Tail of an embryo of ScorpionFig. 207. Tail of an advanced embryo of the Scorpion to illustrate the structure of the mesoblastic somites.(After Metschnikoff.)al.alimentary tract;an.i.anal invagination;ep.epiblast;me.s.mesoblastic somite.
Fig. 207. Tail of an advanced embryo of the Scorpion to illustrate the structure of the mesoblastic somites.(After Metschnikoff.)
al.alimentary tract;an.i.anal invagination;ep.epiblast;me.s.mesoblastic somite.
For a considerable period it is composed of the polygonal yolk cells already described and shewn in figs. 203, 204, and 205. The yolk cells divide and become somewhat smaller as development proceeds; but the main products of the division of the yolk nuclei and the protoplasm around them are undoubtedly cells which join the mesoblast (fig. 203A). The permanent alimentary tract is formed of three sections,viz.stomodæum, proctodæum, and mesenteron. The stomodæum and proctodæum are both formed before the mesenteron. The stomodæum is formed as an epiblastic pit between the two procephalic lobes (figs. 200and205,st). It becomes deeper, and by the latest stage figured is a deep pit lined by a cuticle and ending blindly. To its hinder section, which forms the suctorial apparatus of the adult, three powerful muscles (a dorsal and two lateral) are attached.
The proctodæum is formed considerably later than the stomodæum. It is a comparatively shallow involution, which forms the rectum of the adult. It is dilated at its extremity, and two Malpighian vessels early grow out from it.
The mesenteron is formedin the interior of the yolk. Its walls are derived from the cellular elements of the yolk, and the first section to be formed is the hinder extremity, which appears as a short tube ending blindly behind in contact with the proctodæum, and open to the yolk in front. The later history of the mesenteron has not been followed, but it undoubtedly includesthe whole of the abdominal section of the alimentary canal of the adult, except the rectum, and probably also the thoracic section. The later history of the yolk which encloses the mesenteron has not been satisfactorily studied, though it no doubt gives rise to the hepatic tubes, and probably also to the thoracic diverticula of the alimentary tract.
The general history of the alimentary tract in Scorpio is much the same as in Spiders. The hypoblast, the origin of which as mentioned above is somewhat uncertain, first appears on the ventral side and gradually spreads so as to envelop the yolk, and form the wall of the mesenteron, from which the liver is formed as a pair of lateral outgrowths. The proctodæum and stomodæum are both short, especially the former (videfig. 207).
Summary and general conclusions.
The embryonic forms of Scorpio and Spiders are very similar, but in spite of the general similarity of Chelifer to Scorpio, the embryo of the former differs far more from that of Scorpio than the latter does from Spiders. This peculiarity is probably to be explained by the early period at which Chelifer is hatched; and though a more thorough investigation of this interesting form is much to be desired, it does not seem probable that its larva is a primitive type.
The larvæ of the Acarina with their peculiar ecdyses are to be regarded as much modified larval forms. It is not however easy to assign a meaning to the hexapodous stage through which they generally pass.
With reference to the segments and appendages, some interesting points are brought out by the embryological study of these forms.
The maximum number of segments is present in the Scorpion, in which nineteen segments (not including the præ-oral lobes, but including the telson) are developed. Of these the first twelve segments have traces of appendages, but the appendages of the six last of these (unless the pecten is an appendage) atrophy. In Spiders there are indications in the embryo of sixteen segments and in all the Arachnida, except the Acarina, at the least four segments bear appendages in the embryo which are without them in the adult. The morphological bearings of this fact are obvious.
It deserves to be noted that, in both Scorpio and the Spider, the cheliceræ are borne in the embryo by the first post-oral segment, and provided with a distinct ganglion, so that they cannot correspond (as they are usually supposed to do) with the antennæ of Insects, which are always developed on the præ-oral lobes, and never supplied by an independent ganglion.
The cheliceræ would seem probably to correspond with the mandibles of Insects, and the antennæ to be absent. In favour of this view is the fact that the embryonic ganglion of the mandibles of Insects is stated (cf.Lepidoptera,Hatschek,p.340) to become, like the ganglion of the cheliceræ, converted into part of the œsophageal commissure.
If the above considerations are correct, the appendages of the Arachnida retain in many respects a very much more primitive condition than those of Insects. In the first place, both the cheliceræ and pedipalpi are much less differentiated than the mandibles and first pair of maxillæ with which they correspond. In the second place, the first pair of ambulatory limbs must be equivalent to the second pair of maxillæ of Insects, which, for reasons stated above, were probably originally ambulatory. It seems in fact a necessary deduction from the arguments stated that the ancestors of the present Insecta and Arachnida must have diverged from a common stem of the Tracheata at a time when the second pair of maxillæ were still ambulatory in function.
