Part 6 (1/2)
2. The segmentation of the abdomen; ”like the labium, the abdomen also very generally retains its original segmentation, which is shown in the development of eleven segments” (Gerstacker). The Orthoptera with eleven segments in the abdomen, agree perfectly in the number of their body-segments with the Prawn-larva represented in Figure 33, or indeed, with the higher Crustacea (Podophthalma and Edriophthalma) in general, in which the historically youngest last thoracic segment (see page 123), which is sometimes late-developed, or dest.i.tute of appendages, or even deficient, is still wanting.
3. That, as in the Crustacea, the s.e.xual orifice and a.n.u.s are placed upon different segments; ”whilst the former is situated in the ninth segment, the latter occurs in the eleventh” (Gerstacker).
4. Their palaeontological occurrence; ”in a fossil state the Orthoptera make their appearance the earliest of all Insects, namely as early as the Carboniferous formation, in which they exceed all others in number”
(Gerstacker).
5. The absence of uniformity of habit at the present day in an order so small when compared with the Coleoptera, Hymenoptera, etc. For this also is usually a phenomenon characteristic of very ancient groups of forms which have already overstepped the climax of their development, and is explicable by extinction in ma.s.s. A Beetle or a b.u.t.terfly is to be recognised as such at the first glance, but only a thorough investigation can demonstrate the mutual relations.h.i.+ps of Termes, Blatta, Mantis, Forficula, Ephemera, Libellula, etc. I may refer to a corresponding remarkable example from the vegetable world: amongst Ferns the genera Aneimia, Schizaea and LyG.o.dium, belonging to the group Schizaeaceae which is very poor in species, differ much more from each other than any two forms of the group Polypodiaceae which numbers its thousands of species.
If, from all this, it seems right to regard the Orthoptera as the order of Insects approaching most nearly to the common primitive form, we must also expect that their mode of development will agree better with that of the primitive form, than, for example, that of the Lepidoptera, in the same way that some of the Prawns (Peneus) approaching most closely the primitive form of the Decapoda, have most truly preserved their original mode of development. Now, the majority of the Orthoptera quit the egg in a form which is distinguished from that of the adult Insect almost solely by the want of wings; these larvae then soon acquire rudiments of wings, which appear more strongly developed after every moult. Even this perfectly gradual transition from the youngest larva to the s.e.xually mature Insect, preserves in a far higher degree the picture of an original mode of development, than does the so-called complete metamorphosis of the Coleoptera, Lepidoptera, or Diptera, with its abruptly separated larva-, pupa- and imago-states.
The most ancient Insects would probably have most resembled these wingless larvae of the existing Orthoptera. The circ.u.mstance that there are still numerous wingless species among the Orthoptera, and that some of these (Blattidae) are so like certain Crustacea (Isopods) in habit that both are indicated by the same name (”Baratta”) by the people in this country, can scarcely be regarded as of any importance.
The contrary supposition that the oldest Insects possessed a ”complete metamorphosis,” and that the ”incomplete metamorphosis” of the Orthoptera and Hemiptera is only of later origin, is met by serious difficulties. If all the cla.s.ses of Arthropoda (Crustacea, Insecta, Myriopoda and Arachnida) are indeed all branches of a common stem (and of this there can scarcely be a doubt), it is evident that the water-inhabiting and water-breathing Crustacea must be regarded as the original stem from which the other terrestrial cla.s.ses, with their tracheal respiration, have branched off. But nowhere among the Crustacea is there a mode of development comparable to the ”complete metamorphosis” of the Insecta, nowhere among the young or adult Crustacea are there forms which might resemble the maggots of the Diptera or Hymenoptera, the larvae of the Coleoptera, or the caterpillars of the Lepidoptera, still less any bearing even a distant resemblance to the quiescent pupae of these animals. The pupae, indeed, cannot at all be regarded as members of an original developmental series, the individual stages of which represent permanent ancestral states, for an animal like the mouthless and footless pupa of the Silkworm, enclosed by a thick coc.o.o.n, can never have formed the final, s.e.xually mature state of an Arthropod.
In the development of the Insecta we never see new segments added to those already present in the youngest larvae, but we do see segments which were distinct in the larva afterwards become fused together or disappear. Considering the parallelism which prevails throughout organic nature between palaeontological and embryonic development, it is therefore improbable that the oldest Insects should have possessed fewer segments than some of their descendants. But the larva of the Coleoptera, Lepidoptera, etc., never have more than nine abdominal segments, it is therefore not probable that they represent the original young form of the oldest Insects, and that the Orthoptera, with an abdomen of eleven segments, should have been subsequently developed from them.
Taking into consideration on the one hand these difficulties, and on the other the arguments which indicate the Orthoptera as the order most nearly approaching the primitive form, it is my opinion that the ”incomplete metamorphosis” of the Orthoptera is the primitive one, INHERITED from the original parents of all Insects, and the ”complete metamorphosis” of the Coleoptera, Diptera, etc., a subsequently ACQUIRED one.)
