Stages in the development of CyprisFig. 235. Stages in the development of Cypris.(From Claus.)A. Fourth stage. B. Fifth stage.Mx´.first maxilla;Mx´´.andf´.second maxilla;f´´.first pair of feet;L.liver.
Fig. 235. Stages in the development of Cypris.(From Claus.)A. Fourth stage. B. Fifth stage.
Mx´.first maxilla;Mx´´.andf´.second maxilla;f´´.first pair of feet;L.liver.
The second stage (fig. 234B), inaugurated by the first moult, is mainly characterized by the appearance of two fresh pairs of appendages,viz.the first pair of maxillæ and the first pair of feet; the second pair of maxillæ not being developed till later. The first pair appear as leaf-like curved plates (Mx´) more or less like Phyllopod appendages (Claus) though at this stage without an exopodite. The first pair of feet (f´´) terminates in a curved claw and is used for adhering. The mandibles have by this stage fully developed blades, and have practically attained their adult form, consisting of a powerful toothed blade and a four-jointed palp.
During the third and fourth stages the first pair of maxillæ acquire their pectinated gill plate (epipodite) and four blades; and in the fourth stage (fig. 235A) the second pair of maxillæ (Mx´´) arises, as a pair of curved plates, similar to the first pair of maxillæ at their first appearance. The forked tail is indicated during the fourth stage by two bristles. During the fifth stage (fig. 235B) the number of joints of the first pair of antennæ becomes increased, and the posterior maxillæ develop a blade and becomefour-jointed ambulatory appendages terminating in a hook. The caudal fork becomes more distinct.
In the sixth stage (fig. 236) the second and hindermost pair of feet becomes formed (f´´´) and the maxillæ of the second pair lose their ambulatory function, and begin to be converted into definite masticatory appendages by the reduced jointing of their palp, and the increase of their cutting blades. By the seventh stage all the appendages have practically attained their permanent form; the second pair of maxillæ has acquired small branchial plates, and the two following feet have become jointed. In the eighth and ninth stages the generative organs attain their mature form.
Sixth stage in the development of CyprisFig. 236. Sixth stage in the development of Cypris.(From Claus.)Mx´.first maxilla;Mx´´.f´.second maxilla;f´´.andf´´´.first and second pair of feet;Fu.caudal fork;L.liver;S.D.shell-gland.
Fig. 236. Sixth stage in the development of Cypris.(From Claus.)
Mx´.first maxilla;Mx´´.f´.second maxilla;f´´.andf´´´.first and second pair of feet;Fu.caudal fork;L.liver;S.D.shell-gland.
The larva of Cythere at the time of birth has rudiments of all the limbs, but the mandibular palp still functions as a limb, and the three feet (2nd pair of maxillæ and two following appendages) are very rudimentary.
The larvæ of Cypridina are hatched in a condition which to all intents and purposes resembles the adult.
Phylogeny of the Crustacea.
The classical work of Fritz Müller (No.452) on the phylogeny of the Crustacea has given a great impetus to the study of their larval forms, and the interpretations of these forms which he has offered have been the subject of a very large amount of criticism and discussion. A great step forward in this discussion has been recently made by Claus in his memoir (No.448).
The most fundamental question concerns the meaning of the Nauplius. Is the Nauplius the ancestral form of the Crustacea, as is believed by Fritz Müller and Claus, or are its peculiarities and constant occurrence due to some other cause? The most plausible explanation on the second hypothesiswould seem to be the following. The segments with their appendages of Arthropoda and Annelida are normally formed from before backwards, therefore every member of these two groups with more than three segments must necessarily pass through a stage withonly three segments, and the fact that in a particular group this stage is often reached on the larva being hatched is in itself no proof that the ancestor of the group had only three segments with their appendages. This explanation appears to me, so far as it goes, quite valid; but though it relieves us from the necessity of supposing that the primitive Crustacea had only three pairs of appendages, it does not explain several other peculiarities of the Nauplius[203]. The more important of these are the following.
1. That the mandibles have the form of biramous swimming feet and are not provided with a cutting blade.
2. That the second pair of antennæ are biramous swimming feet with a hook used in mastication, and are innervated (?) from the subœsophageal ganglion.
3. The absence of segmentation in the Nauplius body. An absence which is the more striking in that before the Nauplius stage is fully reached the body of the embryo is frequently divided into three segments,e.g.Copepoda and Cirripedia.
4. The absence of a heart.
5. The presence of a median single eye as the sole organ of vision.
Of these points the first, second, and fifth appear only to be capable of being explained phylogenetically, while with reference to the absence of a heart it appears very improbable that the ancestral Crustacea were without a central organ of circulation. If the above positions are accepted the conclusion would seem to follow that in a certain sense the Nauplius is an ancestral form—but that, while it no doubt had its three anterior pairs of appendages similar to those of existing Nauplii, it may perhaps have been provided with a segmented body behind provided with simple biramous appendages. A heart and cephalo-thoracic shield may also have been present, though the existence of the latter is perhaps doubtful. There was no doubt a median single eye, but it is difficult to decide whether or no paired compound eyes were also present. The tail ended in a fork between the prongs of which the anus opened; and the mouth was protected by a large upper lip. In fact, it may very probably turn out that the most primitive Crustacea more resembled an Apus larva at the moult immediately before the appendages lose their Nauplius characters (fig. 208B), or a Cyclops larva just before the Cyclops stage (fig. 229), than the earliest Nauplius of either of these forms.
