. Neapel,Vol.I.1879.
Gephyrea tubicola (Actinotrocha).
(371)A. Krohn.“Ueb. Pilidium u. Actinotrocha.”Müller’sArchiv, 1858.(372)A. Kowalevsky. “On anatomy and development of Phoronis,”Pétersbourg, 1867. 2Pl.Russian.VideLeuckart’sBericht, 1866‑7.(373)E. Metschnikoff.“Ueber d. Metamorphose einiger Seethiere (Actinotrocha).”Zeit. f. wiss. Zool.Bd.XXI.1871.(374)J. Müller.“Bericht üb. ein. Thierformen d. Nordsee.”Müller’sArchiv, 1846.(375)An. Schneider.“Ueb. d. Metamorphose d. Actinotrocha branchiata.”Müller’sArch.1862.
[149]The following scheme shews the classification of the Gephyrea adopted in the present chapter:I.Gephyrea nuda.(1)Inermia.(2)Armata.II.Gephyrea tubicola.(Phoronis).[150]The fact that these pouches are outgrowths of the alimentary tract appears to preclude the possibility of their being homologous with excretory tubes of the Platyelminthes and Rotifera.[151]Kowalevsky states that what I have called the mouth is the anus, but his subsequent descriptions shew that he has transposed the mouth and anus in the embryo, and that the opening, which he asserts to be the anus, is in reality the mouth.
[149]The following scheme shews the classification of the Gephyrea adopted in the present chapter:
I.Gephyrea nuda.
(1)Inermia.
(2)Armata.
II.Gephyrea tubicola.(Phoronis).
[150]The fact that these pouches are outgrowths of the alimentary tract appears to preclude the possibility of their being homologous with excretory tubes of the Platyelminthes and Rotifera.
[151]Kowalevsky states that what I have called the mouth is the anus, but his subsequent descriptions shew that he has transposed the mouth and anus in the embryo, and that the opening, which he asserts to be the anus, is in reality the mouth.
The present chapter deals with three small isolated groups, which only resemble each other in that the systematic position of all of them is equally obscure.
Chætognatha.
The discoveries of Kowalevsky (No.378) confirmed by Bütschli (No.376) with reference to the development of Sagitta, though they have not brought us nearer to a knowledge of the systematic position of this remarkable form, are nevertheless ofgreat value for the more general problems of embryology. The development commences after the eggs are laid. The segmentation is uniform, and a blastosphere, formed of a single layer of columnar cells, is the product of it. An invagination takes place, the opening of which narrows to a blastopore situated at the pole of the embryo opposite that at which the mouth subsequently appears (fig. 164A). The simple archenteron soon becomes anteriorly divided into three lobes, which communicate freely with the still single cavity behind (fig. 164B). The two lateral lobes are destined to form the body cavity, and the median lobe the alimentary tract of the adult. An invagination soon arises at the opposite pole of the embryo to the blastopore and forms the mouth and œsophagus (fig. 164B and C,m).
Illustration: TitleFig. 164. Three stages in the development of Sagitta.(A and C after Bütschli and B after Kowalevsky.) The three embryos are represented in the same positions.A. The gastrula stage.B. A succeeding stage in which the primitive archenteron is commencing to be divided into three parts, the two lateral of which are destined to form the body cavity.C. A later stage in which the mouth involution (m) has become continuous with the alimentary tract, and the blastopore has become closed.m.mouth; al. alimentary canal;ae.archenteron;bl.p.blastopore;pv.perivisceral cavity;sp.splanchnopleuric mesoblast;so.somatopleuric mesoblast;ge.generative organs.
Fig. 164. Three stages in the development of Sagitta.(A and C after Bütschli and B after Kowalevsky.) The three embryos are represented in the same positions.
A. The gastrula stage.B. A succeeding stage in which the primitive archenteron is commencing to be divided into three parts, the two lateral of which are destined to form the body cavity.C. A later stage in which the mouth involution (m) has become continuous with the alimentary tract, and the blastopore has become closed.
m.mouth; al. alimentary canal;ae.archenteron;bl.p.blastopore;pv.perivisceral cavity;sp.splanchnopleuric mesoblast;so.somatopleuric mesoblast;ge.generative organs.