Which of the different modes of development at present occurring in a cla.s.s of animals may claim to be that approaching most nearly to the original one, is easy to judge from the above statements.
The primitive history of a species will be preserved in its developmental history the more perfectly, the longer the series of young states through which it pa.s.ses by uniform steps; and the more truly, the less the mode of life of the young departs from that of the adults, and the less the peculiarities of the individual young states can be conceived as transferred back from later ones in previous periods of life, or as independently acquired.
Let us apply this to the Crustacea.
CHAPTER 12. PROGRESS OF EVOLUTION IN CRUSTACEA.
According to all the characters established in the last paragraph, the Prawn that we traced from the Nauplius through states a.n.a.logous to Zoea and Mysis to the form of a Macrurous Crustacean appears at present to be the animal, which in the section of the higher Crustacea (Malacostraca) furnishes the truest and most complete indications of its primitive history. That it is the most complete is at once evident. That it is the truest must be a.s.sumed, in the first place, because the mode of life of the various ages is less different than in the majority of the other Podophthalma; for from the Nauplius to the young Prawn they were found swimming freely in the sea, whilst Crabs, Porcellanae, the Tatuira, Squilla, and many Macrura, when adult usually reside under stones, in the clefts of rocks, holes in the earth, subterranean galleries, sand, etc., not to mention other deviations in habits such as are presented by the Hermit Crabs, Pinnotheres, etc.,--and secondly and especially because the peculiarities which distinguish the Zoea of this species particularly from other Zoeae (the employment of the anterior limbs for swimming, the furcate tail, the simple heart, the deficiency of the paired eyes and abdomen at first, etc.) are neither to be deduced from a retro-transfer of late-acquired advantages to this early period of life, nor to be regarded at all as advantages over other Zoeae which the larva might have acquired in the struggle for existence.
A similar development must have been once pa.s.sed through by the primitive ancestor of all Malacostraca, probably differing from that of our Prawn, especially in the circ.u.mstance that it would go on more uniformly without the sudden change of form and mode of locomotion produced in the latter by the simultaneous sprouting forth and entering into action in the Nauplius of four and in the Zoea of five pairs of limbs. It is to be supposed that, not only originally but even still, in the larvae of the first Malacostraca, the new body-segments and pairs of limbs are formed singly,--first of all the segments of the fore-body, then those of the abdomen, and finally those of the middle-body,--and, moreover, that in each region of the body the anterior segments were formed earlier than the posterior ones, and therefore last of all the hindermost segment of the middle-body. Of this original mode more or less distinct traces still remain, even in species in which, in other respects, the course of development of their ancestors is already nearly effaced. Thus the abdominal feet of the Prawn-larva represented in Figure 33, are formed singly from before backwards, and after these the last feet of the middle-body; thus, in Palinurus, the last two pairs of feet of the middle-body are formed later than the rest; thus in the young larvae of the Stomapoda the last three abdominal segments are dest.i.tute of limbs, which are still wanting on the last of them in older larvae; and thus, in the Isopoda, the historically newest pair of feet is produced later than all the rest. In the Copepoda this formation of new segments and limbs, gradually advancing from before backwards, is more perfectly preserved than in any of the higher Crustacea.* (* It is well known that, in many cases, even in adult animals the last segment of the middle-body, or some of its last segments, either want their limbs or are themselves deficient (Entoniscus Porcellanae male, Leucifer, etc.). This might be due to the animals having separated from the common stem before these limbs were formed at all. But in those cases with which I am best acquainted, it seems to me more probable that the limbs have been subsequently lost again. That these particular limbs and segments are more easily lost than others is explained by the circ.u.mstance that, as the youngest, they have been less firmly fixed by long-continued inheritance. (”Mr. Dana believes, that in ordinary Crustaceans, the abortion of the segments with their appendages almost always takes place at the posterior end of the cephalothorax.”--Darwin, Balanidae, page 111.))
The original development of the Malacostraca starting from the Nauplius, or the lowest free-living grade with which we are acquainted in the cla.s.s of Crustacea, is now-a-days nearly effaced in the majority of them. That this extinction has actually taken place in the way already deduced as a direct consequence from Darwin's theory, will be the more easily demonstrated, the more this process is still included in the course of life, and the less completely it is already worn out. We may hope to obtain the most striking examples in the still unknown developmental history of the various Schizopoda, Peneidae, and, indeed, of the Macrura in general. At present the multifarious Zoea-forms appear to be particularly instructive. Almost all the peculiarities by which they depart from the primitive form of the Zoea of Peneus (Figures 29, 30 and 32), may in fact be conceived as transferred back from a later period into this early period of life. This is the case with the large compound eyes,--with the structure of the heart,--with the raptorial feet in Squilla,--and with the powerful, muscular, straightly-extended abdomen in Palaemon, Alpheus, Hippolyte, and the Hermit Crabs. (In the latter, indeed, the abdomen of the adult animal is a shapeless sac filled with the liver and generative organs, but it is still tolerably powerful in the Glaucothoe-stage, and was certainly still more powerful when this stage was still the permanent form of the animal.) It is also the case with the abdomen of the Zoeae of the Crabs, the Porcellanae, and the Tatuira, which is still powerful, although usually bent under the breast; the two last swim tolerably by means of the abdomen, even when adult, as do the true Crabs in the young state known as Megalops.