If the Nauplius ancestor thus reconstructed is admitted to have existed, the next question in the phylogeny of the Crustacea concerns the relations of the various phyla to the Nauplius. Are the different phyla descended from the Nauplius direct, or have they branched at a later period fromsome central stem? It is perhaps hardly possible as yet to give a full and satisfactory answer to this question, which requires to be dealt with for each separate phylum; but it may probably be safely maintained that the existing Phyllopods are members of a group which was previously much larger, and the most central of all the Crustacean groups; and which more nearly retains in the characters of the second pair of antennæ etc. the Nauplius peculiarities. This view is shared both by Claus and Dohrn, and appears to be in accordance with all the evidence we have both palæontological and morphological. Claus indeed carries this view still further, and believes that the later Nauplius stages of the different Entomostracan groups and the Malacostraca (Penæus larva) exhibit undoubted Phyllopod affinities. He therefore postulates the earlier existence of a Protophyllopod form, which would correspond very closely with the Nauplius as reconstructed above, from which he believes all the Crustacean groups to have diverged.
It is beyond the scope of this work to attempt to grapple with all the difficulties which arise in connection with the origin and relationships of the various phyla, but I confine myself to a few suggestions arising out of the developmental histories recorded above.
Malacostraca.In attempting to reconstitute from the evidence in our possession the ancestral history of the Malacostraca we may omit from consideration the larval history of all those types which leave the egg in nearly the adult form, and confine our attention to those types in which the larval history is most completely preserved.
There are three forms which are of special value in this respect,viz.Euphausia, Penæus and Squilla. From the history of these which has already been given it appears that in the case of the Decapoda four stages (Claus) may be traced in the best preserved larval histories.
1. A Nauplius stage with the usual Nauplius characters.
2. A Protozoæa stage in which the maxillæ and first pair of maxillipeds are formed behind the Nauplius appendages; but in which the tail is still unsegmented. This stage is comparatively rarely preserved and usually not very well marked.
3. A Zoæa stage the chief features of which have already been fully characterised (videp.465). Three more or less distinct types of Zoæa are distinguished by Claus. (a) That of Penæus, in which the appendages up to the third pair of maxillipeds are formed, and the thorax and abdomen are segmented, the former being however very short. The heart is oval, with one pair of ostia. From this type the Zoæa forms of the other Decapoda are believed by Claus to be derived. (b) That of Euphausia, with but one pair of maxillipeds and those short and Phyllopod-like. The heart oval with one pair of ostia. (c) That of Squilla, with an elongated many-chambered heart, two pairs of maxillipeds and the abdominal appendages in full activity.
4. A Mysis stage, which is only found in the macrurous Decapod larvæ.
The embryological questions requiring to be settled concern the valueof the above stages. Do they represent stages in the actual evolution of the present types, or have their characters been secondarily acquired in larval life?
With reference to the first stage this question has already been discussed, and the conclusion arrived at, that the Nauplius does in a much modified form represent an ancestral type. As to the fourth stage there can be no doubt that it is also ancestral, considering that it is almost the repetition of an actually existing form.
The second stage can clearly only be regarded as an embryonic preparation for the third; and the great difficulty concerns the third stage.
The natural view is that this stage like the others has an ancestral value, and this view was originally put forward by Fritz Müller and has been argued for also by Dohrn. On the other hand the opposite side has been taken by Claus, who has dealt with the question very ably and at great length, and has clearly shewn that some of Fritz Müller’s positions are untenable. Though Claus’ opinion is entitled to very great weight, an answer can perhaps be given to some of his objections. The view adopted in this section can best be explained by setting forth the chief points which Claus urges against Fritz Müller’s view.
The primary question which needs to be settled is whether the Malacostraca have diverged very early from the Nauplius root, or later in the history of the Crustacea from the Phyllopod stem. On this question Claus[204]brings arguments, which appear to me very conclusive, to shew that the Malacostraca are derived from a late Protophyllopod type, and Claus’ view on this point is shared also by Dohrn. The Phyllopoda present so many characters (not possessed by the Nauplius) in common with the Malacostraca or their larval forms, that it is incredible that the whole of these should have originated independently in the two groups. The more important of these characters are the following.
1. The compound eyes, so often stalked in both groups.
2. The absence of a palp on the mandible, a very marked character of the Zoæa as well as of the Phyllopoda.
3. The presence of a pair of frontal sense knobs.
4. The Phyllopod character of many of the appendages.Cf.first pair of maxillipeds of the Euphausia Zoæa.
5. The presence of gill pouches (epipodites) on many of the appendages[205].
In addition to these points, to which others might be added, Claus attempts to shew that Nebalia must be regarded as a type intermediate between the Phyllopods and Malacostraca. This view seems fairly established, and if true is conclusive in favour of the Phyllopod origin of the Malacostraca. If the Protophyllopod origin of the Malacostraca is admitted, it seems clear that the ancestral forms of the Malacostraca must have developed their segments regularly from before backwards, and been provided with nearly similar appendages on all the segments. This however is far from the case in existing Malacostraca, and Fritz Müller commences his summary of the characters of the Zoæa in the following words[206]. “The middle body with its appendages, those five pairs of feet to which these animals owe their name, is either entirely wanting or scarcely indicated.” This he regards as an ancestral character of the Malacostraca, and is of opinion that their thorax is to be regarded as a later acquirement than the head or abdomen. Claus’ answer on this point is that in the most primitive Zoææ,viz.those already spoken of as types, the thoracic and abdominal segments actually develop in regular succession from before backwards, and he therefore concludes that the late development of the thorax in the majority of Zoæa forms is secondary and not an ancestral Phyllopod peculiarity.
This is the main argument used by Claus against the Zoæa having any ancestral meaning. His view as to the meaning of the Zoæa may be gathered from the following passage. After assuming that none of the existing Zoæa types could have been adult animals, he says—“Much more probably the process of alteration of the metamorphosis, which the Malacostracan phylum underwent in the course of time and in conjunction with the divergence of the later Malacostracan groups, led secondarily to the three different Zoæa configurations to which probably later modifications were added, as for instance in the young form of the Cumaceæ. We might with the same justice conclude that adult Insects existed as caterpillars or pupæ as that the primitive form of the Malacostraca was a Protozoæa or Zoæa.”