At the gastrula stage there is formed a paired mass destined to give rise to the generative organs. It arises as a prominence of six cells, projecting from the hypoblast at the anterior pole of the archenteron, and soon separates itself as a mass, or probably a pair of masses, lying freely in the cavity of the archenteron (fig. 164A,ge). When the folding of the primitive cavity takes place the generative rudiment is situated at the hind end of the median lobe of the archenteron in the position represented infig. 164C,ge.
An elongation of the posterior end of the embryo now takes place, and the embryo becomes coiled up in the egg, and when eventually hatched sufficiently resembles the adult to be recognisable as a young Sagitta.
Before hatching takes place various important changes become manifest. The blastopore disappears after being carried to the ventral surface. The middle section of the trilobed region of the archenteron becomes separated from the unpaired posterior part, and forms a tube, blind behind, but opening in front by the mouth (fig. 165A,al). It constitutes the permanent alimentary tract, and is formed of a pharyngeal epiblastic invagination, and a posterior hypoblastic section derived from the primitive archenteron. The anus is apparently not formed till comparatively late. After the isolation of the alimentary tract the remainder of the archenteron is formed of two cavities in front, which open freely into a single cavity behind (fig. 165A).The whole of it constitutes the body cavity and its wallsthe mesoblast.The anterior paired part becomes partitioned off into a head section and a trunk section (fig. 165A and B). The former constitutes a pair of distinct cavities (c.pv) in the head, and the latter two cavities opening freely into the unpaired portion behind. At the junction of the paired cavities with the unpaired cavity are situated the generative organs (ge). The inner wall of each of the paired cavities forms the splanchnopleuric mesoblast, and the outer wall of the whole the somatic mesoblast. The inner walls of the posterior cavities unite above and below the alimentary tract, and form the dorsal and ventral mesenteries, which divide the body cavity into two compartments in the adult. Before the hatching of the embryo takes place this mesentery is continued backwards so as to divide the primitively unpaired caudal part of the body cavity in the same way.
Late embryo of SagittaFig. 165. Two views of a late embryo of Sagitta.A. from the dorsal surface. B. from the side. (After Bütschli.)m.mouth;al.alimentary canal;v.g.ventral ganglion (thickening of epiblast);ep.epiblast;c.pv.cephalic section of body cavity;so.somatopleure;sp.splanchnopleure;ge.generative organs.
Fig. 165. Two views of a late embryo of Sagitta.A. from the dorsal surface. B. from the side. (After Bütschli.)
m.mouth;al.alimentary canal;v.g.ventral ganglion (thickening of epiblast);ep.epiblast;c.pv.cephalic section of body cavity;so.somatopleure;sp.splanchnopleure;ge.generative organs.
From the somatic mesoblast of the trunk is derived the single layer of longitudinal muscles of Sagitta, and part of the epithelioid lining of the body cavity. The anterior termination of the trunk division of the body cavity is marked in the adult by the mesentery dividing into two laminæ, which bend outwards to join the body wall.
The cephalic section of the body cavity seems to atrophy, and its walls to become converted into the complicated system of muscles present in the head of the adult Sagitta.
In the presence of a section of the body cavity in the head the embryo of Sagitta resembles Lumbricus, Spiders, etc.
The generative rudiment of each side divides into an anterior and a posterior part(fig. 165,ge). The former constitutes the ovary, and is situated in front of the septum dividing the tail from the body; and the latter, in the caudal region of the trunk, forms the testis.
The nervous system originates from the epiblast. There is a ventral thickening (fig. 165B,v.g) in the anterior region of the trunk, and a dorsal one in the head. The two are at first continuous, and on becoming separated from the epiblast remain united by thin cords.
The ventral ganglion is far more prominent during embryonic life than in the adult. Its position and early prominence in the embryo perhaps indicate that it is the homologue of the ventral cord of Chætopoda[152].
Bibliography.
