Part 6 (2/2)

The mode of development of Peneus and Palinurus, as also several peculiar larvae of unknown origin, but which are in all probability to be attributed to Macrurous Crustacea, necessitate on the contrary the opposite supposition, namely, that the different groups of the Macrura have pa.s.sed from their original to their present mode of development independently of each other and also independently of the Crabs.” To this we may answer that the occurrence of the Zoea-form in all the above-mentioned Decapoda, its existence in Peneus during the whole of that period of life which is richest in progress and in which the wide gap between the Nauplius and the Decapod is filled up, its recurrence even in the development of the Stomapoda, the occurrence of a larval form closely approaching the youngest Zoea of Peneus in the Schizopod genus Euphausia, and the reminiscence of the structure of Zoea, which even the adult Tanais has preserved in its mode of respiration,--all indicate Zoea as one of those steps in development which persisted as a permanent form throughout a long period of repose, perhaps through a whole series of geological formations, and thus has also made a deeper impression upon the development of its descendants, and formed a firmer nucleus in the midst of other and more readily effaced young states. It cannot, therefore, surprise us that in transitions from the original mode of metamorphosis to direct development, even when produced independently, the larval life commences in the same way with this Zoea-form in different families, in which the earlier stages of development are effaced. But except what is common to all Zoeae, and what may easily be explained as being transferred back from a later into this stage, the Zoeae of the Crabs, for example, agree with those of Pagurus and Palaemon in no single peculiarity of structure which leads us to suppose a common inheritance. Consequently we may apparently a.s.sume, without hesitation, that when the Brachyura and Macrura separated, the primitive ancestors of each of these groups pa.s.sed through a more complete metamorphosis, and that the transition to the present mode of development belongs to a later period. With regard to the Brachyura, it may be added that in them this transition occurred only a little later and indeed before the existing families separated.

The arrangement of the processes of the carapace, and, still more, the similar number of the caudal setae in the most different Zoeae of Crabs (Figures 19 to 23) prove this. Such an accordance in the number of organs apparently so unimportant is only explicable by common inheritance. We may predict with certainty that amongst the Brachyura no species will occur which, like Peneus, still produces Nauplius-brood.*

(* I must not omit remarking that what has been said as to the development of the Crabs applies essentially only to the groups Cyclometopa, Catometopa and Oxyrhyncha, placed together by Alph.

Milne-Edwards as ”Eustomes.” Among the Oxystomata, as also among the ”Anomura apterura,” Edw., which approach so nearly to the Crabs, I am unacquainted with the earliest young states of any of the species.)

As we have already seen, Mysis and the Isopoda depart from all other Crustacea very remarkably by the fact that their embryos are curved upwards, instead of, as elsewhere, downwards. Does not so isolated a phenomenon as this, it might be asked, in the sense of Darwin's theory, indicate a common inheritance? Does it not necessitate that we should unite as the descendants of the same primitive ancestors, Mysis with the Isopoda on the one hand, and on the other the rest of the Podophthalma with the Amphipoda? I think not. Such a necessity exists only for those who estimate a peculiarity at a higher value because it makes its appearance at an earlier period of the egg-life. Whoever regards species as not created independently and unchangeably, but as having gradually become what they are, will say to himself that, when the ancestors of our Mysides came (probably much later than those of the Amphipoda and Isopoda) to develop numerous body-segments and limbs whilst still embryos, as they could no longer find room in the egg when extended straight out, and were therefore compelled to bend themselves, this could only take place either upwards or downwards, and whatever conditions may have decided the direction actually adopted, any near relations.h.i.+p to either of the two orders of Edriophthalma could hardly have taken part in it.