Granting Claus’ two main positions,viz.that the Malacostraca are derived from Protophyllopods, and that the segments were in the primary ancestral forms developed from before backwards, it does not appear impossible that a secondary and later ancestral form may have existed with a reduced thorax. This reduction may only have been partial, so that the Zoæa ancestor would have had the following form. A large cephalo-thorax and well-developed tail (?) with swimming appendages. The appendages up to the second pair of maxillipeds fully developed, but the thorax veryimperfect and provided only with delicate foliaceous appendages not projecting beyond the edge of the cephalo-thoracic shield.
Another hypothesis for which there is perhaps still more to be said is that there was a true ancestral Zoæa stage in which the thoracic appendages were completely aborted. Claus maintains that the Zoæa form with aborted thorax is only a larval form; but he would probably admit that its larval characters were acquired to enable the larva to swim better. If this much be admitted it is not easy to see why an actual member of the ancestral series of Crustacea should not have developed the Zoæa peculiarities when the mud-dwelling habits of the Phyllopod ancestors were abandoned, and a swimming mode of life adopted. This view, which involves the supposition that the five (or six including the third maxillipeds) thoracic appendages were lost in the adult (for they may be supposed to have been retained in the larva) for a series of generations, and reappeared again in the adult condition, at a later period, may at first sight appear very improbable, but there are, especially in the larval history of the Stomatopoda, some actual facts which receive their most plausible explanation on this hypothesis.
These facts consist in cases of the actual loss of appendages during development, and their subsequent reappearance. The two most striking cases are the following.
1. In the Erichthus form of the Squilla larva the appendages corresponding to the third pair of maxillipeds and first two pairs of ambulatory appendages of the Decapoda are developed in the Protozoæa stage, but completely aborted in the Zoæa stage, and subsequently redeveloped.
2. In the case of the larva of Sergestes in the passage from the Acanthosoma (Mysis) stage to the Mastigopus stage the two hindermost thoracic appendages become atrophied and redevelop again later.
Both of these cases clearly fit in very well with the view that there was an actual period in the history of the Malacostraca in which the ancestors of the present forms were without the appendages which are aborted and redeveloped again in these larval forms. Claus’ hypothesis affords no explanation of these remarkable cases.
It is however always possible to maintain that the loss and reappearance of the appendages in these cases may have no ancestral meaning; and the abortion of the first pair of maxillipeds and reduction of some of the other appendages in the case of the Loricata is in favour of this explanation. Similar examples of the abortion and reappearance of appendages, which cannot be explained in the way attempted above, are afforded by the Mites and also by the Insects,e.g.Bees.
On the other hand there is almost a conclusive indication that the loss of the appendages in Sergestes has really the meaning assigned to it, in that in the allied genius Leucifer the two appendages in question are actually absent in the adult, so that the stage with these appendages absent is permanently retained in an adult form. In the absence of the mandibular palp in all the Zoæa forms, its actual atrophy in the Penæus Zoæa, and itsuniversal reappearance in adult Malacostraca, are cases which tell in favour of the above explanation. The mandibular palp is permanently absent in Phyllopods, which clearly shews that its absence in the Zoæa stage is due to the retention of an ancestral peculiarity, and that its reappearance in the adult forms was a late occurrence in the Malacostracan history.
The chief obvious difficulty of this view is the redevelopment of the thoracic feet after their disappearance for a certain number of generations. The possibility of such an occurrence appears to me however clearly demonstrated by the case of the mandibular palp, which has undoubtedly been reacquired by the Malacostraca, and by the case of the two last thoracic appendages of Sergestes just mentioned. The above difficulty may be diminished if we suppose that the larvæ of the Zoæa ancestors always developed the appendages in question. Such appendages might first only partially atrophy in a particular Zoæa form and then gradually come to be functional again; so that, as a form with functional thoracic limbs came to be developed out of the Zoæa, we should find in the larval history of this form that the limbs were developed in the pre-zoæal larval stages, partially atrophied in the Zoæa stage, and redeveloped in the adult. From this condition it would not be difficult to pass to a further one in which the development of the thoracic limbs became deferred till after the Zoæa stage.
The general arguments in favour of a Zoæa ancestor with partially or completely aborted thoracic appendages having actually existed in the past appear to me very powerful. In all the Malacostracan groups in which the larva leaves the egg in an imperfect form a true Zoæa stage is found. That the forms of these Zoææ should differ considerably is only what might be expected, considering that they lead a free existence and are liable to be acted upon by natural selection, and it is probable that none of those at present existing closely resemble the ancestral form. The spines from their carapace, which vary so much, were probably originally developed, as suggested by Fritz Müller, as a means of defence. The simplicity of the heart—so different from that of Phyllopods—in most forms of Zoæa is a difficulty, but the reduction in the length of the heart may very probably be a secondary modification; the primitive condition being retained in the Squilla Zoæa. In any case this difficulty is not greater on the hypothesis of the Zoæa being an ancestral form, than on that of its being a purely larval one.
The points of agreement in the number and character of the appendages, form of the abdomen, etc. between the various types of Zoæa appear to me too striking to be explained in the manner attempted by Claus. It seems improbable that a peculiarity of form acquired by the larva of some ancestral Malacostracan should have been retained so permanently in so many groups[207]—morepermanently indeed than undoubtedly ancestral forms like that of Mysis—and it would be still more remarkable that a Zoæa form should have been two or more times independently developed.
There are perhaps not sufficient materials to reconstruct the characters of the Zoæa ancestor, but it probably was provided with the anterior appendages up to the second pair of maxillipeds, and (?) with abdominal swimming feet. The heart may very likely have been many-chambered. Whether gill pouches were present on the maxillipeds and abdominal feet does not appear to me capable of being decided. The carapace and general shape were probably the same as in existing Zoæas. It must be left an open question whether the six hindermost thoracic appendages were absent or only very much reduced in size.