(376)O. Bütschli.“Zur Entwicklungsgeschichte der Sagitta.”Zeitschrift f. wiss. Zool.,Vol.XXIII.1873.(377)C. Gegenbaur.“Über die Entwicklung der Sagitta.”Abhand. d. naturforschenden Gesellschaft in Halle,1857.(378)A. Kowalevsky.“Embryologische Studien an Würmern u. Arthropoden.”Mém. Acad. Pétersbourg,VII.sér., Tom.XVI., No.12. 1871.
Myzostomea.
The development of these peculiar parasites on Crinoids has been investigated by Metschnikoff (No.380), Semper (No.381), and Graff (No.379).
The segmentation is unequal, and would appear to be followed by an epibolic invagination. The outer layer of cells (epiblast) becomes covered with cilia, and the inner is transformed into a non-cellular (?) central yolk mass. At this stage the larva is hatched, and commences to lead a free existence. In the next stage observed by Metschnikoff, the mouth, œsophagus, stomach, and anus had become developed; and two pairs of feet were present. In both of these feet Chætopod-like setæ were present, which in the hinder pair were simple fine bristles without a terminal hook. The papilliform portion of the foot is at first undeveloped. The feet become successively added, like Chætopod segments, and the stomach does not become dendriform till the whole complement of feet (5 pairs) are present.
In the primitive covering of cilia, combined with a subsequent indicationof segments in the formation of the feet and setæ, the larva of the Myzostomea shews an approximation to the Chætopoda, and the group is probably to be regarded as an early Chætopod type specially modified in connection with its parasitic habits.
Bibliography.
(379)L. Graff.Das Genus Myzostoma.Leipzig, 1877.(380)E. Metschnikoff.“Zur Entwicklungsgeschichte d. Myzostomum.”Zeit. f. wiss. Zool.,Vol.XVI.1866.(381)C. Semper.“Z. Anat. u. Entwick. d. Gat. Myzostomum.”Zeit. f. wiss. Zool.,Vol.IX.1858.
Gastrotricha.
A few observations of Ludwig on the winter eggs of Ichthydium larus shew that the segmentation is a total and apparently a regular one. It leads to the formation of a solid morula. The embryo has a ventral curvature, and the caudal forks are early formed as cuticular structures. By the time the embryo leaves the egg, it has almost reached the adult state. The ventral cilia arise some little time prior to the hatching.
Bibliography.
(382)H. Ludwig.“Ueber die Ordnung GastrotrichaMetschn.”Zeit. f. wiss. Zool.,Vol.XXVI.1876.
[152]Langerhans has recently made some important investigations on the nervous system of Sagitta, and identifies the ventral ganglion with the parieto-splanchnio ganglia of Molluscs, while he has found a pair of new ganglia, the development of which is unknown, which he calls the subœsophageal or pedal ganglia. The embryological facts do not appear to be in favour of these interpretations.
[152]Langerhans has recently made some important investigations on the nervous system of Sagitta, and identifies the ventral ganglion with the parieto-splanchnio ganglia of Molluscs, while he has found a pair of new ganglia, the development of which is unknown, which he calls the subœsophageal or pedal ganglia. The embryological facts do not appear to be in favour of these interpretations.
Nematelminthes[153].
Nematoidea.Although the ova of various Nematodes have formed some of the earliest, as well as the most frequent objects of embryological observation, their development is still but very imperfectly known. Both viviparous and oviparous forms are common, and in the case of the oviparous forms the eggs are usually enveloped in a hard shell. The segmentation is total and nearly regular, though the two first segments are often unequal. The relation of the segmentation spheres to the germinal layers is however only satisfactorily established (through the researches of Bütschli (No.383)) in the case of Cucullanus elegans, a form parasitic in the Perch[154].