With reference to the order of the development of the appendages and segments, very considerable differences are noticeable in the different Arachnoid types. This fact alone appears to me to be sufficient to prove that the order of appearance of the appendages is often a matter of embryonic convenience, without any deep morphological significance. In Scorpio the segments develop successively, except perhaps the first post-oral, which is developed after some of the succeeded segments have been formed. In Spiders the segment of the cheliceræ, and probably also of the pedipalpi, appears later than the next three or four. In both these types the segments arise before the appendages, but the reverse appears to be the case in Chelifer. The permanent appendages, except the cheliceræ, appear simultaneously in Scorpions and Spiders. The second pair appears long before the others in Chelifer, then the third, next the first, and finally the three hindermost.
Bibliography.
Scorpionidæ.
(434)El. Metschnikoff.“Embryologie des Scorpions.”Zeit. f. wiss. Zool.Bd.XXI.1870.(435)H. Rathke.Reisebemerkungen aus Taurien(Scorpio). Leipzig, 1837.
Pseudoscorpionidæ.
(436)El. Metschnikoff.“Entwicklungsgeschichte d. Chelifer.”Zeit. f. wiss. Zool.,Bd.XXI.1870.(437)A. Stecker.“Entwicklung der Chthonius-Eier im Mutterleibe und die Bildung des Blastoderms.”Sitzung. königl. böhmisch. Gesellschaft Wissensch.,1876, 3. Heft, andAnnal. and Mag. Nat. History, 1876,XVIII.197.
Phalangidæ.
(438)M. Balbiani.“Mémoire sur le développement des Phalangides.”Ann. Scien. Nat.SeriesV.Vol.XVI.1872.
Araneina.
(439)M. Balbiani.“Mémoire sur le développement des Aranéides.”Ann. Scien. Nat. SeriesV.Vol.XVII.1873.(440)F. M. Balfour. “Notes on the development of the Araneina.”Quart. Journ. of Micr. Science,Vol.XX.1880.(441)J. Barrois.“Recherches s. l. développement des Araignées.”Journal de l’Anat. et de la Physiol.1878.(442)E. Claparède.Recherches s. l’évolution des Araignées.Utrecht, 1862.(443)Herold.De generatione Araneorum in Ovo.Marburg, 1824.(444)H. Ludwig.“Ueber die Bildung des Blastoderms bei den Spinnen.”Zeit. f. wiss. Zool.,Vol.XXVI.1876.
Acarina.
(445) P. van Beneden.“Développement de l’Atax ypsilophora.”Acad. Bruxelles,t.XXIV.(446) Ed. Claparède.“Studien über Acarinen.”Zeit. f. wiss. Zool.,Bd.XVIII.1868.
Formation of the layers and the embryonic envelopes in the Tracheata.
There is a striking constancy in the mode of formation of the layers throughout the group. In the first place the hypoblast is not formed by a process which can be reduced to invagination: in other words, there is no gastrula stage.
Efforts have been made to shew that the mesoblastic groove of Insects implies a modified gastrula, but since it is the essence of a gastrula that it should directly or indirectly give rise to the archenteron, the groove in question cannot fall under this category. Although the mesoblastic groove of Insects is not a gastrula, it is quite possible that it is the rudiment of a blastopore, the gastrula corresponding to which has now vanished from the development. It would thus be analogous to the primitive streak of Vertebrates[184].
The growth of the blastoderm over the yolk in Scorpions admits no doubt of being regarded as an epibolic gastrula. The blastopore would however be situated dorsally, a position which it does not occupy in any gastrula type so far dealt with. This fact, coupled with the consideration that the partial segmentation of Scorpio can be derived without difficulty from the ordinary Arachnidan type (videp.120), seems to shew that there is no true epibolic invagination in the development of Scorpio.
On the formation of the blastoderm traces of two embryonic layers are established. The blastoderm itself is essentially the epiblast, while the central yolk is the hypoblast. The formation of the embryo commences in connection with a thickening of the blastoderm, known as the ventral plate. The mesoblast is formed as an unpaired plate split off from the epiblast of the ventral plate. This process takes place in at any rate two ways. In Insects a groove is formed, which becomes constricted off to form the mesoblastic plate: in Spiders there is a keel-like thickening of the blastoderm, which takes the place of the groove.
The unpaired mesoblastic plate becomes in all forms very soon divided into twomesoblastic bands.
The mesoblastic bands are very similar to, and probably homologous with, those of Chætopoda; but the different modes by which they arise in these two groups are very striking, and probably indicate that profound modifications have taken place in the early development of the Tracheata. In the Chætopoda the bands are from the first widely separated, and gradually approach each other ventrally, though without meeting. In the Tracheata they arise from the division of an unpaired ventral plate.
