Side and dorsal view of a larva of StrongylocentrusFig. 266. Side and dorsal view of a larva of Strongylocentrus.(From Agassiz.) General reference letters as in figs. 264 and 265.e´´.anterolateral arms;v´´. ciliated epaulettes;w´. invagination to form the disc of Echinus.
Fig. 266. Side and dorsal view of a larva of Strongylocentrus.(From Agassiz.) General reference letters as in figs. 264 and 265.
e´´.anterolateral arms;v´´. ciliated epaulettes;w´. invagination to form the disc of Echinus.
The water-vascular ring is from the first complete, so that, as in Asterias, it is perforated through the centre by a new œsophagus. According to Agassiz the first five tentacles or tube feet grow into the radial canals, and form the odd terminal tentacles exactly as in Asterias[226]. Spatangus only differs in development from Echinus in the fact that the opening of the invagination to form the ventral disc becomes completely closed, and that the tube feet have eventually to force their way through the larval epidermis of the amnion, which is ruptured in the process and eventually thrown off.
Full-grown larva of StrongylocentrusFig. 267. Full-grown larva of Strongylocentrus.(From Agassiz.)The figure shews the largely-developed abactinal disc of the young Echinus enclosing the larval stomach. Reference letters as in previous figs.
Fig. 267. Full-grown larva of Strongylocentrus.(From Agassiz.)
The figure shews the largely-developed abactinal disc of the young Echinus enclosing the larval stomach. Reference letters as in previous figs.
Crinoidea.The larva of Antedon, while still within the egg-shell, assumes an oval form and uniform ciliation. Before itbecomes hatched the uniform layer of cilia is replaced by four transverse bands of cilia, and a tuft of cilia at the posterior extremity. In this condition it escapes from the egg-shell (fig. 268A), and becomes bilateral, owing to a flattening of the ventral surface. On the flattened surface appears a ciliateddepression corresponding in position with the now closed blastopore (videp.550). The third ciliated band bends forward to pass in front of this (fig. 269). Behind the last ciliated band there is present a small depression of unknown function, alsosituated on the ventral surface. The posterior extremity of the embryo elongates to form the rudiment of the future stem, and a fresh depression, marking the position of the future mouth, makes its appearance on the anterior and ventral part.
Three stages in the development of Antedon (Comatula.)Fig. 268. Three stages in the development of Antedon (Comatula.)(From Lubbock; after Thomson.)A. larva just hatched; B. larva with rudiment of the calcareous plates; C. Pentacrinoid larva.
Fig. 268. Three stages in the development of Antedon (Comatula.)(From Lubbock; after Thomson.)
A. larva just hatched; B. larva with rudiment of the calcareous plates; C. Pentacrinoid larva.
While the ciliated bands are still at their full development, the calcareous skeleton of the future calyx makes its appearance in the form of two rows, each of five plates, formed of a network of spicula (figs. 268B and269). The plates of the anterior ring are known as the orals, those of the posterior as the basals. The former surround the left,i.e. anterior peritoneal sack; the latter the right,i.e.posterior peritoneal sack. The two rows of plates are at first not quite transverse, but form two oblique circles, the dorsal end being in advance of the ventral. The rows soon become transverse, while the originally somewhat ventral oral surface is carried into the centre of the area enclosed by the oral plates.
Larva of AntedonFig. 269. Larva of Antedon with rudiments of calcareous skeleton.(From Carpenter; after Thomson.)1. Terminal plate at the end of the stem; 3. basals;or.orals;bl.position of blastopore.
Fig. 269. Larva of Antedon with rudiments of calcareous skeleton.(From Carpenter; after Thomson.)
1. Terminal plate at the end of the stem; 3. basals;or.orals;bl.position of blastopore.
By the change in position of the original ventral surface relatively to the axis of the body, the bilateral symmetry of the larva passes into a radial symmetry. While the first skeletal elements of the calyx are being formed, the skeleton of the stem is also established. The terminal plate is first of all established, then the joints, eight at first, of the stem. The centro-dorsal plate is stated by Thomson to be formed as the uppermost joint of the stem[227]. The larva, after the completion of the above changes, is shewn infig. 268B, and somewhat more diagrammatically infig. 269.
