Diagrammatic section of CymothoaFig. 242. Diagrammatic section of Cymothoa shewing the dorsal organ.(From Bullar.)
Fig. 242. Diagrammatic section of Cymothoa shewing the dorsal organ.(From Bullar.)
In Cymothoa (Bullar,No.499) there appears on the dorsal surface, in the region which afterwards becomes the first thoracic segment, an unpaired linear thickening of the blastoderm. This soon becomes a circular patch, the central part of which is invaginated so as to communicate with the exterior by a narrow opening only (fig. 242). It becomes at the same time attached to the inner egg membrane. It retains this condition till the close of larval life.
In Oniscus (Dohrn,No.500; Bobretzky,No.498) there appears very early a dorsal patch of thickened cells. These cells become attached at their edge to the inner egg membrane and gradually separated from the embryo, with which they finally only remain in connection by a hollow column of cells (fig. 241A,do). The original patch now gradually spreads over the inner egg membrane, and forms a transverse saddle-shaped band of flattened cells which engirths the embryo on all but the ventral side.
In the Amphipods the epiblast cells remain attached for a small area on the dorsal surface to the first larval skin, when this is formed. This patch of cells, often spoken of as a micropyle apparatus, forms a dorsal organ equivalent to that in Oniscus. A perforation is formed in it at a laterperiod. A perhaps homologous structure is found in the embryos of Euphausia, Cuma, etc.
Section of an embryo of Asellus AquaticusFig. 243. Diagrammatic Section of an embryo of Asellus Aquaticus To Shew the Paired Dorsal Organ.(From Bullar; after E. van Beneden.)
Fig. 243. Diagrammatic Section of an embryo of Asellus Aquaticus To Shew the Paired Dorsal Organ.(From Bullar; after E. van Beneden.)
In many Branchiopoda a dorsal organ is found. Its development has been studied by Grobben in Moina. It persists in the adult in Branchipus, Limnadia, Estherea, etc.
In the Copepoda a dorsal organ is sometimes found in the embryo; Grobben at any rate believes that he has detected an organ of this nature in the embryo of Cyclops serrulatus.
A paired organ which appears to be of the same nature has been found in Asellus and Mysis.
In Asellus (Rathke (No.501), Dohrn (No.500), Van Beneden (No.497)) this organ originates as two cellular masses at the sides of the body just behind the region of the procephalic lobes. Each of them becomes trifoliate and bends towards the ventral surface. In each of their lobes a cavity arises and finally the three cavities unite, forming a trilobed cavity open to the yolk. This organ eventually becomes so large that it breaks through the egg membranes and projects at the sides of the embryo (fig. 243). Though formed before the appendages it does not attain its full development till considerably after the latter have become well established.
In Mysis it appears during the Nauplius stage as a pair of cavities lined by columnar cells, which atrophy very early.
Various attempts have been made to identify organs in other Arthropod embryos with the dorsal organ of the Crustacea, but the only organ at all similar which has so far been described is one found in the embryo of Linguatula (videChapterXIX.), but there is no reason to think that this organ is really homologous with the dorsal organ of the Crustacea.
The mesoblast.The mesoblast in the types so far investigated arises from the same cells as the hypoblast, and appears as a somewhat irregular layer between the epiblast and the hypoblast. It gives rise to the same parts as in other forms, but it is remarkable that it does not, in most Decapods and Isopods(and so far we do not know about other forms), become divided into somites, at any rate with the same distinctness that is usual in Annelids and Arthropods. Not only so, but there is at first no marked division into a somatic and splanchnic layer with an intervening body cavity. Some of the cells become differentiated into the muscles of the body wall and limbs; and other cells, usually in the form of a very thin layer, into the muscles of the alimentary tract. In the tail ofPalæmonBobretzky noticed that the cells about to form the muscles of the body were imperfectly divided into cubical masses corresponding with the segments; which however, in the absence of a central cavity, differed from typical mesoblastic somites. InMysisMetschnikoff states that the mesoblast becomes broken up into distinct somites. Further investigations on this subject are required. The body cavity has the form of irregular blood sinuses amongst the internal organs.