The further history of the mesoblastic bands is nearly thesame for all the Tracheata so far investigated, and is also very much the same as for the Chætopoda. There is a division into somites, each containing a section of the body cavity. In the cephalic section of the mesoblastic bands a section of the body cavity is also formed. In Arachnida, Myriapoda, and probably also Insecta, the body cavity is primitively prolonged into the limbs.
In Spiders at any rate, and very probably in the other groups of the Tracheata, a large part of the mesoblast is not derived from the mesoblastic plate, but is secondarily added from the yolk cells.
In all Tracheata the yolk cells give rise to the mesenteron which, in opposition, as will hereafter appear, to the mesenteron of the Crustacea, forms the main section of the permanent alimentary tract.
One of the points which is still most obscure in connection with the embryology of the Tracheata is the origin of the embryonic membranes. Amongst Insects, with the exception of the Thysanura, such membranes are well developed. In the other groups definite membranes like those of Insects are never found, but in the Scorpion a cellular envelope appears to be formed round the embryo from the cells of the blastoderm, and more or less similar structures have been described in some Myriapods (videp.390). These structures no doubt further require investigation, but may provisionally be regarded as homologous with the amnion and serous membrane of Insects. In the present state of our knowledge it does not seem easy to give any explanation of the origin of these membranes, but they may be in some way derived from an early ecdysis.
[160]F. M. Balfour, “On certain points in the Anatomy of Peripatus capensis.”Quart. Journ. of Micros. Science,Vol.XIX.1879.[161]This figure is taken from Moseley. The epiblastic invaginations are represented in it very accurately, and though not mentioned in the text of the paper, Moseley informs me that he has long been aware of the homology of these folds with those in various other Tracheata.[162]The specimens shewing tracheæ which Moseley has placed in my hands are quite sufficient to leave no doubt whatever in my mind as to the general accuracy of his description of the tracheal system.[163]The classification of the Myriapoda employed in the present section is:I.Chilognatha.(Millipedes.)II.Chilopoda.(Centipedes.)[164]Stecker’s (No.400) observations were made on the eggs of Julus fasciatus, Julus fœtidus, Craspedosoma marmoratum, Polydesmus complanatus, and Strongylosoma pallipes, and though carried on by means of sections, still leave some points very obscure, and do not appear to me deserving of much confidence. The two species of Julus and Craspedosoma undergo, according to Stecker, a nearly identical development. The egg before segmentation is constituted of two substances, a central protoplasmic, and a peripheral deutoplastic. It first divides into two equal segments, and coincidentally with their formation part of the central protoplasm travels to the surface as two clear fluid segments. The ovum is thus composed of two yolk segments to two protoplasmic segments. The two former next divide into four, with the production of two fresh protoplasmic segments. The four protoplasmic segments now constitute the upper or animal pole of the egg, and occupy the position of the future ventral plate. The yolk segments form the lower pole, which is howeverdorsalin relation to the future animal. The protoplasmic segments increase in number by a regular division, and arrange themselves in three rows, of which the two outermost rapidly grow over the yolk segments. A large segmentation cavity is stated to be present in the interior of the ovum.It would appear from Stecker’s description that the yolk segments (hypoblast) next become regularly invaginated, so as to enclose a gastric cavity, opening externally by a blastopore; but it is difficult to believe that a typical gastrula, such as that represented by Stecker, really comes into the cycle of development of the Chilognatha.The mesoblast is stated to be derived mainly from the epiblast. This layer in the region of the future ventral plate becomes reduced to two rows of cells, and the inner of these by the division of its constituent elements gives rise to the mesoblast. The development of Polydesmus and Strongylosoma is not very different from that of Julus. The protoplasm at the upper pole occupies from the first a superficial position. Segmentation commences at the lower pole, where the food-yolk is mainly present! The gastrula is stated to be similar to that of Julus. The mesoblast is formed in Polydesmus as a layer of cells split off from the epiblast, but in Strongylosoma as an outgrowth from the lips of the blastopore. Stecker, in spite of the statements in his paper as to the origin of the mesoblast from the epiblast, sums up at the end to the effect that both the primary layers have a share in the formation of the mesoblast, which originates by a process of endogenous cell division!It may be noted that the closure of the blastopore takes place, according to Stecker, on the dorsal side of the embryo.