It may, however, be remarked, that the different curvature of the embryo in the Amphipoda and Isopoda is so far instructive, as it proves that their present mode of development was adopted only after the separation of these orders, and that, in the primitive stock of the Edriophthalma, the embryos were, if not Nauplii, at least short enough in the body to find room in the egg in an extended position, like the larvae of Achtheres enclosed by the Nauplius-skin. On the other hand the uniformity of development that prevails in each of the two orders--which is expressed in the Amphipoda for example in the formation of the ”micropylar apparatus,” in the Isopoda in the want of the last pair of ambulatory feet--testifies that the present mode of development has come down from a very early period and extends back beyond the separation of the present families. In these two orders also, as well as in the Crabs, we can hardly hope to find traces of earlier young states, unless it be in the family of the Tanaidae.* (* Whether the want of the abdominal feet in the young of Tanais be an inheritance from the time of the primitive Isopoda, or a subsequently acquired peculiarity, which appears to me the more admissible view at present, may perhaps be decided with some certainty, when we become acquainted with the development and mode of life of its family allies, Apseudes and Rhoea. The latter, as is well known, is the only Isopod which possesses a secondary flagellum on the anterior antennae. I have recently obtained a new and unexpected proof that the Tanaidae (”Asellotes heteropodes” M.-Edw.) of all known Crustacea approach most closely to the primitive form of the Edriophthalma. Mr. C. Spence Bate writes to me: ”Apseudes, as far as I know, is the ONLY Isopod in which the antennal scale so common in the Macrura is present on the lower antenna.”) If any one will furnish me with an Amphipod or an Isopod with Nauplius-brood, the existence of which would not be more remarkable in independently produced species than that of a Prawn with Nauplius-brood, I will abandon the whole Darwinian theory.

With regard to the Crabs, and also to the Isopoda and Amphipoda, we were led to the a.s.sumption that, about the period when these groups started from the common stem, a simplification of their process of development took place. This also seems to be intelligible from Darwin's theory.

When any circ.u.mstances favourable to a group of animals caused its wider diffusion and divergence into forms adapting themselves to new and various conditions of existence, this greater variability, which betrays itself in the production of new forms, will also favour the simplification of the development which is almost always advantageous, and moreover, exactly at this period, during adaptation to new circ.u.mstances, as has already been indicated with regard to fresh-water animals, this simplification will be doubly beneficial, and therefore, in connexion with this, a doubly strict selection will take place.

So much for the development of the higher Crustacea.

A closer examination of the developmental history of the lower Crustacea is unnecessary after what has been said in general upon the historical significance of the young states, and the application of this which has just been made to the Malacostraca. We may see, without further discussion, how the representation given by Claus of the development of the Copepoda may pa.s.s almost word for word as the primitive history of those animals; we may find in the Nauplius-skin of the larvae of Achtheres and in the egg-like larva of Cryptophialus, precisely similar traces of a transition towards direct development, as were presented by the Nauplius-envelope of the embryos of Mysis and the maggot-like larva of Ligia, etc.

It will be sufficient to indicate an essential difference in the process of development in the higher and lower Crustacea. In the latter all new body-segments and limbs which insert themselves between the two terminal regions of the Nauplius, are formed in uninterrupted sequence from before backwards; in the former there is further a new formation in the middle of the body (the middle-body), which pushes itself in between the fore-body and the abdomen in the same way, as these have done on their part between the head and tail of the Nauplius. Thus, that which appears probable even from the comparison of the limbs of the adult animal, finds fresh support in the developmental history, namely, that the lower Crustacea, like the Insects, are entirely dest.i.tute of the region of the body corresponding to the middle-body of the Malacostraca. It seems probable that the swimming feet of the Copepoda, as also of the pupae of Cirripedia and Rhizocephala, represent the abdominal feet of the Malacostraca, that is to say, are derived by inheritance from the same source with them.

It would be easy to weave together the separate threads furnished by the young forms of the various Crustacea, into a general picture of the primitive history of this cla.s.s. Such a picture, drawn with a little skill, and finished in lively colours, would certainly be more attractive than the dry discussions which I have tacked on to the developmental history of these animals. But the mode of weaving in the loose threads would still in many cases be arbitrary, and to be effected with equal justice in various ways; and many gaps would still have to be filled up by means of more or less bold a.s.sumptions. Those who have not wandered much in this region of research would then readily believe that they were standing upon firm ground, where mere fancy had thrown an airy bridge; those acquainted with the subject, on the other hand, would soon find out these weak points in the structure, but would then be easily led to regard even what was founded upon well considered facts, as merely floating in the air. To obviate these misconceptions of its true contents from either side, it would be necessary to accompany such a picture throughout with lengthy, dry explanations. This has deterred me from further filling in the outline which I had already sketched.