It is the case, lastly, with the conversion of the two anterior pairs of limbs into antennae. The second pair of antennae, which, in the various Zoeae always remains a step behind that of the adult animal, is particularly remarkable. In the Crabs the ”scale” is entirely wanting; their Zoeae have it indicated in the form of a moveable appendage, which is often exceedingly minute. In the Hermit Crabs a similar, usually moveable, spiniform process occurs as the remains of the scale; their Zoeae have a well-developed but inarticulate scale. A precisely similar scale is possessed by the adult Prawns, in the Zoeae of which it exists still in a jointed form, like the outer branch of the second pair of feet of the Nauplius or Peneus-Zoea.
The long, spiniform processes on the carapace of the Zoeae of the Crabs and Porcellanae are not to be explained in this way, but their advantage to the larvae is evident. Thus, for example, if the body of the Zoea of Porcellana stellicola (Figure 24), without the processes of the carapace and without the abdomen, which however is not rigidly extensible, is scarcely half a line in length, whilst with the processes it is four lines long, a mouth of eight times the width is necessary in order to swallow the little animal when thus armed.* (* Persephone, a rare Crab, belonging to the family Leucosiidae, is served in the same manner by its long chelate feet. If we seize the animal, it extends them most obstinately straight downwards, so that in all probability we should more easily break than bend them.) Consequently these processes of the carapace may be regarded as acquired by the Zoea itself in the struggle for existence.
The formation of new limbs beneath the skin of the larvae is also to be referred to an earlier occurrence of processes which originally took place at a later period. The original course must have been that they sprouted forth in a free form upon the ventral surface of the larva in the next stage after the change of skin; whilst now they are developed before the change of skin, and thus only come into action a stage earlier. In larvae which, for other reasons, must be regarded as more nearly approaching the primitive form, the original mode usually prevails in this particular also. Thus the caudal feet (the ”lateral caudal lamellae”) are formed freely on the ventral surface in Euphausia and the Prawns with Nauplius-brood, and within the caudal lamellae in the Prawns with Zoea-brood, in Pagurus and Porcellana.
A compression of several stages into one, and thereby an abridgement and simplification of the course of development, is expressed in the simultaneous appearance of several new pairs of limbs.
How earlier young states may gradually be completely lost, is shown by Mysis and the Isopoda. In Mysis there is still a trace of the Nauplius-stage; being transferred back to a period when it had not to provide for itself, the Nauplius has become degraded into a mere skin; in Ligia (Figures 36 and 37) this larva-skin has lost the last traces of limbs, and in Philoscia (Figure 38) it is scarcely demonstrable.
Like the spinous processes of the Zoeae, the chelae on the penultimate pair of feet of the young Brachyscelus are to be regarded as acquired by the larva itself. The adult animals swim admirably and are not confined to their host; as soon as the specimens of Chrysaora Blossevillei, Less., or Rhizostoma cruciatum, Less., on which they are seated, become the sport of the waves in the neighbourhood of the sh.o.r.e, they escape from them, and are only to be obtained from lively Acalephs. The young are helpless creatures and bad swimmers; a special apparatus for adhesion must be of great service to them.
To review the developmental history of the different Malacostraca in detail would furnish no results at all correspondent to the time occupied by it,--if our knowledge was more complete it would be more profitable. I therefore abandon it, but will not omit to mention that in it many difficulties which cannot at present be satisfactorily solved would present themselves. To these isolated difficulties I ascribe the less importance, however, because even a little while ago, before the discovery of the Prawn-Nauplius, this entire domain of the development of the Malacostraca was almost inaccessible to Darwin's theory.
Nor will I dwell upon the contradictions which appear to result from the application of the Darwinian theory to this department. I leave it to our opponents to find them out. Most of them may easily be proved to be only apparent. There are two of these objections, however, which lie so much on the surface that they can hardly escape being brought forward, and these, I think, I must get rid of.
”The peculiarities in which the Zoeae of the Crabs, the Porcellanae, the Tatuira, the Hermit Crabs, and the Prawns with Zoea-brood agree, and by which they are in common distinguished from the larvae of Peneus produced from Nauplii, forces us (it might be said) to the supposition that the common ancestor of these various Decapods quitted the egg in a similar Zoea-form. But then neither Peneus with its Nauplius-brood, nor even apparently the Palinuri could be referred back to this ancestor.