Pentacrinoid larva of AntedonFig. 270. Young Pentacrinoid larva of Antedon.(From Carpenter; after Wyville Thomson.)1. terminal plate of stem;cd.centro-dorsal plate; 3. basals; 4. radials;or.orals.
Fig. 270. Young Pentacrinoid larva of Antedon.(From Carpenter; after Wyville Thomson.)
1. terminal plate of stem;cd.centro-dorsal plate; 3. basals; 4. radials;or.orals.
After the above elements of the skeleton have become established the ciliated bands undergo atrophy, and shortly afterwardsthe larva becomes attached by the terminal plate of its stem. It then passes into the Pentacrinoid stage. The larva in this stage is shewn infig. 268C andfig. 270. New joints are added at the upper end of the stem next the calyx, and a new element—the radials—makes its appearance as a ring of five small plates, placed in the space between the basals and orals, and in the intervals alternating with them (fig. 270, 4). The roof of the oral vestibule (videfig. 253andp.551) has in the meantime become ruptured; and the external opening of the mouth thus becomes established. Surrounding the mouth are five petal-like lobes, each of them supported by an oral plate (fig. 268C). In the intervals between them five branched and highly contractile tentacles, which were previously enclosed within the vestibule, now sprout out: they mark the position of the future radial canals, and are outgrowths of the water-vascular ring. At the base of each of them a pair of additional tentacles is soon formed. Each primary tentacle corresponds to one of the radials. These latter are therefore, as their name implies, radial in position; while the basals and orals are interradial. In addition to the contractile radial tentacles ten non-contractile tentacles, also diverticula of the water-vascular ring, are soon formed, two for each interradius.
In the course of the further development the equatorial space between the orals and the basals enlarges, and gives rise to a wide oral disc, the sides of which are formed by the radials resting on the basals; while in the centre of it are placed the five orals, each with its special lobe.
The anus, which is formed on the ventral side in the positionof the blastopore (p.551), becomes surrounded by an anal plate, which is interradial in position, and lies on the surface of the oral disc between the orals and radials. On the oral plate in the next interradius is placed the opening of a single funnel leading into the body cavity, which Ludwig regards as equivalent to the opening of the madreporic canal (videp.551)[228].
From the edge of the vestibule the arms grow out, carrying with them the tentacular prolongation of the water-vascular ring. Two additional rows of radials are soon added.
The stalked Pentacrinoid larva becomes converted, on the absorption of the stalk, into the adult Antedon. The stalk is functionally replaced by a number of short cirri springing from the centro-dorsal plate. The five basals coalesce into a single plate, known as the rosette, and the five orals disappear, though the lobes on which they were placed persist. In some stalked forms,e.g.Rhizocrinus Hyocrinus, the orals are permanently retained. The arms bifurcate at the end of the third radial, and the first radial becomes in Antedon rosacea (though not in all species of Antedon) concealed from the surface by the growth of the centro-dorsal plate. An immense number of funnels, leading into the body cavity, are formed in addition to the single one present in the young larva. These are regarded by Ludwig as equivalent to so many openings of the madreporic canal; and there are developed, in correspondence with them, diverticula of the water-vascular ring.
Comparison of Echinoderm Larvæ and General Conclusions.
In any comparison of the various types of Echinoderm larvæ it is necessary to distinguish between the free-swimming forms, and the viviparous or fixed forms. A very superficial examination suffices to shew that the free-swimming forms agree very much more closely amongst themselves than the viviparousforms. We are therefore justified in concluding that in the viviparous forms the development is abbreviated and modified.