[165]Though the superficially hexapodous larva of Strongylosoma and other Chilognatha has a striking resemblance to some larval Insects, no real comparison is possible between them, even on the assumption that the three functional appendages of both are homologous, because Embryology clearly proves that the hexapodous Insect type has originated from an ancestor with numerous appendages by the atrophy of those appendages, and not from an hexapodous larval form prior to the development of the full number of adult appendages.[166]Newport states however that a pair of limbs is present on the first, second, and fourth post-oral segments, but that the third segment is apodous; and this is undoubtedly the case in the adult.[167]The following classification of the Insecta is employed in this chapter.I.Aptera.(1) Collembola.(2) Thysanura.II.Orthoptera.(1) Orthoptera genuina (Blatta,Locusta, etc.).(2) " pseudoneuroptera (Termes,Ephemera,Libellula).III.Hemiptera.(1) Hemiptera heteroptera (Cimex,Notonecta, etc.).(2) " homoptera (Aphis,Cicada, etc.).(3) " parasita (Pediculus, etc.).IV.Diptera.(1) Diptera genuina (Musca,Tipula, etc.).(2) " aphaniptera (Pulex, etc.).(3) " pupipara (Braula, etc.).V.Neuroptera.(1) Neuroptera planipennia (Myrmeleon, etc.).(2) " trichoptera (Phryganea, etc.).VI.Coleoptera.VII.Lepidoptera.VIII.Hymenoptera.(1) Hymenoptera aculeata (Apis,Formica, etc.).(2) " entomophaga (Ichneumon,Platygaster, etc.).(3) " phytophaga (Tenthredo,Sirex, etc.).[168]The reverse nomenclature to this is rather inconveniently employed by Metschnikoff.[169]This has not been shewn in the case of Hydrophilus.[170]According to Kowalevsky the history of the dorsal plate is somewhat different. He believes that on the absorption of the amnion the ventral plate unites with the serous membrane, and that the latter directly gives rise to the dorsal integument, while the thickened part of it becomes involuted to form the dorsal tube already described.[171]Tichomiroff (No.420) denies the existence of a true invagination to form the mesoblast, and also asserts that a separation of mesoblast cells from the epiblast can take place at other parts besides the median ventral line.[172]If these structures are equivalent to appendages, they may correspond to one of the pairs of antennæ of Crustacea. From a figure by Fritz Müller of the larva of Calotermes (Jenaische Zeit.Vol.XI.pl.11, fig. 12) it would appear that they lie in front of the true antennæ, and would therefore on the above hypothesis correspond to the first pair of antennæ of Crustacea. Bütschli (No.405) describes in the Bee a pair of prominences immediately in front of the mandibles which eventually unite to form a kind of underlip; they in some ways resemble true appendages.[173]In Smynthurus, one of the Collembola, there are, according to Lubbock, only two stigmata, which are placed on the head.[174]Graber’s view on this point may probably be explained by supposing that he has mistaken a passage of yolk cells into the blastoderm for a passage of blastoderm cells into the yolk. The former occurrence takes place, as I have found, largely in Spiders, and probably therefore also occurs in Insects.[175]This point requires further observation.[176]For a systematic account of this subject the reader is referred to Lubbock (No.420) and to Graber (No.411). He will find in Weismann (Nos. 430 and 431) a detailed account of the internal changes which take place.[177]Brauer and Lubbock (No.421) have pointed out the primitive characters of these forms, especially of Campodea.[178]Petites Nouvelles Entomologiques, May, 1878.[179]The classification of the Arachnida adopted in the present work is shewn below.I.Arthrogastra.Scorpionidæ.Pedipalpi.Pseudoscorpionidæ.Solifugæ.Phalangidæ.II.Araneina.Tetrapneumones.Dipneumones.III.Araneina.[180]The origin of the hypoblast cells, if such these cells are, is obscure. Metschnikoff doubtfully derives them from the blastoderm cells; from my investigations on Spiders it appears to me more probable that they originate in the yolk.[181]The exact fate of the three original segments is left somewhat obscure by Metschnikoff. He believes however that the anterior segment forms the procephalic lobes, the posterior probably the telson and five adjoining caudal segments, and the middle one the remainder of the body. This view does not appear to me quite satisfactory, since on the analogy of Spiders and other Arthropoda the fresh somites ought to be added by a continuous segmentation of the posterior lobe.[182]Various views have been put forward by Claparède and Balbiani about the position and significance of the primitive cumulus. For a discussion of whichvideself,No.440.[183]For further detailsvideself,No.440.[184]The primitive streak of Vertebrates, as will appear in the sequel, has no connection with the medullary groove, and is the rudiment of the blastopore.
[160]F. M. Balfour, “On certain points in the Anatomy of Peripatus capensis.”Quart. Journ. of Micros. Science,Vol.XIX.1879.
[161]This figure is taken from Moseley. The epiblastic invaginations are represented in it very accurately, and though not mentioned in the text of the paper, Moseley informs me that he has long been aware of the homology of these folds with those in various other Tracheata.
[162]The specimens shewing tracheæ which Moseley has placed in my hands are quite sufficient to leave no doubt whatever in my mind as to the general accuracy of his description of the tracheal system.