Heart.The origin and development of the heart and vascular system are but very imperfectly known.
In Phyllopods (Branchipus) Claus (No.454) has shewn that the heart is formed by the coalescence of the lateral parts of the mesoblast of the ventral plates. The chambers are formed successively as the segments to which they belong are established, and the anterior chambers are in full activity while the posterior are not yet formed.
In Astacus and Palæmon, Bobretzky finds that at the stage before the heart definitely appears there may be seen a solid mass of mesoblast cells in the position which it eventually occupies[209]; and considers it probable that the heart originates from this mass. At the time when the heart can first be made out and before it has begun to beat, it has the form of an oval sack with delicate walls separated from the mesenteron by a layer of splanchnic mesoblast. Its cavity is filled with a peculiar plasma which also fills up the various cavities in the mesoblast. Around it a pericardial sack is soon formed, and the walls of the heart become greatly thickened. Four bands pass off from the heart, two dorsalwards which become fixed to the integument, and two ventralwards. There is also a median band of cells connecting the heart with the dorsal integument. The main arteries arise as direct prolongations of the heart. Dohrn’s observations on Asellus greatly strengthen the view that the heart originates from a solid mesoblastic mass, in that he was able to observe the hollowing out of the mass inthe living embryo (cf.the development of the heart in Spiders). Some of the central cells (nuclei, Dohrn) become blood corpuscles. The formation of these is not, according to Dohrn, confined to the heart, but takes placein situin all the parts of the body (antennæ, appendages, etc.). The corpuscles are formed as free nuclei and are primarily derived from the yolk, which at first freely communicates with the cavities of the appendages.
Alimentary tract.In Astacus the formation of the mesenteron by invagination, and the absorption of the yolk by the hypoblast cells, have already been described. On the absorption of the yolk the mesenteron has the form of a sack, the walls of which are formed of immensely long cells—the yolk pyramids—at the base of which the nucleus is placed (fig. 238B). This sack gives rise both to the portion of the alimentary canal between the abdomen and the stomach and to the liver. The epithelial wall of both of these parts is formed by the outermost portions of the pyramids with the nuclei and protoplasm becoming separated off from the yolk as a layer of flat epithelial cells. The yolk then breaks up and forms a mass of nutritive material filling up the cavity of the mesenteron.
The differentiation both of the liver and alimentary tract proper first takes place on the ventral side, and commences close to the point where the proctodæum ends, and extends forward from this point. A layer of epithelial cells is thus formed on the ventral side of the mesenteron which very soon becomes raised into a series of longitudinal folds, one of which in the middle line is very conspicuous. The median fold eventually, by uniting with a corresponding fold on the dorsal side, gives rise to the true mesenteron; while the lateral folds form parallel hepatic cylinders, which in front are not constricted off from the alimentary tract. The lateral parts of the dorsal side of the mesenteron similarly give rise to hepatic cylinders. The yolk pyramids of the anterior part of the mesenteron, which projects forwards as a pair of diverticula on each side to the level of the stomach, are not converted into hepatic cylinders till after the larva is hatched.
The proctodæum very early opens into the mesenteron, but the stomodæum remains closed till the differentiation of the mid-gut is nearly completed. The proctodæum gives rise to the abdominal part of the intestine, and the stomodæum to the œsophagus and stomach. The commencement of the masticatory apparatus in the latter appears very early as a dorsal thickening of the epithelium.