All the free forms are nearly alike in their earliest stage after the formation of the archenteron. The surface between the anus and the future mouth becomes flattened, and (except in Antedon, Cucumaria, Psolinus, etc. which practically have an abbreviated development like that of the viviparous forms) a ridge of cilia becomes established in front of the mouth, and a second ridge between the mouth and the anus. This larval form, which is shewn infig. 264A, is the type from which the various forms of Echinoderm larvæ start.
In all cases, except in Bipinnaria, the two ciliated ridges soon become united, and constitute a single longitudinal post-oral ciliated ring.
The larvæ in their further growth undergo various changes, and in the later stages they may be divided into two groups:
(1) The Pluteus larva of Echinoids and Ophiuroids.(2) The Auricularia (Holothuroids) and Bipinnaria (Asteroids) type.
The first group is characterized by the growth of a number of arms more or less surrounding the mouth, and supported by calcareous rods. The ciliated band retains its primitive condition as a simple longitudinal band throughout larval life. There is a very small præ-oral lobe, while an anal lobe is very largely developed.
Larva of Holothuroid and AsteriasFig. 271. A. The larva of a Holothuroid. B. The larva of an Asterias.m.mouth;st.stomach;a.anus;l.c.primitive longitudinal ciliated band;pr.c.præ-oral ciliated band.
Fig. 271. A. The larva of a Holothuroid. B. The larva of an Asterias.
m.mouth;st.stomach;a.anus;l.c.primitive longitudinal ciliated band;pr.c.præ-oral ciliated band.
The Auricularia and Bipinnaria resemble each other in shape, in the development of a large præ-oral lobe, and in the absence of provisional calcareous rods; but differ in the fact that the ciliated band is single in Auricularia (fig. 271A), and is double in Bipinnaria (fig. 271B).
The Bipinnaria larva shews a great tendency to develop soft arms; while in the Auricularia the longitudinal ciliatedband breaks up into a number of transverse ciliated bands. This condition is in some instances reached directly, and such larvæ undoubtedly approximate to the larvæ of Antedon, in which the uniformly ciliated condition is succeeded by one with four transverse bands, of which one is præ-oral.
All or nearly all Echinoderm larvæ are bilaterally symmetrical, and since all Echinodermata eventually attain a radial symmetry, a change necessarily takes place from the bilateral to the radial type.
In the case of the Holothurians and Antedon, and generally in the viviparous types, this change is more or less completely effected in the embryonic condition; but in the Bipinnaria and Pluteus types a radial symmetry does not become apparent till after the absorption of the larval appendages. It is a remarkable fact, which seems to hold for the Asteroids, Ophiuroids, Echinoids, and Crinoids, that the dorsal side of the larva is not directly converted into the dorsal disc of the adult; but the dorsal and right side becomes the adult dorsal or abactinal surface, while the ventral and left becomes the actinal or ventral surface.
It is interesting to note with reference to the larvæ of the Echinodermata that the various existing types of larvæ must have been formed after the differentiation of the existing groups of the Echinodermata; otherwise it would be necessary to adopt the impossible position that the different groups of Echinodermata were severally descended from the different types of larvæ. The various special appendages, etc. of the different larvæ have therefore a purely secondary significance; and their atrophy at the time of the passage of the larva into the adult, which is nothing else but a complicated metamorphosis, is easily explained.
Originally, no doubt, the transition from the larva to the adult was very simple, as it is at present in most Holothurians; but as the larvæ developed various provisional appendages, it became necessary that these should be absorbed in the passage to the adult state.
It would obviously be advantageous that their absorption should be as rapid as possible, since the larva in a state of transition to the adult would be in a very disadvantageousposition. The rapid metamorphosis, which we find in Asteroids, Ophiuroids, and Echinoids in the passage from the larval to the adult state, has no doubt arisen for this reason.