I will only give, as an example, the probable history of the production of a single group of Crustacea, and indeed of the most abnormal of all, the RHIZOCEPHALA, which in the s.e.xually mature state differ so enormously even from their nearest allies, the Cirripedia, and from their peculiar mode of nourishment stand quite alone in the entire animal kingdom.

I must preface this with a few words upon the h.o.m.ology of the roots of the Rhizocephala, i.e. the tubules which penetrate from its point of adhesion into the body of the host, ramify amongst the viscera of the latter, and terminate in caecal branchlets. In the pupae of the Rhizocephala (Figure 58) the foremost limbs (”prehensile antennae”) bear, on each of the two terminal joints, a tongue-like, thin-skinned appendage, in which we may generally observe a few small strongly refractive granules, like those seen in the roots of the adult animal. I have therefore supposed these appendages to be the rudiments of the future roots. A perfectly similar appendage, ”a most delicate tube or ribbon,” was found by Darwin in free-swimming pupae of Lepas australis on the last joints of the ”prehensile antennae.” From the perfect accordance in their entire structure shown by the pupae of the Rhizocephala and Cirripedia, there can be no doubt that the appendages of Sacculina and Lepas, which are so like each other and spring from the same spot, are h.o.m.ologous structures.

Now in three species of Lepas, in Dichelaspis Warwickii and in Scalpellum Peronii, Darwin saw, on tearing recently-affixed animals from their point or support, that a long narrow band issued from the same point of the antennae; its end was torn away, and in Dichelaspis, judging from its ragged appearance, it had attached itself firmly to the support. From this it follows that this appendage in Lepas australis can hardly be anything but a young cement-duct. If, therefore, the supposition that the appendages on the antennae of the pupae of Rhizocephala are young roots be correct, the roots of the Rhizocephala are h.o.m.ologous with the cement-ducts of the Cirripedia. And this, strange as it may appear at the first glance, seems to me scarcely doubtful. It is true that the act of adhesion of the Rhizocephala has never yet been observed, but it is more than probable that they attach themselves, just like the Cirripedia, by means of the antennae, and that therefore the points of attachment in the two groups indicate h.o.m.ologous parts of the body. From the point of attachment in the Rhizocephala the roots penetrate into the body of the host, whilst in the Cirripedia, the cement-ducts issue from the same point. The roots are blind tubes, ramified in different ways in different species. The cement-ducts in the basis of the Balanidae likewise const.i.tute a generally remarkably complicated system of ramified tubes, with regard to the mode of termination of which nothing certain has yet been made out. Individual caecal branches are not unfrequently seen even in the vicinity of the carina; and, at least in some species, in which the cement-ducts divide into extremely numerous and fine branchlets, forming a network which gradually becomes denser towards the circ.u.mference of the basis, these seem nowhere to possess an orifice.

Now as to the question: How were Cirripedia converted by natural selection into Rhizocephala?

A considerable number of existing Cirripedia settle exclusively or chiefly upon living animals;--on Sponges, Corals, Mollusks, Cetaceans, Turtles, Sea-Snakes, Sharks, Crustaceans, Sea Urchins, and even on Acalephs. Dichelaspis Darwinii was found by Filippi in the branchial cavity of Palinurus vulgaris, and I have met with another species of the same genus in the branchial cavity of Lupea diacantha.

The same thing may have taken place in primitive times. The supposition that certain Cirripedes might once upon a time have selected the soft ventral surface of a Crab, Porcellana or Pagurus, for its dwelling-place, has certainly nothing improbable about it. If then the cement-ducts of such a Cirripede instead of merely spreading on the surface, pierced or pushed before them the soft ventral skin and penetrated into the interior of the host, this must have been beneficial to the animal, because it would be thereby more securely attached and protected from being thrown off during the moulting of its host.

Variations in this direction were preserved as advantageous.