On the whole then it may be regarded as probable that the Malacostraca are descended from Protophyllopod forms, in which, on the adoption of swimming habits, six appendages of the middle region of the body were reduced or aborted, and a Zoæa form acquired, and that subsequently the lost appendages were redeveloped in the descendants of these forms, and have finally become the most typical appendages of the group.
The relationship of the various Malacostracan groups is too difficult a subject to be discussed here, but it seems to me most likely that in addition to the groups with a Zoæa stage the Edriophthalmata and Cumaceæ are also post-zoæal forms which have lost the Zoæa stage. Nebalia is however very probably to be regarded as a præ-zoæal form which has survived to the present day; and one might easily fancy that its eight thin thoracic segments with their small Phyllopod-like feet might become nearly aborted.
Copepoda.The Copepoda certainly appear to have diverged very early from the main stem, as is shewn by their simple biramous feet and the retention of the median eye as the sole organ of vision. It may be argued that they have lost the eye by retrogressive changes, and in favour of this view cases of the Pontellidæ and of Argulus may be cited. It is however more than doubtful whether the lateral eyes of the Pontellidæ are related to the compound Phyllopod eye, and the affinities of Argulus are still uncertain. It would moreover be a great paradox if in a large group of Crustacea the lateral eyes had been retained in a parasitic form only (Argulus), but lost in all the free forms.
Cirripedia.The Cirripedia are believed by Claus to belong to the same phylum as the Copepoda. This view does not appear to be completely borne out by their larval history. The Nauplius differs very markedly from that of the Copepoda, and this is still more true of the Cypris stage. The Copepod-like appendages of this stage are chiefly relied upon to support the above view, but this form of appendages was probably very primitive and general, and the number (without taking into consideration the doubtful case of Cryptophialus) does not correspond to that in Copepoda. On the other hand the paired eyes and the bivalve shell form great difficulties in the way of Claus’ view. It is clear that the Cypris stage represents more or lessclosely an ancestral form of the Cirripedia, and that both the large bivalve shell and the compound eyes were ancestral characters. These characters would seem incompatible with Copepod affinities, but point to the independent derivation of the Cirripedia from some early bivalve Phyllopod form.
Figures illustrating the development of AstacusFig. 237. Figures illustrating the development of Astacus.(From Parker; after Reichenbach.)A. Section through part of the ovum during segmentation.n.nuclei;w.y.white yolk;y.p.yolk pyramids;c.central yolk mass.B and C. Longitudinal sections during the gastrula stage.a.archenteron;b.blastopore;ms.mesoblast;ec.epiblast;en.hypoblast distinguished from epiblast by shading.D. Highly magnified view of the anterior lip of blastopore to shew the origin of the primary mesoblast from the wall of the archenteron.p.ms.primary mesoblast;ec.epiblast;en.hypoblast.E. Two hypoblast cells to shew the amœba-like absorption of yolk spheres.y.yolk;n.nucleus;p.pseudopodial process.F. Hypoblast cells giving rise endogenously to the secondary mesoblast (s.ms.,).n.nuclei.
Fig. 237. Figures illustrating the development of Astacus.(From Parker; after Reichenbach.)
A. Section through part of the ovum during segmentation.n.nuclei;w.y.white yolk;y.p.yolk pyramids;c.central yolk mass.B and C. Longitudinal sections during the gastrula stage.a.archenteron;b.blastopore;ms.mesoblast;ec.epiblast;en.hypoblast distinguished from epiblast by shading.D. Highly magnified view of the anterior lip of blastopore to shew the origin of the primary mesoblast from the wall of the archenteron.p.ms.primary mesoblast;ec.epiblast;en.hypoblast.E. Two hypoblast cells to shew the amœba-like absorption of yolk spheres.y.yolk;n.nucleus;p.pseudopodial process.F. Hypoblast cells giving rise endogenously to the secondary mesoblast (s.ms.,).n.nuclei.
Ostracoda.The independent origin of the Ostracoda from the main Crustacean stem seems probable. Claus points out that the Ostracoda present by no means a simple organisation, and concludes that they were not descended from a form with a more complex organisation and a larger number of appendages. Some simplifications have however undoubtedly taken place, as the loss of the heart, and of the compound eyes in many forms. These simplifications are probably to be explained (as is done by Claus) as adaptations due to the small size of body and its enclosure in a thick bivalve shell. Although Claus is strongly opposed to the view thatthe number of the appendages has been reduced, yet the very fact of the (in some respects) complex organisation of this group might seem to indicate that it cannot have diverged from the Phyllopod stem at so early a stage as (on Claus’ view of the Nauplius) would seem to be implied by the very small number of appendages which is characteristic of it, and it therefore appears most probable that the present number may be smaller than that of the ancestral forms.
The formation of the germinal layers.
The formation of the germinal layers has been more fully studied in various Malacostraca, more especially in the Decapoda, than in other groups.
Decapoda.To Bobretzky (No.472) is due the credit of having been the pioneer in this line of investigation; and his researches have been followed up and enlarged by Haeckel, Reichenbach (No.488), and Mayer (No.482). The segmentation is centrolecithal and regular (fig. 237A). At its close the blastoderm is formed of a single uniform layer of lens-shaped cells enclosing a central sphere of yolk, in which as a rule all trace of the division into columns, present during the earlier stages of segmentation, has disappeared; though in Palæmon the columns remain for a long period distinct. The cells of the blastoderm are at first uniform, but in Astacus, Eupagurus, and most Decapoda, soon become more columnar for a small area, and form a circular patch. The whole patch either becomes at once invaginated (Eupagurus, Palæmon,fig. 239A) or else the edge of it is invaginated as a roughly speaking circular groove deeper anteriorly than posteriorly, within which the remainder of the patch forms a kind of central plug, which does not become invaginated till a somewhat later period (Astacus,fig. 237B and C). After the invagination of the above patch the remainder of the blastoderm cells form the epiblast.