The primitive mesenteron in Palæmon differentiates itself into the permanent mid-gut and liver in a manner generally similar to that in Astacus, though the process is considerably less complicated. A distinct layer of cells separates itself from the outer part of the yolk pyramids, and gives rise to the glandular lining both of the mid-gut and of the liver. The differentiation of this layer commences behind, and the mid-gut very soon communicates freely with the proctodæum. The lateral parts of the primitive mesenteron become constricted into four wings, two directed forwards and two backwards; these, after the yolk in them has become absorbed, constitute the liver. The median part simply becomes the mesenteron.The stomachic end of the stomodæum lies in contact with the mesenteron close to the point where it is continued into the hepatic diverticula, and, though the partition wall between the two becomes early very thin, a free communication is not established till the yolk has been completely absorbed.
The alimentary tract in the Isopoda is mainly if not entirely formed from the proctodæum and stomodæum, both of which arise before any other part of the alimentary system as epiblastic invaginations, and gradually grow inwards (fig. 244). In Oniscus the liver is formed as two discs at the surface of the yolk on each side of the anterior part of the body. Their walls are composed of cubical cells derived from the yolk cells, the origin of which was spoken of onp.516. These two discs gradually take the form of sacks (fig. 244B,li.) freely open on their inner side to the yolk. As these sacks continue to grow the stomodæum and proctodæum do not remain passive. The stomodæum, which gives rise to the œsophagus and stomach of the adult, soon exhibits a posterior dilatation destined to become the stomach, on the dorsal wall of which a well-marked prominence—the earliest trace of the future armature—is soon formed (fig. 244B,zp). The proctodæum (pr) grows with much greater rapidity than the stomodæum, and its end adjoining the yolk becomes extremely thin or even broken through. In the earliest stages it was surrounded by the yolk cells, but in its later growth the yolk cells become gradually reduced in number and appear to recede before it—so much so that one is led to conclude that the later growth of the proctodæum takes place at the expense of the yolk cells.
Longitudinal sections through the embryo of Oniscus murariusFig. 244. Two longitudinal sections through the embryo of Oniscus murarius.(After Bobretzky.)st.stomodæum;pr.proctodæum;hy.hypoblast formed of large nucleated cells imbedded in yolk;m.mesoblast;vg.ventral nerve cord;sg.supra-œsophageal ganglion;li.liver;do.dorsal organ;zp.rudiment of masticatory apparatus.
Fig. 244. Two longitudinal sections through the embryo of Oniscus murarius.(After Bobretzky.)
st.stomodæum;pr.proctodæum;hy.hypoblast formed of large nucleated cells imbedded in yolk;m.mesoblast;vg.ventral nerve cord;sg.supra-œsophageal ganglion;li.liver;do.dorsal organ;zp.rudiment of masticatory apparatus.
The liver sacks become filled with a granular material without a trace of cells; their posterior wall is continuous with the yolk cells, and their anterior lies close behind the stomach. The proctodæum continually grows forwards till it approaches close to the stomodæum, and the twoliver sacks, now united into one at their base, become directly continuous with the proctodæum. By the stage when this junction is effected the yolk cells have completely disappeared. It seems then that in Oniscus the yolk cells (hypoblast) are mainly employed in giving rise to the walls of the liver; but that they probably also supply the material for the later growth of the apparent proctodæum. It becomes therefore necessary to conclude that the latter, which might seem, together with the stomodæum, to form the whole alimentary tract, does in reality correspond to the proctodæum and mesenteron together, though the digestive fluids are no doubt mainly secreted not in the mesenteron but in the hepatic diverticula. The proctodæum and stomodæum at first meet each other without communicating, but before long the partition between the two is broken through.
In Cymothoa (Bullar,No.499) the proctodæum and stomodæum develop in the same manner as in Oniscus, but the hypoblast has quite a different form. The main mass of the yolk, which is much greater than in Oniscus, is not contained in definite yolk cells, but the hypoblast is represented by (1) two solid masses of cells, derived apparently from the inner layer of blastoderm cells, which give rise to the liver; and (2) by a membrane enclosing the yolk in which nuclei are present.