In spite of the varying provisional appendages possessed by Echinoderm larvæ it is possible, as stated above (p.574), to recognise a type of larva, of which all the existing Echinoderm larval forms are modifications. This type does not appear to me to be closely related to that of the larvæ of any group described in the preceding pages. It has no doubt certain resemblances to the trochosphere larva of Chætopoda, Mollusca, etc., but the differences between the two types are more striking than the resemblances. It firstly differs from the trochosphere larva in the character of the ciliation. Both larvæ start from the uniformly ciliated condition, but while the præ-oral ring is almost invariable, and a peri-anal ring very common in the trochosphere; in the Echinoderm larva such rings are rarely found; and even when present,i.e.the præ-oral ring of Bipinnaria and the terminal though hardly peri-anal patch of Antedon, do not resemble closely the more or less similar structures of the trochosphere. The two ciliated ridges (fig. 264A) common to all the Echinoderm larvæ, and subsequently continued into a longitudinal ring, have not yet been found in any trochosphere. The transverse ciliated rings of the Holothurian and Crinoid larvæ are of no importance in the comparison between the trochosphere larvæ and the larvæ of Echinodermata, since such rings are frequently secondarily developed.Cf.Pneumodermon and Dentalium amongst Mollusca.
In the character of the præ-oral lobe the two types again differ. Though the præ-oral lobe is often found in Echinoderm larvæ it is never the seat of an important (supra-œsophageal) ganglion and organs of special sense, as it invariably is in the trochosphere.
Nothing like the vaso-peritoneal vesicles of the Echinoderm larvæ has been found in the trochosphere; nor have the characteristic trochosphere excretory organs been found in the Echinoderm larvæ.
The larva which most nearly approaches those of the Echinodermata is the larva of Balanoglossus described in the next chapter.
Bibliography.
(542)Alex. Agassiz.Revision of the Echini.Cambridge, U.S. 1872‑74.(543)Alex. Agassiz. “North American Starfishes.”Memoirs of the Museum of Comparative Anatomy and Zoology at Harvard College,Vol.V.,No.1. 1877 (originally published in 1864).(544)J. Barrois.“Embryogénie de l’Asteriscus verruculatus.”Journal de l’Anat. et Phys.1879.(545)A. Baur.Beiträge zur Naturgeschichte d. Synapta digitata.Dresden, 1864.(546)H. G. Bronn.Klassen u. Ordnungen etc. Strahlenthiere,Vol.II.1860.(547)W. B. Carpenter. “Researches on the structure, physiology and development of Antedon.”Phil. Trans.CLVI.1866, andProceedings of the Roy. Soc.,No.166. 1876.(548)P. H. Carpenter. “On the oral and apical systems of the Echinoderms.”Quart. J. of Micr. Science,Vol.XVIII.andXIX.1878‑9.(549)A. Götte.“Vergleichende Entwicklungsgeschichte d. Comatula mediterranea.”Arch. für micr. Anat.,Vol.XII.1876.(550)R. Greeff.“Ueber die Entwicklung des Asteracanthion rubens vom Ei bis zur Bipinnaria u. Brachiolaria.”Schriften d. Gesellschaft zur Beförderung d. gesammten Naturwissenschaften zu Marburg,Bd.XII.1876.(551)R. Greeff.“Ueber den Bau u. die Entwicklung d. Echinodermen.”Sitz. d. Gesell. z. Beförderung d. gesam. Naturwiss. zu Marburg,No.4. 1879.(552)T. H. Huxley. “Report upon the researches of Müller into the anat. and devel. of the Echinoderms.”Ann. and Mag. of Nat. Hist., 2ndSer., Vol.VIII.1851.(553)KorenandDanielssen.“Observations sur la Bipinnaria asterigera.”Ann. Scien. Nat.,Ser.III., Vol.VII.1847.(554)KorenandDanielssen. “Observations on the development of the Starfishes.”Ann. and Mag. of Nat. Hist.,Vol.XX.1857.(555)A. Kowalevsky.“Entwicklungsgeschichte d. Holothurien.”Mém. Ac. Pétersbourg,Ser.VII., Tom.XI., No.6.(556)A. Krohn.