The invaginated sack appears to be the archenteron and its mouth the blastopore. The mouth finally becomes closed[208], and the sack itself then forms the mesenteron.
Sections of the embryo of AstacusFig. 238. Two longitudinal sections of the embryo of Astacus.(From Parker; after Bobretzky.)A. Nauplius stage. B. Stage after the hypoblast cells have absorbed the food-yolk. The ventral surface is turned upwards.fg.stomodæum;hg.proctodæum;an.anus;m.mouth;mg.mesenteron;abd.abdomen;h.heart.
Fig. 238. Two longitudinal sections of the embryo of Astacus.(From Parker; after Bobretzky.)
A. Nauplius stage. B. Stage after the hypoblast cells have absorbed the food-yolk. The ventral surface is turned upwards.fg.stomodæum;hg.proctodæum;an.anus;m.mouth;mg.mesenteron;abd.abdomen;h.heart.
In Astacus the archenteron gradually grows forwards, its opening is at first wide, but becomes continuously narrowedand is finally obliterated. Very shortly after this occurrence there is formed, slightly in front of the point where the last trace of the blastopore was observable, a fresh epiblastic invagination, which gives rise to the proctodæum, and the opening of which remains as the definite anus. The proctodæum (fig. 238A,hg)is very soon placed in communication with the mesenteron (mg). The stomodæum (fg) is formed during the same stage as the proctodæum. It gives rise to the œsophagus and stomach. The hypoblast cells which form the wall of the archenteron grow with remarkable rapidity at the expense of the yolk; the spherules of which they absorb and digest in an amœba-like fashion by means of their pseudopodia. They become longer and longer, and finally, after absorbing the whole yolk, acquire a form almost exactly similar to that of the yolk pyramids during segmentation (fig. 238B). They enclose the cavity of the mesenteron, and their nuclei and protoplasm are situated externally. The cells of the mesenteron close to its junction with the proctodæum differ from those elsewhere in being nearly flat.
In Palæmon (Bobretzky) the primitive invagination (fig. 239A) has far smaller dimensions than in Astacus, and appears before the blastoderm cells have separated from the yolk pyramids. The cells which are situated at the bottom of it pass into the yolk, increase in number, and absorb the whole yolk, forming a solid mass of hypoblast in which the outlines of the individual cells would seem at first not to be distinct. The blastopore in the meantimebecomes closed. Some of the nuclei now pass to the periphery of the yolk mass; the cells appertaining to them gradually become distinct and assume a pyramidal form (fig. 239B,hy), the inner ends of the cells losing themselves in a central mass of yolk, in the interior of which nuclei are at first present but soon disappear. The mesenteron thus becomes constituted of a layer of pyramidal cells which merge into a central mass of yolk. Some of the hypoblast cells adjoining the junction of the proctodæum and mesenteron become flattened, and in the neighbourhood of these cells a lumen first appears. The stomodæum and proctodæum are formed as in Astacus.Fig. 239B shews the relative positions of the proctodæum, stomodæum, and mesenteron. Although the process of formation of the hypoblast and mesenteron is essentially the same in Astacus and Palæmon, yet the differences between these two forms are very interesting, in that the yolk isexternalto the mesenteron in Astacus, butenclosed within itin Palæmon. This difference in the position of the yolk is rendered possible by the fact that the invaginated hypoblast cells in Palæmon do not, at first, form a continuous layer enclosing a central cavity, while they do so in Astacus.
Two stages in the development of PalæmonFig. 239. Two stages in the development of Palæmon seen in section.(After Bobretzky.)A. Gastrula stage.B. Longitudinal section through a late stage.hy.hypoblast;sg.supra-œsophageal ganglion;vg.ventral nerve cord; hd. proctodæum;st.stomodæum.
Fig. 239. Two stages in the development of Palæmon seen in section.(After Bobretzky.)
A. Gastrula stage.B. Longitudinal section through a late stage.hy.hypoblast;sg.supra-œsophageal ganglion;vg.ventral nerve cord; hd. proctodæum;st.stomodæum.
The mesoblast appears to be formed of cells budded off from the anterior wall of the archenteron (Astacus,fig. 237D),or from its lateral walls generally (Palæmon). They make their first appearance soon after the invagination of the hypoblast has commenced. The mesoblast cells are at first spherical, and gradually spread, especially in an anterior direction, from their point of origin.
According to Reichenbach there are formed in Astacus at the Nauplius stage a number of peculiar cells which he speaks of as ‘secondary mesoblast cells.’ His account is not very clear or satisfactory, but it appears that they originate (fig. 237F) in the hypoblast cells by a kind of endogenous growth, and though they have at first certain peculiar characters they soon become indistinguishable from the remaining mesoblast cells.
Towards the end of the Nauplius period the secondary mesoblast cells aggregate themselves into a rod close to the epiblast in the median ventral line, and even bifurcate round the mouth and extend forwards to the extremity of the procephalic lobes. This rod of cells very soon vanishes, and the secondary mesoblast cells become indistinguishable from the primary. Reichenbach believes, on not very clear evidence, that these cells have to do with the formation of the blood.
General form of the body.The ventral thickening of epiblast or ventral plate, continuous with the invaginated patch already mentioned, forms the first indication of the embryo. It is at first oval, but soon becomes elongated and extended anteriorly into two lateral lobes—the procephalic lobes. Its bilateral symmetry is further indicated by a median longitudinal furrow. The posterior end of the ventral plate next becomes raised into a distinct lobe—the abdomen—which in Astacus at first liesin frontof the still open blastopore. This lobe rapidly grows in size, and at its extremity is placed the narrow anal opening. It soon forms a well-marked abdomen bent forwards over the region in front (figs. 239B, and240A and B). Its early development as a distinct outgrowth causes it to be without yolk; and so to contrast very forcibly with the anterior thoracic and cephalic regions of the body. In most cases this process corresponds to the future abdomen, but in some cases (Loricata) it appears to include part of the thorax. Before it has reached a considerable development, three pairs of appendages spring from the region of the head,viz.two pairs of antennæ and the mandibles, and inaugurate a so-called Nauplius stage (fig. 240A). These three appendages are formed nearly simultaneously, but the hindermost appears to become visible slightly before the two others(Bobretzky). The mouth lies slightly behind the anterior pair of antennæ, but distinctly in front of the posterior pair. The other appendages, the number of which at the time of hatching varies greatly in the different Decapods (videsection on larval development), sprout in succession from before backwards (fig. 240B). The food-yolk in the head and thoracic region gradually becomes reduced in quantity with the growth of the embryo, and by the time of hatching the disparity in size between the thorax and abdomen has ceased to exist.