[163]The classification of the Myriapoda employed in the present section is:
I.Chilognatha.(Millipedes.)
II.Chilopoda.(Centipedes.)
[164]Stecker’s (No.400) observations were made on the eggs of Julus fasciatus, Julus fœtidus, Craspedosoma marmoratum, Polydesmus complanatus, and Strongylosoma pallipes, and though carried on by means of sections, still leave some points very obscure, and do not appear to me deserving of much confidence. The two species of Julus and Craspedosoma undergo, according to Stecker, a nearly identical development. The egg before segmentation is constituted of two substances, a central protoplasmic, and a peripheral deutoplastic. It first divides into two equal segments, and coincidentally with their formation part of the central protoplasm travels to the surface as two clear fluid segments. The ovum is thus composed of two yolk segments to two protoplasmic segments. The two former next divide into four, with the production of two fresh protoplasmic segments. The four protoplasmic segments now constitute the upper or animal pole of the egg, and occupy the position of the future ventral plate. The yolk segments form the lower pole, which is howeverdorsalin relation to the future animal. The protoplasmic segments increase in number by a regular division, and arrange themselves in three rows, of which the two outermost rapidly grow over the yolk segments. A large segmentation cavity is stated to be present in the interior of the ovum.
It would appear from Stecker’s description that the yolk segments (hypoblast) next become regularly invaginated, so as to enclose a gastric cavity, opening externally by a blastopore; but it is difficult to believe that a typical gastrula, such as that represented by Stecker, really comes into the cycle of development of the Chilognatha.
The mesoblast is stated to be derived mainly from the epiblast. This layer in the region of the future ventral plate becomes reduced to two rows of cells, and the inner of these by the division of its constituent elements gives rise to the mesoblast. The development of Polydesmus and Strongylosoma is not very different from that of Julus. The protoplasm at the upper pole occupies from the first a superficial position. Segmentation commences at the lower pole, where the food-yolk is mainly present! The gastrula is stated to be similar to that of Julus. The mesoblast is formed in Polydesmus as a layer of cells split off from the epiblast, but in Strongylosoma as an outgrowth from the lips of the blastopore. Stecker, in spite of the statements in his paper as to the origin of the mesoblast from the epiblast, sums up at the end to the effect that both the primary layers have a share in the formation of the mesoblast, which originates by a process of endogenous cell division!
It may be noted that the closure of the blastopore takes place, according to Stecker, on the dorsal side of the embryo.
[165]Though the superficially hexapodous larva of Strongylosoma and other Chilognatha has a striking resemblance to some larval Insects, no real comparison is possible between them, even on the assumption that the three functional appendages of both are homologous, because Embryology clearly proves that the hexapodous Insect type has originated from an ancestor with numerous appendages by the atrophy of those appendages, and not from an hexapodous larval form prior to the development of the full number of adult appendages.
[166]Newport states however that a pair of limbs is present on the first, second, and fourth post-oral segments, but that the third segment is apodous; and this is undoubtedly the case in the adult.
[167]The following classification of the Insecta is employed in this chapter.
I.Aptera.
(1) Collembola.
(2) Thysanura.
II.Orthoptera.
(1) Orthoptera genuina (Blatta,Locusta, etc.).
(2) " pseudoneuroptera (Termes,Ephemera,Libellula).
III.Hemiptera.
(1) Hemiptera heteroptera (Cimex,Notonecta, etc.).
(2) " homoptera (Aphis,Cicada, etc.).
(3) " parasita (Pediculus, etc.).
IV.Diptera.
(1) Diptera genuina (Musca,Tipula, etc.).
(2) " aphaniptera (Pulex, etc.).
(3) " pupipara (Braula, etc.).
V.Neuroptera.
(1) Neuroptera planipennia (Myrmeleon, etc.).
(2) " trichoptera (Phryganea, etc.).
VI.Coleoptera.
VII.Lepidoptera.
VIII.Hymenoptera.
(1) Hymenoptera aculeata (Apis,Formica, etc.).
(2) " entomophaga (Ichneumon,Platygaster, etc.).
(3) " phytophaga (Tenthredo,Sirex, etc.).
[168]The reverse nomenclature to this is rather inconveniently employed by Metschnikoff.
[169]This has not been shewn in the case of Hydrophilus.
[170]According to Kowalevsky the history of the dorsal plate is somewhat different. He believes that on the absorption of the amnion the ventral plate unites with the serous membrane, and that the latter directly gives rise to the dorsal integument, while the thickened part of it becomes involuted to form the dorsal tube already described.
[171]Tichomiroff (No.420) denies the existence of a true invagination to form the mesoblast, and also asserts that a separation of mesoblast cells from the epiblast can take place at other parts besides the median ventral line.
