Facts and Arguments for Darwin by Fritz Muller (read along books .TXT) 📕

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 shore, 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. 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 passed 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 assume, without hesitation, that when the Brachyura and Macrura separated, the primitive ancestors of each of these groups passed 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 relationship 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