Isopoda.The early embryonic phases of the Isopoda have been studied by means of sections by Bobretzky (No.498) and Bullar (No.499) and have been found to present considerable variations. When laid the egg is enclosed in a chorion, but shortly after the commencement of segmentation (Ed. van Beneden and Bullar) a second membrane appears, which is probably of the nature of a larval membrane.
Two stages in the development of PalæmonFig. 240. Two stages in the development of Palæmon.A. Nauplius stage.B. Stage with eight pairs of appendages.op.eyes;at1.andat2.first and second antennæ;md.mandibles;mx1,mx2.,first and second maxillæ;mxp3.third maxillipeds;lb.upper lip.
Fig. 240. Two stages in the development of Palæmon.
A. Nauplius stage.B. Stage with eight pairs of appendages.op.eyes;at1.andat2.first and second antennæ;md.mandibles;mx1,mx2.,first and second maxillæ;mxp3.third maxillipeds;lb.upper lip.
In all the forms the segmentation is followed by the formation of a blastoderm, completely enclosing the yolk, and thickened along an area which will become the ventral surface of the embryo. In this area the blastoderm is formed of at least two layers of cells—an external columnar epiblast, and an internal layer of scattered cells which form the mesoblast and probably in part also the hypoblast (Oniscus,Bobretzky; Cymothoa,Bullar).
In Asellus aquaticus there is a centrolecithal segmentation, ending in the formation of a blastoderm, which appears first on the ventral surface and subsequently extends to the dorsal.
In Oniscus murarius, and Cymothoa the segmentation is partial [for its peculiarities and relationshipvidep.120] and a disc, formed of a single layer of cells, appears at a pole of the egg which corresponds to the future ventral surface (Bobretzky). This layer gradually grows round the yolk partly by division of its cells, though a formation of fresh cells from the yolk may also take place. Before it has extended far round the yolk, the central part of it becomes two or more layers deep, and the cells of the deeper layers rapidly increase in number, and are destined to give rise to the mesoblast and probably also to part or the whole of the hypoblast. In Cymothoa this layer does not at first undergo any important change, but in Oniscus it becomes very thick, and its innermost cells (Bobretzky) become imbedded in the yolk, which they rapidly absorb; and increasing in number first of all form a layer in the periphery of the yolk, and finally fill up the whole of the interior of the yolk (fig. 241A), absorbing it in the process.
It appears possible that these cells do not, as Bobretzky believes, originate from the blastoderm, but from nuclei in the yolk which have escaped his observation. This mode of origin would be similar to that by which yolk cells originate in the eggs of the Insecta, etc. If Bobretzky’s account is correct we must look to Palæmon, as he himself suggests, to find an explanation of the passage of the hypoblast cells into the yolk. The thickening of the primitive germinal disc would, according to this view, be equivalent to the invagination of the archenteron in Astacus, Palæmon, etc.
Two longitudinal sections through the embryo of Oniscus murariusFig. 241. Two longitudinal sections through the embryo of Oniscus murarius.(After Bobretzky.)st.stomodæum;pr.proctodæum;hy.hypoblast formed of large nucleated cells imbedded in the yolk;m.mesoblast;vg.ventral nerve cord;sg.supra-œsophageal ganglion;li.liver;do.dorsal organ;zp.rudiment of masticatory apparatus;ol.upper lip.
Fig. 241. Two longitudinal sections through the embryo of Oniscus murarius.(After Bobretzky.)
st.stomodæum;pr.proctodæum;hy.hypoblast formed of large nucleated cells imbedded in the yolk;m.mesoblast;vg.ventral nerve cord;sg.supra-œsophageal ganglion;li.liver;do.dorsal organ;zp.rudiment of masticatory apparatus;ol.upper lip.
Whatever may be the origin of the cells in the yolk they no doubt correspond to the hypoblast of other types. In Cymothoa nothing similar to them has been met with, but the hypoblast has a somewhat different origin being apparently formed from some of the indifferent cells below the epiblast, which collect as a solid mass on the ventral surface, and then divide into two masses which become hollow and give rise to the liver cæca. Their fate, as well as that of the hypoblast in Oniscus, is dealt with in connection with the alimentary tract. The completion of the enclosure of the yolk by the blastoderm takes place on the dorsal surface. In all the Isopods which have been carefullystudied, there appears before any other organ a provisional structure formed from the epiblast and known as the dorsal organ. An account of it is given in connection with the development of the organs. The general external changes undergone by the larva in its development are as follows. The ventral thickened area of the blastoderm (ventral plate) shapes itself and girths nearly the whole circumference of the ovum in Oniscus (fig. 241A) but is relatively much shorter in Cymothoa. Anteriorly it dilates into the two procephalic lobes. In Cymothoa it next becomes segmented; and the anterior segments are formed nearly simultaneously, and those of the abdomen somewhat later. At the same time a median depression appears dividing the blastoderm longitudinally into two halves. The appendages are formed later than their segments, and the whole of them are formed nearly simultaneously, with the exception of the last thoracic, which does not appear till comparatively late after the hatching of the embryo. The late development of the seventh thoracic segment and appendage is a feature common to the majority of the Isopoda (Fritz Müller). In Oniscus the limbs are formed in nearly the same way as in Cymothoa, but in Asellus they do not arise quite simultaneously. First of all, the two antennæ and mandibles (the future palp) appear, inaugurating a stage often spoken of as the Nauplius stage, which is supposed to correspond with the free Naupliusstage of Penæus and Euphausia. At this stage a cuticle is shed (Van Beneden) which remains as an envelope surrounding the larva till the time of hatching. Similar cuticular envelopes are formed in many Isopoda. Subsequently the appendages of the thorax appear, and finally those of the abdomen. Later than the appendages there arise behind the mouth two prominences which resemble appendages, but give rise to a bilobed lower lip (Dohrn).