The early development of this embryo takes place within the body of the parent, and the egg is enveloped in a delicate membrane. After the completion of the early stages of segmentation the embryo acquires the form of a thin flat plate composed of two layers of cells (fig. 166A and B). The two layers of this plate give rise respectively to the epiblast and hypoblast, and at a certain stage the hypoblastic layer ceases togrow, while the growth of the epiblastic layer continues. As a consequence of this the sides of the plate begin to fold over towards the side of the hypoblast (fig. 166D.) This folding results in the formation of a remarkably constituted gastrula, which has the form of a hollow two-layered cylinder with an incompletely closed slit on one side (fig. 166E,bl.p). This slit has the value of a blastopore. It becomes closed by the coalescence of the two edges, a process which commences posteriorly, and then gradually extends forwards. In front the blastopore never becomes completely closed, but remains as the permanent mouth. The embryo after these changes has a worm-like form, which becomes the more obvious as it grows in length and becomes curved (fig. 166F).
Illustration: TitleFig. 166. Various stages in the development of Cucullanus elegans.(From Bütschli.)A. Surface view of flattened embryo at an early stage in the segmentation.B. Side view of an embryo at a somewhat later stage, in optical section.C. Flattened embryo at the completion of segmentation.D. Embryo at the commencement of the gastrula stage.E. Embryo when the blastopore is reduced to a mere slit.F. Vermiform embryo after the division of the alimentary tract into œsophageal and glandular divisions.m.mouth;ep.epiblast;hy.hypoblast;me.mesoblast;œ.œsophagus;bl.p.blastopore.
Fig. 166. Various stages in the development of Cucullanus elegans.(From Bütschli.)
A. Surface view of flattened embryo at an early stage in the segmentation.B. Side view of an embryo at a somewhat later stage, in optical section.C. Flattened embryo at the completion of segmentation.D. Embryo at the commencement of the gastrula stage.E. Embryo when the blastopore is reduced to a mere slit.F. Vermiform embryo after the division of the alimentary tract into œsophageal and glandular divisions.
m.mouth;ep.epiblast;hy.hypoblast;me.mesoblast;œ.œsophagus;bl.p.blastopore.
The hypoblast of the embryo gives rise to the alimentarycanal, and soon becomes divided into an œsophageal section (fig. 166F,œ) formed of granular cells, and a posterior division formed of clear cells. The mesoblast (fig. 166,me) takes its origin from certain special hypoblast cells around the mouth, and thence grows backwards towards the posterior end of the body.
The young Cucullanus becomes hatched while still in the generative ducts of its parent, and is distinguished by the presence of a remarkable thread-like tail. On the dorsal surface is a provisional boring apparatus in the form of a conical papilla. A firm cuticle enveloping the body is already present. In this condition it leaves its parent and host, and leads for a time a free existence in the water. Its metamorphosis is dealt with in another section.
The ova of the Oxyuridæ parasitic in Insects are stated by Galeb (No.386) to take the form of a blastosphere at the close of segmentation. An inner layer is then formed by delamination. What the inner layer gives rise to is not clear, since the whole alimentary canal is stated to be derived from two buds, which arise at opposite ends of the body, and grow inwards till they meet.
The generative organs.The study of the development of the generative organs of Nematodes has led to some interesting results. In the case of both sexes the generative organs originate (Schneider, No.390) from a single cell. This cell elongates and its nuclei multiply. After assuming a somewhat columnar form, it divides into (1) a superficial investing layer, and (2) an axial portion.
In the female the superficial layer is only developed distinctly in the median part of the column. In the course of the further development the two ends of the column become the blind ends of the ovary, and the axial tissue they contain forms the germinal tissue of nucleated protoplasm. The superficial layer gives rise to the epithelium of the uterus and oviduct. The germinal tissue, which is originally continuous, is interrupted in the middle part (where the superficial layer gives rise to the uterus and oviduct), and is confined to the two blind extremities of the tube.
In the male the superficial layer, which gives rise to the epithelium of the vas deferens, is only formed at the hinder end ofthe original column. In other respects the development takes place as in the female.
Gordioidea.The ovum of Gordius undergoes a regular segmentation. According to Villot (No.391) it forms at the close of segmentation a morula, which becomes two-layered by delamination. The embryo is at first spherical, but soon becomes elongated.