“Beobacht. a. d. Entwick. d. Holothurien u. Seeigel.”Müller’sArchiv, 1851.(557)A. Krohn.“Ueb. d. Entwick. d. Seesterne u. Holothurien.”Müller’sArchiv, 1853.(558)A. Krohn.“Beobacht. üb. Echinodermenlarven.”Müller’sArchiv, 1854.(559)H. Ludwig.“Ueb. d. primar. Steinkanal d. Crinoideen, nebst vergl. anat. Bemerk. üb. d. Echinodermen.”Zeit. f. wiss. Zool.,Vol.XXXIV.1880.(560)E. Metschnikoff.“Studien üb. d. Entwick. d. Echinodermen u. Nemertinen.”Mém. Ac. Pétersbourg, SeriesVII., Tom.XIV., No.8. 1869.(561)[229]Joh. Müller.“Ueb. d. Larven u. d. Metamorphose d. Echinodermen.”Abhandlungen d. Berlin. Akad.(Five Memoirs), 1848, 49, 50, 52 (two Memoirs).(562)Joh. Müller.“Allgemeiner Plan d. Entwicklung d. Echinodermen.”Abhandl. d. Berlin. Akad.,1853.(563)E. Selenka.“Zur Entwicklung d. Holothurien.”Zeit. f. wiss. Zool.,Bd.XXVII.1876.(564)E. Selenka.“Keimblätter u. Organanlage bei Echiniden.”Zeit. f. wiss. Zool.,Vol.XXXIII.1879.(565)Sir Wyville Thomson. “On the Embryology of the Echinodermata.”Natural History Review, 1864.(566)Sir Wyville Thomson. “On the Embryogeny of Antedon rosaceus.”Phil. Trans.1865.
[217]The following classification of the Echinodermata is employed in this chapter:I.Holothuroidea.II.Asteroidea.III.Ophiuroidea.IV.Echinoidea.V.Crinoidea.[218]The origin of the vaso-peritoneal vesicle is not quite the same in all the species. In Holothuria tubulosa it is separated from the cæcal end of the archenteron; the remainder of which then grows towards the oral invagination. In Cucumaria the archenteron forks (fig. 249); and one fork forms the vaso-peritoneal vesicle, and the other the major part of the mesenteron.[219]There appears to be some uncertainty as to how much of the larval œsophagus is derived from the stomodæal invagination.[220]VideP. H. Carpenter, “On the genus Actinometra.”Linnean Trans., 2nd Series, Zoology,Vol.II., PartI.,1879.[221]The exact position of the madreporic tubercle in relation to the abactinal plates does not seem to have been made out. It might have been anticipated that it would be placed in one of the primary interradial plates, but this does not seem to be the case. The position of the anus is also obscure.[222]The following statements are taken from the abstract in Bronn’sThierreichs.[223]Whether interradial plates are developed as in Asterias is not clear. They seem to be found in Ophiopholis bellis, Agassiz, but have not been recognised in other forms (videCarpenter,No.548, p.369).[224]Videespecially Müller, Agassiz, and Metschnikoff.[225]For viviparous EchinivideAgassiz,Proc. Amer. Acad. 1876.[226]Götte (No.549) supported by Müller’s and Krohn’s older, and in some points extremely erroneous observations, has enunciated the view that the radial canals in Echinoids and Holothuroids have a different nature from those in Asteroids and Ophiuroids.[227]Götte (No.549) on the other hand holds that the centro-dorsal plate is developed by the coalescence of a series of at first independent rods, which originate simultaneously with, and close to, the lower edges of the basals, and that it is therefore similar in its origin to the basals.[228]I have made no attempt to discuss the homologies of the plates of the larval Echinodermata because the criteria for such a discussion are still in dispute. The suggestive memoirs of P. H. Carpenter (No.548) on this subject may be consulted by the reader. Carpenter attempts to found his homologies on the relation of the plates to the primitive peritoneal vesicles, and I am inclined to believe that this method of dealing with these homologies is the right one. Ludwig (No.559) by regarding the opening of the madreporic canal as a fixed point has arrived at very different results.[229]The dates in this reference are the dates of publication.