The two hepatic masses lie on the surface of the yolk, and each of them becomes divided into three short cæcal tubes freely open to the yolk. The stomodæum soon reaches its full length, but the proctodæum grows forwards above the yolk till it meets the stomodæum. By the time this takes place the liver cæca have grown into three large tubes filled with fluid, and provided with a muscular wall. They now lie above the yolk, and no longer communicate directly with the cavity of the yolk-sack, but open together with the yolk-sack into the point of junction of the proctodæum and stomodæum. The yolk-sack of Cymothoa no doubt represents part of the mesenteron, but there is no evidence in favour of any part of the apparent proctodæum representing it also, though it is quite possible that it may do so. The relations of the yolk-sack and hepatic diverticula in Cymothoa appear to hold good for Asellus and probably for most Isopoda.
The differences between the Decapods and Isopods in the development of the mesenteron are not inconsiderable, but they are probably to be explained by the relatively larger amount of food-yolk in the latter forms. The solid yolk in the Isopods on this view represents the primitive mesenteron of Decapods after the yolk has been absorbed by the hypoblast cells. Starting from this standpoint we find that in both groups the lateral parts of the mesenteron become the liver. In Decapods the middle part becomes directly converted into the mid-gut, the differentiation of it commencing behind and proceeding forwards. In the Isopods, owing to the mesenteron not having a distinct cavity, the differentiation of it, which proceeds forwards as in Decapods, appears simply like a prolongation forwards of the proctodæum, the cells for the prolongation being probably supplied from the yolk. In Cymothoa the food-yolk is so bulky that a special yolk-sack is developedfor its retention, which is not completely absorbed till some time after the alimentary canal has the form of a continuous tube. The walls of this yolk-sack are morphologically a specially developed part of the mesenteron.
Bibliography.
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(516)C. Spence Bate. “On the development of the Cirripedia.”Annals and Mag. of Natur. History.Second Series,VIII.,1851.(517)E. van Beneden.“Développement des Sacculines.”Bull. de l’Acad. roy. de Belg.,1870.(518)C. Claus.Die Cypris-ähnliche Larve der Cirripedien.Marburg, 1869.(519)Ch. Darwin.A monograph of the sub-class Cirripedia, 2Vols., Ray Society, 1851‑4.(520)A. Dohrn.“Untersuchungen über Bau u. Entwicklung d. ArthropodenIX.Eine neue Naupliusform (Archizoëa gigas).”Zeit. f. wiss. Zool.,Bd.XX.,1870.(521)P. P. C. Hoek.“Zur Entwicklungsgeschichte der EntomostrakenI.Embryologie von Balanus.”Niederländisches Archiv für Zoologie,Vol.III., 1876‑7.(522)R. Kossmann.“Suctoria u. Lepadidæ.”Arbeiten a. d. zool.-zoot. Institute d. Univer. Würz.,Vol.I., 1873.(523)Aug. Krohn.“Beobachtungen über die Entwicklung der Cirripedien.”Wiegmann’sArchiv für Naturgesch.,XXVI.,1860.(524)E. Metschnikoff.Sitzungsberichte d. Versammlung deutscher Naturforscher zu Hannover, 1865. (Balanus balanoides.)(525)Fritz Müller.“Die Rhizocephalen.”Archiv f. Naturgeschichte,1862‑3.(526)F. C. Noll.“Kochlorine hamata, ein bohrendes Cirriped.”Zeit. f. wiss. Zool.,Bd.XXV.,1875.(527)A. Pagenstecher.“Beiträge zur Anatomie und Entwicklungsgeschichte von Lepas pectinata.”Zeit. f. wiss. Zool.,Vol.XIII., 1863.(528)J. V. Thompson.Zoological Researches and Illustrations,Vol.I., PartI.MemoirIV.On the Cirripedes or Barnacles.8vo. Cork, 1830.(529)J. V. Thompson. “Discovery of the Metamorphosis in the second type of the Cirripedes,viz.the Lepades completing the natural history of these singular animals, and confirming their affinity with the Crustacea.”Phil. Trans.1835. PartII.(530)R. von Willemoes Suhm. “On the development of Lepas fascicularis.”Phil. Trans.,Vol.166, 1876.