By an invagination at the anterior extremity the head is formed. It consists of a basal portion, armed with three rings of stylets, and a conical proboscis, armed with three large stylets. When the larva becomes free the head becomes everted, though it remains retractile. By the time the embryo is hatched a complete alimentary tract is formed with an oral opening at the end of the proboscis, and a subterminal ventral anal opening. It is divided into an œsophagus and stomach, and a large gland opens into it at the base of the proboscis.
The body has a number of transverse folds, which give it a ringed appearance.
Metamorphosis and life history.
Nematoidea.Although a large number of Nematodes have a free existence and simple life history, yet the greater number of known genera are parasitic, and undergo a more or less complicated metamorphosis[155]. According to this metamorphosis they may be divided into two groups (which by no means closely correspond with the natural divisions),viz.those which have a single host, and those with two hosts. Each of these main divisions may be subdivided again into two.
In the first group with one host the simplest cases are those in which the adult sexual form of parasite lays its eggs in the alimentary tract of its host, and the eggs are thence transported to the exterior. The embryo still in the egg, if favoured by sufficient warmth and moisture, completes its development up to a certain point, and, if then swallowed by an individual of the species in which it is parasitic in the adult condition, it is denuded of its shell by the action of the gastric juice, and develops directly into the sexual form.
Leuckart has experimentally established this metamorphosis in the case of Trichocephalus affinis, Oxyurus ambigua, and Heterakis vermicularis. The Oxyuridæ of Blatta and Hydrophilus have a similar life history(Galeb,No.386), and it is almost certain that the metamorphosis of the human parasites, Ascaris lumbricoides and Oxyurus vermicularis, is of this nature.
A slightly more complicated metamorphosis is common in the genera Ascaris and Strongylus. In these cases the egg-shell is thin, and the embryo becomes free externally, and enjoys for a shorter or longer period a free existence in water or moist earth. During this period it grows in size, and though not sexual usually closely resembles the adult form of the permanently free genus Rhabditis. In some cases the free larva becomes parasitic in a freshwater Mollusc, but without thereby undergoing any change. It eventually enters the alimentary tract of its proper host and there become sexual.
As examples of this form of development worked out by Leuckart may be mentioned Dochmius trigonocephalus, parasitic in the dog, and Ascaris acuminata, in the frog. The human parasite Dochmius duodenale undergoes the same metamorphosis as Dochmius trigonocephalus.
A remarkable modification of this type of metamorphosis is found in Ascaris (Rhabdonema) nigrovenosa, which in its most developed condition is parasitic in the lungs of the frog (Metschnikoff, Leuckart,No.388). The embryos pass through their first developmental phases in the body of the parent. They have the typical Rhabditis form, and make their way after birth into the frog’s rectum. From this they pass to the exterior, and then living either in moist earth, or the fæces of the frog, develop into a sexual form, but are very much smaller than in the adult condition. The sexes are distinct, and the males are distinguished from the females by their smaller size, shorter and rounded tails, and thinner bodies. The females have paired ovaries with a very small number of eggs, but the testis of the males is unpaired. Impregnation takes place in the usual way, and in summer time about four embryos are developed in each female, which soon burst their egg-capsules, and then move freely in the uterus. Their active movements soon burst the uterine walls, and they then come to lie freely in the body cavity. The remaining viscera of the mother are next reduced to a finely granular material, which serves for the nutrition of the young forms which continue to live in the maternal skin. The larvæ eventually become free, and though in many respects different from the parent form which gave rise to them, have nevertheless the Rhabditis form. They live in water or slime, and sometimes become parasitic in water-snails; in neither case however do they undergo important changes unless eventually swallowed by a frog. They then pass down the trachea into the lungs and there rapidly develop into the adult form. No separate males have been found in the lungs of the frog, but it has been shewn by Schneider (No.390) that the so-called females are really hermaphrodites; the same gland giving originto both spermatozoa and ova, the former being developed before the latter[156]. The remarkable feature of the above life history is the fact that in the stage corresponding with the free larval stage of the previous forms the larvæ of this species become sexual, and give rise to a second free larval generation, which develops into the adult form on again becoming parasitic in the original host. It constitutes a somewhat exceptional case of heterogamy as defined in the introduction.