[217]The following classification of the Echinodermata is employed in this chapter:
I.Holothuroidea.
II.Asteroidea.
III.Ophiuroidea.
IV.Echinoidea.
V.Crinoidea.
[218]The origin of the vaso-peritoneal vesicle is not quite the same in all the species. In Holothuria tubulosa it is separated from the cæcal end of the archenteron; the remainder of which then grows towards the oral invagination. In Cucumaria the archenteron forks (fig. 249); and one fork forms the vaso-peritoneal vesicle, and the other the major part of the mesenteron.
[219]There appears to be some uncertainty as to how much of the larval œsophagus is derived from the stomodæal invagination.
[220]VideP. H. Carpenter, “On the genus Actinometra.”Linnean Trans., 2nd Series, Zoology,Vol.II., PartI.,1879.
[221]The exact position of the madreporic tubercle in relation to the abactinal plates does not seem to have been made out. It might have been anticipated that it would be placed in one of the primary interradial plates, but this does not seem to be the case. The position of the anus is also obscure.
[222]The following statements are taken from the abstract in Bronn’sThierreichs.
[223]Whether interradial plates are developed as in Asterias is not clear. They seem to be found in Ophiopholis bellis, Agassiz, but have not been recognised in other forms (videCarpenter,No.548, p.369).
[224]Videespecially Müller, Agassiz, and Metschnikoff.
[225]For viviparous EchinivideAgassiz,Proc. Amer. Acad. 1876.
[226]Götte (No.549) supported by Müller’s and Krohn’s older, and in some points extremely erroneous observations, has enunciated the view that the radial canals in Echinoids and Holothuroids have a different nature from those in Asteroids and Ophiuroids.
[227]Götte (No.549) on the other hand holds that the centro-dorsal plate is developed by the coalescence of a series of at first independent rods, which originate simultaneously with, and close to, the lower edges of the basals, and that it is therefore similar in its origin to the basals.
[228]I have made no attempt to discuss the homologies of the plates of the larval Echinodermata because the criteria for such a discussion are still in dispute. The suggestive memoirs of P. H. Carpenter (No.548) on this subject may be consulted by the reader. Carpenter attempts to found his homologies on the relation of the plates to the primitive peritoneal vesicles, and I am inclined to believe that this method of dealing with these homologies is the right one. Ludwig (No.559) by regarding the opening of the madreporic canal as a fixed point has arrived at very different results.
[229]The dates in this reference are the dates of publication.
The larva of Balanoglossus is known as Tornaria. The præ-larval development is not known, and the youngest stage (fig. 272) so far described (Götte,No.569) has many remarkable points of resemblance to a young Bipinnaria.
Early stage of TornariaFig. 272. Early stage in the development of Tornaria.(After Götte.)W.so-called water-vascular vesicle developing as an outgrowth of the mesenteron;m.mouth;an.anus.
Fig. 272. Early stage in the development of Tornaria.(After Götte.)
W.so-called water-vascular vesicle developing as an outgrowth of the mesenteron;m.mouth;an.anus.
A mouth (m), situated on the ventral surface, leads into an alimentary canal with a terminal anus (an). A præ-oral lobe is well developed, as in Bipinnaria, but there is no post-anal lobe. The bands of cilia have the same general form as in Bipinnaria. There is a præ-oral band, and a longitudinal post-oral band; and the two bands nearly meet at the apex of the præ-oral lobe (fig. 273). A contractile band passes from the œsophagus to the apex of the præ-oral lobe, and a diverticulum (fig. 272,W) from the alimentary tract, directed towards the dorsal surface, is present. Contractile cells are scattered in the space between the body wall and the gut.