[172]If these structures are equivalent to appendages, they may correspond to one of the pairs of antennæ of Crustacea. From a figure by Fritz Müller of the larva of Calotermes (Jenaische Zeit.Vol.XI.pl.11, fig. 12) it would appear that they lie in front of the true antennæ, and would therefore on the above hypothesis correspond to the first pair of antennæ of Crustacea. Bütschli (No.405) describes in the Bee a pair of prominences immediately in front of the mandibles which eventually unite to form a kind of underlip; they in some ways resemble true appendages.
[173]In Smynthurus, one of the Collembola, there are, according to Lubbock, only two stigmata, which are placed on the head.
[174]Graber’s view on this point may probably be explained by supposing that he has mistaken a passage of yolk cells into the blastoderm for a passage of blastoderm cells into the yolk. The former occurrence takes place, as I have found, largely in Spiders, and probably therefore also occurs in Insects.
[175]This point requires further observation.
[176]For a systematic account of this subject the reader is referred to Lubbock (No.420) and to Graber (No.411). He will find in Weismann (Nos. 430 and 431) a detailed account of the internal changes which take place.
[177]Brauer and Lubbock (No.421) have pointed out the primitive characters of these forms, especially of Campodea.
[178]Petites Nouvelles Entomologiques, May, 1878.
[179]The classification of the Arachnida adopted in the present work is shewn below.
I.Arthrogastra.
Scorpionidæ.
Pedipalpi.
Pseudoscorpionidæ.
Solifugæ.
Phalangidæ.
II.Araneina.
Tetrapneumones.
Dipneumones.
III.Araneina.
[180]The origin of the hypoblast cells, if such these cells are, is obscure. Metschnikoff doubtfully derives them from the blastoderm cells; from my investigations on Spiders it appears to me more probable that they originate in the yolk.
[181]The exact fate of the three original segments is left somewhat obscure by Metschnikoff. He believes however that the anterior segment forms the procephalic lobes, the posterior probably the telson and five adjoining caudal segments, and the middle one the remainder of the body. This view does not appear to me quite satisfactory, since on the analogy of Spiders and other Arthropoda the fresh somites ought to be added by a continuous segmentation of the posterior lobe.
[182]Various views have been put forward by Claparède and Balbiani about the position and significance of the primitive cumulus. For a discussion of whichvideself,No.440.
[183]For further detailsvideself,No.440.
[184]The primitive streak of Vertebrates, as will appear in the sequel, has no connection with the medullary groove, and is the rudiment of the blastopore.
History of the larval forms[186].
The larval forms of the Crustacea appear to have more faithfully preserved their primitive characters than those of almost any other group.
Branchiopoda.
The Branchiopoda, comprising under that term the Phyllopoda and Cladocera, contain the Crustacea with the maximum number of segments and the least differentiation of the separate appendages. This and other considerations render it probable that they are to be regarded as the most central group of the Crustaceans, and as in many respects least modified from the ancestral type from which all the groups have originated.
The free larval stages when such exist commence with a larval form known as the Nauplius.
The term Nauplius was applied by O. F. Müller to certain larval forms of the Copepoda (fig. 229) in the belief that they were adult.
Two stages in the development of Apus cancriformisFig. 208. Two stages in the development of Apus cancriformis.(After Claus.)A. Nauplius stage at the time of hatching.B. Stage after first ecdysis.an1.andan2.First and second antennæ;md.mandible;mx.maxilla;l.labrum;fr.frontal sense organ;f.caudal fork;s.segments.
Fig. 208. Two stages in the development of Apus cancriformis.(After Claus.)
A. Nauplius stage at the time of hatching.B. Stage after first ecdysis.
an1.andan2.First and second antennæ;md.mandible;mx.maxilla;l.labrum;fr.frontal sense organ;f.caudal fork;s.segments.
The term has now been extended to a very large number of larvæ which have certain definite characters in common. They are provided (fig. 208A) with three pairs of appendages, the future two pairs of antennæ and mandibles. The first pair of antennæ (an1) is uniramous and mainly sensory in function, the second pair of antennæ (an2) and mandibles (md) are biramous swimming appendages, and the mandibles are without the future cutting blade. The Nauplius mandibles represent in fact the palp. The two posterior appendages are both provided with hook-like prominences on their basal joints, used in mastication. The body in most cases is unsegmented, and bears anteriorly a single median eye. There is a large upper lip, and an alimentary canal formed of œsophagus, stomach and rectum. The anus opens near the hind end of the body. On the dorsal surface small folds of skin frequently represent the commencement of a dorsal shield. One very striking peculiarity of the Nauplius according to Claus and Dohrn is the fact that the second pair of antennæ is innervated froma sub-œsophageal ganglion. A larval form with the above characters occurs with more or less frequency in all the Crustacean groups. In most instances itdoes notexactlyconform to the above type, and the divergences are more considerable in the Phyllopods than in most other groups. Its characters in each case are described in the sequel.