Amongst the Nematodes with but a single host a remarkable parasite in wheat has its place. This form, known as Anguillula scandens, inhabits in the adult condition the ears of wheat, in which it lays its eggs. After hatching, the larvæ become encysted, but become free on the death of the plant. They now inhabit moist earth, but eventually make their way into the ears of the young wheat and become sexually mature.
The second group of parasitic Nematodes with two hosts may be divided into two groups, according to whether the larva has a free existence before passing into its first or intermediate host, or is taken into it while still in the egg. In the majority of cases the larval forms live in special connective-tissue capsules, or sometimes free in the tissues of their intermediate hosts; but the adults, as in the cases of other parasitic Nematodes, inhabit the alimentary tract.
The life history of Spiroptera obtusa may be cited as an example of a Nematode with two hosts in which the embryo is transported into its intermediate host while still within the egg. The adult of this form is parasitic in the mouse, and the ova pass out of the alimentary tract with the excreta, and may commonly be found in barns, etc. If one of the ova is now eaten by the meal-worm (larva of Tenebrio), it passes into the body cavity of this worm and undergoes further development. After about five weeks it becomes encapsuled between the ‘fat bodies’ of the meal-worm. It then undergoes an ecdysis, and, if the meal-worm with its parasites is now eaten by the mouse, the parasites leave their capsule and develop into the sexual form.
As examples of life histories in which a free state intervenes before the intermediate host, Cucullanus elegans and Dracunculus may be selected. The adult Cucullanus elegans is parasitic in the alimentary tract of the Perch and other freshwater fishes. It is a viviparous form, and the young after birth pass out into the water. They next become parasitic in Cyclops, passing in through the mouth, so into the alimentary tract, and thence into the body cavity. They soon undergo an ecdysis, in the course of which the œsophagus becomes divided into a muscular pharynx and true glandularœsophagus. They then grow rapidly in length, and at a second ecdysis acquire a peculiar beaker-like mouth cavity approaching that of the adult. They do not become encapsuled. No further development of the worm takes place so long as it remains in the Cyclops, but, if the Cyclops is now swallowed by a Perch, the worm undergoes a further ecdysis, and rapidly attains to sexual maturity.
The observations of Fedschenko on Dracunculus medinensis[157], which is parasitic in the subcutaneous connective tissue in Man, would seem to shew that it undergoes a metamorphosis very similar to that of Cucullanus. There is moreover a striking resemblance between the larvæ of the two forms. The larvæ of Dracunculus become transported into water, and then make their way into the body cavity of a Cyclops by boring through the soft skin between the segments on the ventral surface of the body. In the body cavity the larvæ undergo an ecdysis and further development. But on reaching a certain stage of development, though they remain a long time in the Cyclops, they grow no further. The remaining history is unknown, but probably the next host is man, in which the larva comes to maturity. In the adult condition only females of Dracunculus are known, and it has been suggested by various writers that the apparent females are in reality hermaphrodites, like Ascaris nigrovenosa, in which the male organs come to maturity before the female.
The sexual form is parasitic in warm climates in the human tissues, and produces multitudes of larvæ which pass into the blood, and are sometimes voided with the urine. The larvæ in the blood do not undergo a further development, and unless transported to an intermediate host die before very long. Some, though as yet hardly sufficient, evidence has been brought forward to shew that if the blood of an infected patient is sucked by a mosquito the larvæ develop further in the alimentary tract of the mosquito, pass through a more or less quiescent stage, and eventually grow considerably in size, and on the death of the mosquito pass into the water. From the water they are probably transported directly or indirectly into the human intestines, and then bore their way into the tissues in which they are parasitic, and become sexually mature.