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(531)C. Claus.“Zur näheren Kenntniss der Jugendformen von Cypris ovum.”Zeit. f. wiss. Zool.,Bd.XV.,1865.(532)C. Claus.“Beiträge zur Kenntniss d. Ostracoden. Entwicklungsgeschichte von Cypris ovum.”Schriften d. Gesell. zur Beförderung d. gesamm. Naturwiss. zu Marburg,Vol.IX., 1868.
[185]The following is the classification of the Crustacea employed in the present chapter:I.Branchiopoda.Phyllopoda.Cladocera.II.Malacostraca.Nebaliadæ.Schizopoda.Decapoda.Stomatopoda.Cumaceæ.Edriophthalmata.III.Copepoda.Eucopepoda.Natantia.Parasita.Branchiura.IV.Cirripedia.Thoracica.Abdominalia.Apoda.Rhizocephala.V.Ostracoda.[186]The importance of the larval history of the Crustacea, coupled with our comparative ignorance of the formation of the layers, has rendered it necessary for me to diverge somewhat from the general plan of the work, and to defer the account of the formation of the layers till after that of the larval forms.[187]Nothing appears to be known with reference to the manner in which it comes about that more than one appendage is borne on each of the segments from the eleventh to the twentieth. An investigation of this point would be of some interest with reference to the meaning of segmentation.[188]The doubts which have been thrown upon Müller’s observations appear to be quite unfounded.[189]From Claus’ observations (No.448) it would appear that the respiratory plate is only the exopodite and not, as is usually stated, the coalesced exopodite and epipodite. Huxley in hisComparative Anatomyreserves this point for embryological elucidation.[190]Fritz Müller has recently (Zoologischer Anzeiger,No.52) described a still more abbreviated development of a Palæmon living in brooks near Blumenau.[191]It has been clearly demonstrated that the majority of land crabs leave the egg in the Zoæa form.[192]These five maxillipeds correspond with the three maxillipeds and two anterior ambulatory appendages of the Decapoda.[193]The observations of Brooks (No.493) render it probable that the Alima larva leaves the egg in a form not very dissimilar to the youngest known larva.[194]His paper is unfortunately in Russian.[195]The Pontellidæ form an exception to this statement, in that they are provided with paired lateral eyes in addition to the median one.[196]The term Nauplius was applied to the larva of Cyclops and allied organisms by O. F. Müller under the impression that they were adult forms.[197]Van Beneden (No.506) in the genera investigated by him finds that the two maxillipeds are really distinct pairs of appendages.[198]It seems not impossible that the appendage regarded by Claus as the mandibular palp may really represent the maxilla, which would otherwise seem to be absent. This mode of interpretation would bring the appendages of Argulus into a much closer agreement with those of the parasitic Copepoda. It does not seem incompatible with the existence of the stylet-like maxillæ detected by Claus in the adult.[199]Alepas squalicola is stated by Koren and Danielssen to form an exception to this rule, and to leave the egg with six pairs of appendages.[200]Willemoes Suhm (No.530) states that the mouth is situated at the free end of the upper lip, and that the œsophagus passes through it. From an examination of some specimens of this Nauplius, for which I am indebted to Moseley, I am inclined to think that this is a mistake, and that a groove on the surface of the upper lip has been taken by Suhm for the œsophagus.[201]The enormous spinous development of the larva of Lepas fascicularis is probably to be explained as a secondary protective adaptation, and has no genetic connection with the somewhat similar spinous armature of the Zoæa.[202]There is considerable confusion about the shell-gland and antennary gland. In my account Willemoes Suhm has been followed. Claus however regards what I have called the antennary gland as the shell-gland, and states that it does not open into the antennæ till a later period. He does not clearly describe its opening, nor the organ which I have called the shell-gland.[203]For the characters of Naupliusvidep.460.