In the following stage (fig. 274A) a conspicuous transverse post-oral band of a single row of long cilia is formed, and the original bands become more sinuous. The alimentary diverticulum of the last stage becomes an independent vesicle opening by a pore on the dorsal surface (fig. 274A,w). The contractile cord is now inserted on this vesicle. Where this cord joins the apex of the præ-oral lobe between the two anterior bands of cilia a thickening of the epiblast (? a ganglion) has becomeestablished, and on it are placed two eye-spots (fig. 273oc, andfig. 274A). A deep bay is formed on the ventral surface of the larva.
Young TornariaFig. 273. Young Tornaria.(After Müller.)m.mouth;an.anus;w.water-vascular vesicle;oc.eye-spots;c.c.contractile cord.
Fig. 273. Young Tornaria.(After Müller.)
m.mouth;an.anus;w.water-vascular vesicle;oc.eye-spots;c.c.contractile cord.
Two stages of TornariaFig. 274. Two stages in the development of Tornaria.(After Metschnikoff.)The black lines represent the ciliated bands.m.mouth;an.anus;br.branchial cleft;ht.heart;c.body cavity between splanchnic and somatic mesoblast layers;w.water-vascular vesicle;v.circular blood-vessel.
Fig. 274. Two stages in the development of Tornaria.(After Metschnikoff.)
The black lines represent the ciliated bands.m.mouth;an.anus;br.branchial cleft;ht.heart;c.body cavity between splanchnic and somatic mesoblast layers;w.water-vascular vesicle;v.circular blood-vessel.
As the larva grows older the original bands of cilia become more sinuous, and a second transverse band with small cilia is formed (in the Mediterranean larva) between the previous transverse band and the anus. The water-vascular vesicle is prolonged into two spurs, one on each side of the stomach. A pulsating vesicle or heart is also formed (fig. 274B,ht), and arises, according to Spengel (No.572), as a thickening of the epidermis. It subsequently becomes enveloped in a pericardium, and is placed in a depression in the water-vascular vesicle. Two pairs of diverticula, one behind the other, grow out (Agassiz,No.568) from the gastric region of the alimentary canal. The two parts of each pair form flattened compartments, which together give rise to a complete investment of the adjoining parts of the alimentary tract. The two parts of each coalesce, and thus forma double-walled cylinder round the alimentary tract, but their cavities remain separated by a dorsal and ventral septum.
Eventually (Spengel) the cavity of the anterior cylinder forms the section of the body cavity in the collar of the adult, and that of the posterior (fig. 274B,c) the remainder of the body cavity. The septa, separating the two halves of each, remain as dorsal and ventral mesenteries.
The conversion of Tornaria (fig. 274A) into Balanoglossus (fig. 274B) is effected in a few hours, and consists mainly in certain changes in configuration, and in the disappearance of the longitudinal ciliated band.
The body of the young Balanoglossus (fig. 274B) is divided into three regions (1) the proboscidian region, (2) the collar, (3) the trunk proper. The proboscidian region is formed by the elongation of the præ-oral lobe into an oval body with the eye-spots at its extremity, and provided with strong longitudinal muscles. The heart (ht) and water-vascular vesicle lie near its base, but the contractile cord connected with the latter is no longer present. The mouth is placed on the ventral side at the base of the præ-oral lobe, and immediately behind it is the collar. The remainder of the body is more or less conical, and is still girt with the larval transverse ciliated band, which lies in the middle of the gastric region in the Mediterranean species, but in the œsophageal region in the American one.
The whole of the body, including the proboscis, becomes richly ciliated.
One of the most important characters of the adult Balanoglossus consists in the presence of respiratory structures comparable with the vertebrate gill slits. The earliest traces of these structures are distinctly formed while the larva is still in the Tornariacondition, as one pair of pouches from the œsophagus in the Mediterranean species, and four pairs in the American one (fig. 275,br).
Late state of BalanoglossusFig. 275. Late stage in the development of Balanoglossus with four branchial clefts.(After Alex. Agassiz.)m.mouth;an.anus;br.branchial cleft;ht.heart;W.water-vascular vesicle.