In Asellus and Oniscus the ventral plate moulds itself to the shape of the egg, and covers the greater part of the dorsal as well as of the ventral side (fig. 241A). As a result of this the ventral surface of the embryo is throughout convex; and in Asellus a deep fold appears on the back of the embryo, so that the embryo appears coiled up within the egg with its ventral side outwards and its head and tail in contact. In Oniscus the ventral surface is convex, but the dorsal surface is never bent in as in Asellus. In Cymothoa the egg is very big and the ventral plate does not extend nearly so far round to the dorsal side as in Asellus, in consequence of which the ventral surface is not nearly so convex as in other Isopoda. At the same time the telson is early formed, and is bent forwards so as to lie on the under side of the part of the blastoderm in front. In having this ventral curvature of the telson Cymothoa forms an exception amongst Isopods; and in this respect is intermediate between the embryos of Asellus and those of the Amphipoda.
Amphipoda.Amongst the Amphipoda the segmentation is usually centrolecithal. In the case of Gammarus locusta (Ed. van Beneden and Bessels,No.503) it commences with an unequal but total segmentation like that of the Frog (videp.97), and the separation of a central yolk mass is a late occurrence; and it is noticeable that the part of the egg with the small segments eventually becomes the ventral surface. In the fresh-water species of Gammarus (G. pulex and fluviatilis) the segmentation is more like that of Insects; the blastoderm cells being formed nearly simultaneously over a large part of the surface of the egg.
Both forms of segmentation give rise to a blastoderm covering the whole egg, which soon becomes thickened on the ventralsurface. There is formed, as in the Isopoda, a larval membrane at about the time when the blastoderm is completed. Very soon after this the egg loses its spherical shape, and becomes produced into a pointed extremity—the future abdomen—which is immediately bent over the ventral surface of the part in front. The ventral curvature of the hinder part of the embryo at so early an age stands in marked contrast to the usual condition of Isopod embryos, and is only approached in this group, so far as is known, in the case of Cymothoa.
At the formation of the first larval membrane the blastoderm cells separate themselves from it, except at one part on the dorsal surface. The patch of cells adherent at this part gives rise to a dorsal organ, comparable with that in Oniscus, connecting the embryo and its first larval skin. A perforation appears in it at a later period.
The segments and limbs of the Amphipoda are all formed before the larva leaves the egg.
Cladocera.The segmentation (Grobben,No.455) takes place on the normal centrolecithal type, but is somewhat unequal. Before the close of the segmentation there may be seen at the apex of the vegetative pole one cell marked off from the remainder by its granular aspect. It gives rise to the generative organs. One of the cells adjoining it gives rise to the hypoblast, and the other cells which surround it form the commencement of the mesoblast. The remaining cells of the ovum form the epiblast. By a later stage the hypoblast cell is divided into thirty-two cells and the genital cell into four, while the mesoblast forms a circle of twelve cells round the genital mass.
The hypoblast soon becomes involuted; the blastopore probably closes, and the hypoblast forms a solid cord of cells which eventually becomes the mesenteron. The stomodæum is said to be formed at the point of closure of the blastopore. The mesoblast passes inwards and forms a mass adjoining the hypoblast, and somewhat later the genital mass also becomes covered by the epiblast. The proctodæum appears to be formed later than the stomodæum.
The embryo as first shewn by Dohrn passes through a Nauplius stage in the brood-pouch, but is hatched, except in the case of the winter eggs of Leptodora, in a form closely resembling the adult.
Copepoda.Amongst the free Copepoda the segmentation and formation of the layers have recently been investigated by Hoek (No.512). He finds that there is, in both the fresh-water and marine forms studied by him, a centrolecithal segmentation similar to that of Palæmon and Pagurus (videp.112), which might from the surface be supposed to becomplete and nearly regular. After the formation of the blastoderm an invagination of some of its cells takes place and is completed in about a quarter of an hour. The opening becomes closed. This invagination is compared by Hoek to the invagination in Astacus, and is believed by him to give rise to the mesenteron. Its point of closing corresponds with the hind end of the embryo. On the ventral surface there appear two transverse furrows dividing the embryo into three segments, and a median longitudinal furrow which does not extend to the front end of the foremost segment. The three pairs of Nauplius appendages and upper lip become subsequently formed as outgrowths from the sides of the ventral blastodermic thickening.
Amongst the parasitic Copepoda there are found two distinct types of segmentation, analogous to those in the Isopoda. In the case of Condracanthus the segmentation is somewhat irregular, but on the type of Eupagurus, etc. (videp.112). In the other group (Anchorella, Clavella, Congericola, Caligus, Lerneopoda) the segmentation nearly resembles the ordinary meroblastic type (videp.120), and is to be explained in the same manner as in the cases of Oniscus and Cymothoa. The first blastodermic cells sometimes appear in a position corresponding with the head end of the embryo (Anchorella), at other times at the hind end (Clavella), and sometimes in the middle of the ventral surface. The dorsal surface of the yolk is always the latest to be inclosed by the blastoderm cells. A larval cuticle similar to that of the Isopoda is formed at the same time as the blastoderm. At the sides of the ventral thickening of the blastoderm there grow out the Nauplius appendages, of which only the first two appear in Anchorella. In Anchorella and Lerneopoda the embryos are not hatched at the Nauplius stage, but after the Nauplius appendages have been formed a fresh cuticle—the Nauplius cuticle—is shed, and within it the embryo develops till it reaches the so-called Cyclops stage (videp.490). The embryo within the egg has its abdomen curved dorsalwards as amongst the Isopoda.