But as soon as the cement-ducts penetrated into the body-cavity of the host and were bathed by its fluids, an endosmotic interchange must necessarily have been set up between the materials dissolved in these fluids and in the contents of the cement-ducts, and this interchange could not be without influence upon the nourishment of the parasite. The new source of nourishment opened up in this manner was, as constantly flowing, more certain than that offered by the nourishment accidentally whirled into the mouth of the sedentary animal. The individuals favoured in the development of the cement-ducts now converted into nutriferous roots, had more than others the prospect of abundant food, of vigorous growth, and of producing a numerous progeny. With the further development, a.s.sisted by natural selection, of the roots embracing the intestine of the host and spreading amongst its hepatic tubes, the introduction of nourishment through the mouth and all the parts implicated in it, such as the whirling cirri, the buccal organs, and the intestine, gradually lost their importance, became aborted by disuse, and finally disappeared without leaving a trace of their existence.

Protected by the abdomen of the Crab, or by the sh.e.l.l inhabited by the Pagurus, the parasite also no longer required the calcareous test, in which, no doubt, the first Cirripedes settling upon these Decapods rejoiced. This protective covering, having become superfluous, also disappeared, and there remained at last only a soft sack filled with eggs, without limbs, without mouth or alimentary ca.n.a.l, and nourished, like a plant, by means of roots, which it pushed into the body of its host. The Cirripede had become a Rhizocephalon.

If it be desired to form a notion of what our parasite may have looked like when half way in its progress from the one form to the other, we may consult the figures given by Darwin, (Lepadidae Plate 4 figures 1 to 7) of Anelasma squalicola. This Lepadide, which lives upon Sharks in the North Sea, seems, in fact, to be in the best way to lose its cirri and buccal organs in the same manner. The widely-cleft, sh.e.l.l-less test is supported upon a thick peduncle, which is immersed in the skin of the Shark. The surface of the peduncle is beset with much-ramified, hollow filaments, which ”penetrate the Shark's flesh like roots” (Darwin).

Darwin looked in vain for cement-glands and cement. It seems to me hardly doubtful, that the ramified hollow filaments are themselves nothing but the cement-ducts converted into nutritive roots, and that it is just in consequence of the development of this new source of nourishment, that the cirri and buccal organs are in the highest degree aborted. All the parts of the mouth are extremely minute; the palpi and exterior maxillae have almost disappeared; the cirri are thick, inarticulate, and dest.i.tute of bristles; and the muscles both of the mouth and cirri are without transverse striation. Darwin found the stomach perfectly empty in the animal examined by him.

Having reached the Nauplius, the extreme outpost of the cla.s.s, retiring furthest into the gray mist of primitive time, we naturally look round us to see whether ways may not be descried thence towards other bordering regions. By the structure of the abdomen in Nauplius we might be reminded, like Oscar Schmidt, of the moveable caudal fork of the Rotatoria, which many regard as near allies of the Crustacea, or at any rate of the Arthropoda; in the six feet surrounding the mouth we might imagine an originally radiate structure, and so forth. But I can see nothing certain. Even towards the nearer provinces of the Myriopoda and Arachnida I can find no bridge. For the Insecta alone, the development of the Malacostraca may perhaps present a point of union. Like many Zoeae, the Insecta possess three pairs of limbs serving for the reception of nourishment, and three pairs serving for locomotion; like the Zoeae they have an abdomen without appendages; as in all Zoeae the mandibles in Insects are dest.i.tute of palpi. Certainly but little in common, compared with the much which distinguishes these two animal-forms. Nevertheless the supposition that the Insecta had for their common ancestor a Zoea which raised itself into a life on land, may be recommended for further examination.

Much in what has been adduced above may be erroneous, many an interpretation may have failed, and many a fact may not have been placed in its proper light. But in one thing, I hope, I have succeeded,--in convincing UNPREJUDICED readers, that Darwin's theory furnishes the key of intelligibility for the developmental history of the Crustacea, as for so many other facts inexplicable without it. The deficiencies of this attempt, therefore, must not be laid to the charge of the plan drawn out by the sure hand of the master, but solely to the clumsiness of the workman, who did not know how to find the proper place for every portion of his material.

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