The well-known Trichina spiralis has a life history unlike that of other known Nematodes, though there can be little doubt that this form should be classified in respect to its life history with the last-described forms. The peculiarity of the life history of Trichina is that the embryos set free in the alimentary canal pass through the walls into the muscular tissues and there encyst; but do not in a general way pass out from the alimentarycanal of one host and thence into a fresh host to encyst. It occasionally however happens that this migration does take place, and the life history of Trichina spiralis then becomes almost identical with that of some of the forms of the third type. Trichina is parasitic in man, and in swine, and also in the rat, mouse, cat, fox and other forms which feed upon them. Artificially it can be introduced into various herbivorous forms (rabbit, guinea-pig, horse) and even birds.
The sexual form inhabits the alimentary canal. The female is viviparous, and produces myriads of embryos, which pass into the alimentary canal of their host, through the walls of which they make their way, and travelling along lines of connective tissue pass into the muscles. Here the embryos, which are born in a very imperfect condition, rapidly develop, and eventually assume a quiescent condition in a space inclosed by sarcolemma. Within the sarcolemma a firm capsule is developed for each larva, which after some months becomes calcified; and after the atrophy of the sarcolemma a connective-tissue layer is formed around it. Within its capsule the larva can live for many years, even ten or more, without undergoing further development, but if at last the infected flesh is eaten by a suitable form,e.g.the infected flesh of the pig by man, the quiescent state of the larva is brought to a close, and sexual maturity is attained in the alimentary tract of the new host.
Gordioidea.The free larva of Gordius already described usually penetrates into the larva of Chironomus where it becomes encysted. On the Chironomus being eaten by some fish (Villot,No.391) (Phoxinus lævis or Cobitis barbatula), it penetrates into the wall of the intestine of its second host, becomes again encysted and remains quiescent for some time. Eventually in the spring it leaves its capsule, and enters the intestine, and passes to the exterior with the fæces. It then undergoes a gradual metamorphosis, in the course of which it loses its ringed structure and cephalic armature, grows in length, acquires its ventral cord, and on the development of the generative organs loses the greater part of its alimentary tract.
Young examples of Gordius have often been found in various terrestrial carnivorous Insecta, but the meaning of this fact is not yet clear.
Bibliography.
(383)O. Bütschli.“Entwicklungsgeschichte d. Cucullanus elegans.”Zeit. f. wiss. Zool.,B.XXVI.1876.(384)T. S. Cobbold.Entozoa.Groombridge and Son, 1864.(385)T. S. Cobbold. Parasites:A Treatise on the Entozoa of Man and Animals.Churchill, 1879.(386)O. Galeb.“Organisation et développement des Oxyuridés,”&c.Archives de Zool. expér. et génér.,Vol.VII.1878.(387)R. Leuckart.Untersuchungen üb. Trichina spiralis.2nded.Leipzig, 1866.(388)R. Leuckart.Die menschlichen Parasiten,Bd.II.1876.(389)H. A. Pagenstecher.Die Trichinen nach Versuchen dargestellt.Leipzig, 1865.(390)A. Schneider.Monographie d. Nematoden.Berlin, 1866.(391)A. Villot.“Monographie des Dragoneaux”(Gordioidea).Archives de Zool. expér. et génér.,Vol.III.1874.
Acanthocephala.
The Acanthocephala appear to be always viviparous. At the time of impregnation the ovum is a naked cell, and undergoes in this condition the earlier phases of segmentation.
The segmentation is unequal (Leuckart,No.393), but whether there is an epibolic gastrula has not clearly been made out.
Before segmentation is completed there are formed round the ovum thick protecting membranes, which are usually three in number, the middle one being the strongest. After segmentation the central cells of the ovum fuse together to give rise to a granular mass, while the peripheral cells at a slightly later period form a more transparent syncytium. At the anterior end of the embryo there appears a superficial cuticle bearing in front a ring of hooks.
The embryo is now carried out with the excreta from the intestine of the vertebrate host in which its parent lives. It is then swallowed by some invertebrate host[159].
Another very remarkable human parasite belonging to the same group as Dracunculus is the form known as Filaria sanguinis hominis, or Filaria Bancrofti[158].