Phyllopoda..For the Phyllopoda the development of Apus cancriformis may conveniently be taken as type (Claus,No.454). The embryo at the time it leaves the egg (fig. 208A) is somewhat oval in outline, and narrowed posteriorly. There is a slightV-shaped indentation behind, at the apex of which is situated the anus. The body, unlike that of the typical Nauplius, is already divided into two regions, a cephalic and post-cephalic. On the ventral side of the cephalic region there are present the three normal pairs of appendages. Foremost there are the small anterior antennæ (an1), which are simple unjointed rod-like bodies with two moveable hairs at their extremities. They are inserted at the sides of the large upper lip or labrum (l). Behind these are the posterior antennæ, which are enormously developed and serve as the most important larval organs of locomotion. They are biramous, being formed of a basal portion with a strong hook-like bristle projecting from its inner side, an inner unjointed branch with three bristles, and an outer large imperfectly five-jointed branch with five long lateral bristles. The hook-like organ attached to this pair of appendages would seem to imply that it served in some ancestral form as jaws (Claus). This character is apparently universal in the embryos of true Phyllopods, and constantly occurs in the Copepoda, etc.
The third pair of appendages or mandibles (md) is attached close below the upper lip. They are as yet unprovided with cutting blades, and terminate in two short branches, the inner with two and the outer with three bristles.
At the front of the head there is the typical unpaired eye. On the dorsal surface there is already present a rudiment of the cephalic shield, continuous anteriorly with the labrum (l) or upper lip, the extraordinary size of which is characteristic of the larvæ of Phyllopods. The post-cephalic region, which afterwards becomes the thorax and abdomen, contains underneath the skin rudiments of the five anterior thoracic segments and their appendages, and presents in this respect an important variation from the typical Nauplius form. After the first ecdysis thelarva (fig. 208B) loses its oval form, mainly owing to the elongation of the hinder part of the body and the lateral extension of the cephalic shield, which moreover now completely covers over the head and has begun to grow backwards so as to cover over the thoracic region. At the second ecdysis there appears at its side a rudimentary shell-gland. In the cephalic region two small papillæ (fr) are now present at the front of the head close to the unpaired eye. They are of the nature of sense organs, and may be called the frontal sense papillæ. They have been shewn by Claus to be of some phylogenetic importance. The three pairs of Nauplius appendages have not altered much, but a rudimentary cutting blade has grown out from the basal joint of the mandible. A gland opening at the base of the antennæ is now present, which is probably equivalent to the green gland often present in the Malacostraca. Behind the mandibles a pair of simple processes has appeared, which forms the rudiment of the first pair of maxillæ (mx).
In the thoracic region more segments have been added posteriorly, and the appendages of the three anterior segments are very distinctly formed. The tail is distinctly forked. The heart is formed at the second ecdysis, and then extends to the sixth thoracic segment: the posterior chambers are successively added from before backwards.
At the successive ecdyses which the larva undergoes new segments continue to be formed at the posterior end of the body, and limbs arise on the segments already formed. These limbs probably represent the primitive form of an important type of Crustacean appendage, which is of value for interpreting the parts of the various malacostracan appendages. They consist (fig. 209) of a basal portion (protopodite of Huxley) bearing two rami. The basal portion has two projections on the inner side. To the outer side of the basal portion there is attached a dorsally directed branchial sack (br) (epipodite of Huxley). The outer ramus (ex) (exopodite of Huxley) is formed of a single plate with marginal setæ. The inner one (en) (endopodite of Huxley) is four-jointed, and a process similar to those of the basal joint is given off from the inner side of the three proximal joints.
Illustration: TitleFig. 209. Typical phyllopod appendage.(Copied from Claus.)ex.exopodite;en.endopodite;br.branchial appendage (epipodite). The basal portion bearing the two proximal projections is not sharply separated from the endopodite.
Fig. 209. Typical phyllopod appendage.(Copied from Claus.)
ex.exopodite;en.endopodite;br.branchial appendage (epipodite). The basal portion bearing the two proximal projections is not sharply separated from the endopodite.
At the third ecdysis several new features appear in the cephalic region, which becomes more prominent in the succeedingstages. In the first place the paired eyes are formed at each side of and behind the unpaired eye, secondly the posterior pair of maxillæ is formed though it always remains very rudimentary. The shell-gland becomes fully developed opening at the base of the first pair of maxillæ. The dorsal shield gradually grows backwards till it covers its full complement of segments.