Cirripedia.The segmentation of Balanus and Lepas commences by the segregation of the constituents of the egg into a more protoplasmic portion, and a portion formed mainly of food material. The former separates from the latter as a distinct segment, and then divides into two not quite equal portions. The division of the protoplasmic part of the embryo continues, and the resulting segments grow round the single yolk segment. The point where they finally enclose it is situated on the ventral surface (Lang) at about the position of the mouth (?).
After being enclosed by the protoplasmic cells the yolk divides, and gives rise to a number of cells, which probably supply the material for the walls of the mesenteron. The external layer of protoplasm forms the so-called blastoderm, and soon (Arnold, Lang) becomes thickened on the dorsal surface.
The embryo is next divided by two constrictions into three segments; and there are formed the three appendages corresponding to these, which areat first simple. The two posterior soon become biramous. The larva leaves the egg before any further appendages become formed.
Comparative development of the organs.
Central nervous system.The ventral nerve cord of the Crustacea develops as a thickening of the epiblast along the median ventral line; the differentiation of which commences in front, and thence extends backwards. The ventral cord is at first unsegmented. The supra-œsophageal ganglia originate as thickenings of the epiblast of the procephalic lobes.
The details of the above processes are still in most cases very imperfectly known. The fullest account we have is that of Reichenbach (No.488) for Astacus. He finds that the supra-œsophageal ganglia and ventral cord arise as a continuous formation, and not independently as would seem to be the case in Chætopoda. The supra-œsophageal ganglia are formed from the procephalic lobes. The first trace of them is visible in the form of a pair of pits, one on each side of the middle line. These pits become in the Nauplius stage very deep, and their walls are then continued into two ridges where the epiblast is several cells deep, which pass backwards one on each side of the mouth. The walls of the pits are believed by Reichenbach to give rise to the optic portions of the supra-œsophageal ganglia, and the epiblastic ridges to the remainder of the ganglia and to the circum-œsophageal commissures. At a much later stage, when the ambulatory feet have become formed, a median involution of epiblast in front of the mouth and between the two epiblast ridges gives rise to a central part of the supra-œsophageal ganglia. Five elements are thus believed by Reichenbach to be concerned in the formation of these ganglia,viz.two epiblast pits, two epiblast ridges, and an involution of epiblast between the latter. It should be noted however that the fate neither of the pair of pits, nor of the median involution, appears to have been satisfactorily worked out. The two epiblast ridges, which pass back from the supra-œsophageal ganglia on each side of the mouth, are continued as a pair of thickenings of the epiblast along the sides of a median ventral groove. This groove is deep in front and shallows out posteriorly. The thickenings on the sides of this groove no doubt give rise to the lateral halves of the ventral cord, and the cells of the groove itself are believed by Reichenbach, but it appears to me without sufficient evidence, to become invaginated also and to assist in forming the ventral cord. When the ventral cord becomes separated from the epiblast the two halves of it are united in the middle line, but it is markedly bilobed in section.
In the Isopoda it would appear both from Bobretzky’s and Bullar’s observations that the ventral nerve cord arises as an unpaired thickening of the epiblastin which there is no trace of anything like a median involution. After this thickening has become separated from the epiblast a slightmedian furrow indicates its constitution out of two lateral cords. The supra-œsophageal ganglia are stated to be developed quite simply as a pair of thickenings of the procephalic lobes, but whether they are from the first continuous with the ventral cord does not appear to have been determined.
The later stages in the differentiation of the ventral cord are, so far as is known, very similar throughout the Crustacea. The ventral cord is, as has been stated, at first unsegmented (fig. 241A,vg), but soon becomes divided by a series of constrictions into as many ganglia as there are pairs of appendages or segments (fig. 241B,vg).
There appears either on the ventral side (Oniscus) or in the centre (Astacus, Palæmon) of the two halves of each segment or ganglion a space filled with finely punctuated material, which is the commencement of the commissural portion of the cords. The commissural tissue soon becomes continuous through the length of the ventral cord, and is also prolonged into the supra-œsophageal ganglia.
After the formation of the commissural tissue the remaining cells of the cord form the true ganglion cells. A gradual separation of the ganglia next takes place, and the cells become confined to the ganglia, which are finally only connected by a double band of commissural tissue. The commissural tissue not only gives rise to the longitudinal cords connecting the successive ganglia, but also to the transverse commissures which unite the two halves of the individual ganglia.
The ganglia usually, if not always, appear at first to correspond in number with the segments, and the smaller number so often present in the adult is due to the coalescence of originally distinct ganglia.
Organs of special sense.Comparatively little is known on this head. The compound eyes are developed from the coalescence of two structures, both however epiblastic,viz.(1) part of the superficial epiblast of the procephalic lobes; (2) part of the supra-œsophageal ganglia. The former gives rise to the corneal lenses, the crystalline cones, and the pigment surrounding them; the latter to the rhabdoms and the cells which encircle them. Between these two parts a mesoblastic pigment is interposed.
Of the development of the auditory and olfactory organs almost nothing is known.
Dorsal organ.In a considerable number of the Malacostraca and Branchiopoda a peculiar organ is developed from the epiblast in the anterior dorsal region. This organ has been called the dorsal organ. It appears to be of a glandular nature, and is usually very large in the embryo or larva and disappears in the adult; but in some Branchiopoda it persists through life. In most cases it is unpaired, but in some instances a paired organ appears to take its place.
Various views as to its nature have been put forward. There is but little doubt of its being glandular, and it is possible that it is a provisional renal organ, though so far as I know concretions have not yet been found in it.
Its development has been most fully studied in the Isopoda.