[204]Claus speaks of the various Crustacean phyla as having sprung from a Protophyllopod form, and it might be supposed that he considered that they all diverged from the same form. It is clear however from the context that he regards the Protophyllopod type from which the Malacostraca originated as far more like existing Phyllopods than that from which the Entomostracan groups have sprung. It is not quite easy to get a consistent view of his position on the question, since he states (p.77) that the Malacostraca and the Copepods diverged from a similar form, which is represented in their respective developments by the Protozoæa and earliest Cyclops stage. Yet if I understand him rightly, he does not consider the Protozoæa stage to be the Protophyllopod stage from which the Malacostraca have diverged, but states onp.71 that it was not an ancestral form at all.[205]Claus appears to consider it doubtful whether the Malacostracan gills can be compared with the Phyllopod gill pouches.[206]Facts for Darwin,p.49.[207]A secondary larval form is less likely to be repeated in development than an ancestral adult stage, because there is always a strong tendency for the former, which is a secondarily intercalated link in the chain, to drop out by the occurrence of a reversion to the original type of development.[208]Bobretzky first stated that the invagination remained open, but subsequently corrected himself.Zeit. f. Wiss. Zool.,Bd.XXIV.p.186.[209]Reichenbach describes these cells, and states that there is a thickening of the epiblast adjoining them. In one place he states that the heart arises from this thickening of epiblast, and in another that it arises from the mesoblast. An epiblastic origin of the heart is extremely improbable.
[185]The following is the classification of the Crustacea employed in the present chapter:
I.Branchiopoda.
Phyllopoda.
Cladocera.
II.Malacostraca.
Nebaliadæ.
Schizopoda.
Decapoda.
Stomatopoda.
Cumaceæ.
Edriophthalmata.
III.Copepoda.
Eucopepoda.
Natantia.
Parasita.
Branchiura.
IV.Cirripedia.
Thoracica.
Abdominalia.
Apoda.
Rhizocephala.
V.Ostracoda.
[186]The importance of the larval history of the Crustacea, coupled with our comparative ignorance of the formation of the layers, has rendered it necessary for me to diverge somewhat from the general plan of the work, and to defer the account of the formation of the layers till after that of the larval forms.
[187]Nothing appears to be known with reference to the manner in which it comes about that more than one appendage is borne on each of the segments from the eleventh to the twentieth. An investigation of this point would be of some interest with reference to the meaning of segmentation.
[188]The doubts which have been thrown upon Müller’s observations appear to be quite unfounded.
[189]From Claus’ observations (No.448) it would appear that the respiratory plate is only the exopodite and not, as is usually stated, the coalesced exopodite and epipodite. Huxley in hisComparative Anatomyreserves this point for embryological elucidation.
[190]Fritz Müller has recently (Zoologischer Anzeiger,No.52) described a still more abbreviated development of a Palæmon living in brooks near Blumenau.
[191]It has been clearly demonstrated that the majority of land crabs leave the egg in the Zoæa form.
[192]These five maxillipeds correspond with the three maxillipeds and two anterior ambulatory appendages of the Decapoda.
[193]The observations of Brooks (No.493) render it probable that the Alima larva leaves the egg in a form not very dissimilar to the youngest known larva.
[194]His paper is unfortunately in Russian.
[195]The Pontellidæ form an exception to this statement, in that they are provided with paired lateral eyes in addition to the median one.
[196]The term Nauplius was applied to the larva of Cyclops and allied organisms by O. F. Müller under the impression that they were adult forms.
[197]Van Beneden (No.506) in the genera investigated by him finds that the two maxillipeds are really distinct pairs of appendages.
[198]It seems not impossible that the appendage regarded by Claus as the mandibular palp may really represent the maxilla, which would otherwise seem to be absent. This mode of interpretation would bring the appendages of Argulus into a much closer agreement with those of the parasitic Copepoda. It does not seem incompatible with the existence of the stylet-like maxillæ detected by Claus in the adult.