Fig. 275. Late stage in the development of Balanoglossus with four branchial clefts.(After Alex. Agassiz.)
m.mouth;an.anus;br.branchial cleft;ht.heart;W.water-vascular vesicle.
In the Mediterranean Tornaria the two pouches meet the skin dorsally, and in the young Balanoglossus (fig. 274B,br) acquire an external opening on the dorsal side. In the American species the first four pouches are without external openings till additional pouches have been formed. Fresh gill pouches continue to be formed both in the American and probably the Mediterranean species, but the conversion of the simple pouches into the complicated gill structure of the adult has only been studied by Agassiz (No.568) in the American species. It would seem in the first place that the structure of the adult gill slits is much less complicated in the American than in the Mediterranean species. The simple pouches of the young become fairly numerous. They are at first circular; they then become elliptical, and the dorsal wall of each slit becomes folded; subsequently fresh folds are formed which greatly increase the complexity of the gills. The external openings are not acquired till comparatively late.
Our knowledge of the development of the internal organs, mainly derived from Agassiz, is still imperfect. The vascular system appears early in the form of a dorsal and a ventral vessel, both pointed, and apparently ending blindly at their two extremities. The two spurs of the water-vascular vesicle, which in the Tornaria stage rested upon the stomach, now grow round the œsophagus, and form an anterior vascular ring, which Agassiz describes as becoming connected with the heart, though it still communicates with the exterior by the dorsal pore and seems to become connected with the remainder of the vascular system. According to Spengel (No.572) the dorsal vessel becomes connected with the heart, which remains through life in the proboscis: the cavity of the water-vascular vesicle forms the cavity of the proboscis in the adult, and its pore remains as a dorsal (not, as usually stated, ventral) pore leading to the exterior.
The eye-spots disappear.
Tornaria is a very interesting larval form, since it is intermediate in structure between the larva of an Echinoderm and trochosphere type common to the Mollusca, Chætopoda, etc. The shape of the body especially the form of the ventral depression, the character of the longitudinal ciliated band, the structure and derivation of the water-vascular vesicle, and theformation of the walls of the body cavity as gastric diverticula, are all characters which point to a connection with Echinoderm larvæ.
On the other hand the eye-spots at the end of the præ-oral lobe[230], the contractile band passing from the œsophagus to the eye-spots (fig. 273), the two posterior bands of cilia, and the terminal anus are all trochosphere characters.
The persistence of the præ-oral lobe as the proboscis is interesting, as tending to shew that Balanoglossus is the surviving representative of a primitive group.
Bibliography.
(567)A. Agassiz. “Tornaria.”Ann. Lyceum Nat. Hist.VIII.New York, 1866.(568)A. Agassiz. “The History of Balanoglossus and Tornaria.”Mem. Amer. Acad. of Arts and Scien.,Vol.IX.1873.(569)A. Götte.“Entwicklungsgeschichte d. Comatula Mediterranea.”Archiv für mikr. Anat.,Bd.XII.,1876, p. 641.(570)E. Metschnikoff.“Untersuchungen üb d. Metamorphose, etc. (Tornaria).”Zeit. für wiss. Zool.,Bd.XX.1870.(571)J. Müller.“Ueb. d. Larven u. Metamor. d. Echinodermen.”Berlin Akad.,1849 and 1850.(572)J. W. Spengel.“Bau u. Entwicklung von Balanoglossus.”Tagebl. d. Naturf. Vers. München,1877.
[230]It would be interesting to have further information about the fate of the thickening of epiblast in the vicinity of the eye-spots. The thickening should by rights be the supra-œsophageal ganglion, and it does not seem absolutely impossible that it may give rise to the dorso-median cord in the region of the collar, which constitutes, according to Spengel, the main ganglion of the adult.
[230]It would be interesting to have further information about the fate of the thickening of epiblast in the vicinity of the eye-spots. The thickening should by rights be the supra-œsophageal ganglion, and it does not seem absolutely impossible that it may give rise to the dorso-median cord in the region of the collar, which constitutes, according to Spengel, the main ganglion of the adult.