After the fifth ecdysis the Nauplius appendages undergo a rapid atrophy. The second pair of antennæ especially becomes reduced in size, and the mandibular palp—the primitive Nauplius portion of the mandible—is contracted to a mere rudiment, which eventually completely disappears, while the blade is correspondingly enlarged and also becomes toothed. The adult condition is only gradually attained after a very large number of successive changes of skin.
The chief point of interest in the above development is the fact of the primitive Nauplius form becoming gradually converted without any special metamorphosis into the adult condition[187].
Branchipus like Apus is hatched as a somewhat modified Nauplius, which however differs from that of Apus in the hinder region of the body having no indications of segments. It goes through a very similar metamorphosis, but is at no period of its metamorphosis provided with a dorsal shield: the second pair of antennæ does not abort, and in the male is provided with clasping organs, which are perhaps remnants of the embryonic hooks so characteristic of this pair of antennæ.
The larva of Estheria when hatched has a Nauplius form, a large upper lip, caudal fork and single eye. There are two functional pairs of swimming appendages—the second pair of antennæ and mandibles. The first pair of antennæ has not been detected, and a dorsal mantle to form the shell is not developed. At the first moult the anterior pair of antennæ arises as small stump-like structures, and a small dorsal shield is also formed. Rudiments of six or seven pairs of appendages sproutout in the usual way, and continue to increase in number at successive moults: the shell is rapidly developed. The chief point of interest in the development of this form is the close resemblance of the young larva to a typical adult Cladocera (Claus). This is shewn in the form of the shell, which has not reached its full anterior extension, the rudimentary anterior antennæ, the large locomotor second pair of antennæ, which differ however from the corresponding organs in the Cladocera in the presence of typical larval hooks. Even the abdomen resembles that of Daphnia. These features perhaps indicate that the Cladocera are to be derived from some Phyllopod form like Estheria by a process of retrogressive metamorphosis. The posterior antennæ in the adult Estheria are large biramous appendages, and are used for swimming; and though they have lost the embryonic hook, they still retain to a larger extent than in other Phyllopod families their Nauplius characteristics.
The Nauplius form of the Phyllopods is marked by several definite peculiarities. Its body is distinctly divided into a cephalic and post-cephalic region. The upper lip is extraordinarily large, relatively very much more so than at the later stages. The first pair of antennæ is usually rudimentary and sometimes even absent; while the second pair is exceptionally large, and would seem to be capable of functioning not only as a swimming organ, but even as a masticating organ. A dorsal shield is nearly or quite absent.
Nauplius larva of Leptodora hyalinaFig. 209 A. Nauplius larva of Leptodora hyalina from winter egg.(Copied from Bronn; after Sars.)an1.antenna of first pair;an2.antenna of second pair;md.mandible;f.caudal fork.
Fig. 209 A. Nauplius larva of Leptodora hyalina from winter egg.(Copied from Bronn; after Sars.)
an1.antenna of first pair;an2.antenna of second pair;md.mandible;f.caudal fork.
Cladocera.The probable derivation of the Cladocera from a form similar to Estheria has already been mentioned, and it might have been anticipated that the development would be similar to that of the Phyllopods. The development of the majority of the Cladocera takes place however in the egg, and the young when hatched closely resembles their parents, though in the egg they pass through a Nauplius stage (Dohrn). An exception to the general rule is however offered by the case of the winter eggs of Leptodora, one of the most primitive of the Cladoceran families. The summer eggs develop without metamorphosis, but Sars (No.461)has discovered that the larva leaves the winter eggs in the form of a Nauplius (fig. 209). This Nauplius closely resembles that of the Phyllopods. The body is elongated and in addition to normal Nauplius appendages is marked by six pairs of ridges—the indications of the future feet. The anterior antennæ are as usual small; the second large and biramous, but the masticatory bristle characteristic of the Phyllopods is not present. The mandibles are without a cutting blade. A large upper lip and unpaired eye are present.
The adult form is attained in the same manner as amongst the Phyllopods after the third moult.
Malacostraca.
Owing to the size and importance of the various forms included in the Malacostraca, greater attention has been paid to their embryology than to that of any other division of the Crustacea; and the proper interpretation of their larval forms involves some of the most interesting problems in the whole range of Embryology.
The majority of Malacostraca pass through a more or less complicated metamorphosis, though in the Nebaliadæ, the Cumaceæ, some of the Schizopoda, a few Decapoda (Astacus, Gecarcinus, etc.), and in the Edriophthalmata, the larva on leaving the egg has nearly the form of the adult. In contradistinction to the lower groups of Crustacea the Nauplius form of larva is rare, though it occurs in the case of one of the Schizopods (Euphausia,fig. 212), in some of the lower forms of the Decapods (Penæus,fig. 214), and perhaps also, though this has not been made out, in some of the Stomatopoda.