In the intestine of the invertebrate host the larva is freed from its membranes, and is found to have a somewhat elongated conical form, terminating anteriorly in an obliquely placed disc, turned slightly towards the ventral surface and armed with hooks. Between this disc and the granular mass, already described as formed from the central cells of the embryo, is a rather conspicuous solid body. Leuckart supposes that this body may represent a rudimentary functionless pharynx, while the granular mass in his opinion is an equally rudimentary and functionless intestine. The body wall is formed of a semifluid internal layer surrounding the rudimentary intestine, if such it be, and of a firmer outer wall immediately within the cuticle.
The adult Echinorhyncus is formed by a remarkable process of development within the body of the larva, and the skin is the only part of the larva which is carried over to the adult.
In Echinorhyncus proteus the larva remains mobile during the formation of the adult, but in other forms the metamorphosis takes place during a quiescent condition of the larva.
The organs of the adult are differentiated from a mass of cells which appears to be a product of the central embryonic granular mass, and iscalled by Leuckart the embryonic nucleus. The embryonic nucleus becomes divided into four linearly arranged groups of cells, of which the hindermost but one is the largest, and very early differentiates itself into (1) a peripheral layer, and (2) a central mass formed of two distinct bodies. The peripheral layer of this segment grows forwards and backwards, and embraces the other segments, with the exception of the front end of the first one which is left uncovered. The envelope so formed gives rise to the splanchnic and somatic mesoblast of the adult worm. Of the four groups of cells within it the anterior gives rise to the proboscis, the next to the nerve ganglion, the third, formed of two bodies, to the paired generatives, and the fourth to the generative ducts. The whole of the above complex rapidly elongates, and as it does so the enveloping membrane becomes split into two layers; of which the outer forms the muscular wall of the body (somatic mesoblast), and the inner the muscular sheath of the proboscis and the so-called generative ligament enveloping the generative organs. The inner layer may be called the splanchnic mesoblast in spite of the absence of an intestine. The cavity between the two mesoblastic layers forms the body cavity.
The various parts of the adult continue to differentiate themselves as the whole increases in size. The generative masses very early shew traces of becoming differentiated into testes or ovaries. In the male the two generative masses remain spherical, but in the female become elongated: the rudiment of the generative ducts becomes divided into three sections in both sexes. The most remarkable changes are, however, those undergone by the rudiment of the proboscis.
In its interior there is formed a cavity, but the wall bounding the front end of the cavity soon disappears. By the time that this has taken place the body of the adult completely fills up the larval skin, to which it very soon attaches itself. The hollow rudiment of the proboscis then becomes everted, and forms a papilla at the end of the body, immediately adjoining the larval skin. This papilla, with the larval skin covering it, constitutes the permanent proboscis. The original larval cuticle is either now or at an earlier period thrown off and a fresh cuticle developed. The hooks of the proboscis are formed from cells of the above papilla, which grow through the larval skin as conical prominences, on the apex of which a chitinous hook is modelled. The remainder of the larval skin forms the skin of the adult, and at a later period develops in its deeper layer the peculiar plexus of vessels so characteristic of the Acanthocephala. The anterior oval appendages of the adult cutis, known as the lemnisci, are outgrowths from the larval skin.
The Echinorhyncus has with the completion of these changes practically acquired its adult structure; but in the female the ovaries undergo at this period remarkable changes, in that they break up into a number of spherical masses, which lie in the lumen of the generative ligaments, and also make their way into the body cavity.
The young Echinorhyncus requires to be transported to its permanent host, which feeds on its larval host, before attaining to sexual maturity.
Bibliography.
(392)R. Greeff.“Untersuchungen ü. d. Bau u. Entwicklung des Echin. miliarius.”Archiv f. Naturgesch.1864.(393)R. Leuckart.Die menschlichen Parasiten.Vol.II.p.801 et seq. 1876.(394)An. Schneider.“Ueb. d. Bau d. Acanthocephalen.”Archiv f. Anat. u. Phys.1868.(395)G. R. Wagener.Beiträge z. Entwicklungsgeschichte d. Eingeweidewürmer.Haarlem, 1865.