[199]Alepas squalicola is stated by Koren and Danielssen to form an exception to this rule, and to leave the egg with six pairs of appendages.
[200]Willemoes Suhm (No.530) states that the mouth is situated at the free end of the upper lip, and that the œsophagus passes through it. From an examination of some specimens of this Nauplius, for which I am indebted to Moseley, I am inclined to think that this is a mistake, and that a groove on the surface of the upper lip has been taken by Suhm for the œsophagus.
[201]The enormous spinous development of the larva of Lepas fascicularis is probably to be explained as a secondary protective adaptation, and has no genetic connection with the somewhat similar spinous armature of the Zoæa.
[202]There is considerable confusion about the shell-gland and antennary gland. In my account Willemoes Suhm has been followed. Claus however regards what I have called the antennary gland as the shell-gland, and states that it does not open into the antennæ till a later period. He does not clearly describe its opening, nor the organ which I have called the shell-gland.
[203]For the characters of Naupliusvidep.460.
[204]Claus speaks of the various Crustacean phyla as having sprung from a Protophyllopod form, and it might be supposed that he considered that they all diverged from the same form. It is clear however from the context that he regards the Protophyllopod type from which the Malacostraca originated as far more like existing Phyllopods than that from which the Entomostracan groups have sprung. It is not quite easy to get a consistent view of his position on the question, since he states (p.77) that the Malacostraca and the Copepods diverged from a similar form, which is represented in their respective developments by the Protozoæa and earliest Cyclops stage. Yet if I understand him rightly, he does not consider the Protozoæa stage to be the Protophyllopod stage from which the Malacostraca have diverged, but states onp.71 that it was not an ancestral form at all.
[205]Claus appears to consider it doubtful whether the Malacostracan gills can be compared with the Phyllopod gill pouches.
[206]Facts for Darwin,p.49.
[207]A secondary larval form is less likely to be repeated in development than an ancestral adult stage, because there is always a strong tendency for the former, which is a secondarily intercalated link in the chain, to drop out by the occurrence of a reversion to the original type of development.
[208]Bobretzky first stated that the invagination remained open, but subsequently corrected himself.Zeit. f. Wiss. Zool.,Bd.XXIV.p.186.
[209]Reichenbach describes these cells, and states that there is a thickening of the epiblast adjoining them. In one place he states that the heart arises from this thickening of epiblast, and in another that it arises from the mesoblast. An epiblastic origin of the heart is extremely improbable.
The groups dealt with in the present Chapter undoubtedly belong to the Arthropoda. They are not closely related, and in the case of each group it is still uncertain with which of the main phyla they should be united. It is possible that they may all be offshoots from the Arachnidan phylum.
Pœcilopoda.
The development of Limulus has been studied by Dohrn (No.533) and Packard (No.534). The ova are laid in the sand near the spring-tide marks. They are enveloped in a thick chorion formed of several layers; and (during the later stages of development at any rate) there is a membrane within the chorion which exhibits clear indications of cell outlines[210].
There is a centrolecithal segmentation, which ends in the formation of a blastoderm enclosing a central yolk mass. A ventral plate is then formed, which is thicker in the region where the abdomen is eventually developed. Six segments soon become faintly indicated in the cephalothoracic region, the ends of which grow out into prominent appendages (fig. 245A); of these there are six pairs, which increase in size from before backwards. A stomodæum (m) is by this time establishedand is placed well in front of the foremost pair of appendages[211].
In the course of the next few days the two first appendages of the abdominal region become formed (videfig. 245C shewing those abdominal appendages at a later stage), and have a very different shape and direction to those of the cephalothorax. The appendages of the latter becomeflexed in the middle in such a way that their ends become directed towards the median line (fig. 245B). The body of the embryo (fig. 245B) is now distinctly divided into two regions—the cephalothoracic in front, and the abdominal behind, both divided into segments.