Part 4 (2/2)
Darwin describes as ”acoustic orifices” small apertures in the sh.e.l.l of the pupae of the Cirripedia, which, frequently surrounded by a border, are situated, in Lepas pectinata, upon short, horn-like processes. I feel scarcely any hesitation in regarding the apertures as those of the ”sh.e.l.l-glands,” and the horn-like processes as remains of the frontal horns.)
As the Cirripedia and Rhizocephala now in general resemble each other far more than in their Nauplius-state, this is also the case with the individual members of each of the two orders.
The pupae in both orders attach themselves by means of the adherent feet; those of the Cirripedes to rocks, sh.e.l.ls, turtles, drift-wood, s.h.i.+ps, etc.,--those of the Rhizocephala to the abdomen of Crabs, Porcellanae, and Hermit Crabs. The carapace of the Cirripedes becomes converted, as is well-known, into a peculiar test, on account of which they were formerly placed among the Mollusca, and the natatory feet grow into long cirri, which whirl nourishment towards the mouth, which is now open. The Rhizocephala remain astomatous; they lose all their limbs completely, and appear as sausage-like, sack-shaped or discoidal excrescences of their host, filled with ova (Figures 59 and 60); from the point of attachment closed tubes, ramified like roots, sink into the interior of the host, twisting round its intestine, or becoming diffused among the sac-like tubes of its liver. The only manifestations of life which persist in these non plus ultras in the series of retrogressively metamorphosed Crustacea, are powerful contractions of the roots, and an alternate expansion and contraction of the body, in consequence of which water flows into the brood-cavity and is again expelled, through a wide orifice.* (* The roots of Sacculina purpurea (Figure 60) which is parasitic upon a small Hermit Crab, are made use of by two parasitic Isopods, namely a Bopyrus and the before mentioned Cryptoniscus planarioides (Figure 42). These take up their abode beneath the Sacculina and cause it to die away by intercepting the nourishment conveyed by the roots; the roots, however, continue to grow, even without the Sacculina, and frequently attain an extraordinary extension, especially when a Bopyrus obtains its nourishment from them.)
(FIGURE 59. Young of Peltogaster socialis on the abdomen of a small Hermit Crab; in one of them the fasciculately ramified roots in the liver of the Crab are shown. Animal and roots deep yellow.
FIGURE 60. Young Sacculina purpurea with its roots; the animal purple-red, the roots dark gra.s.s-green. Magnified 5 diam.)
Out of several Cirripedes, which are anomalous both in structure and development, Cryptophialus minutus must be mentioned here; Darwin found it in great quant.i.ties together in the sh.e.l.l of Concholepas peruviana on the Chonos Islands. The egg, which is at first elliptical, soon, according to Darwin, becomes broader at the anterior extremity, and acquires three club-shaped horns, one at each anterior angle and one behind; no internal parts can as yet be detected. Subsequently the posterior horn disappears, and the adherent feet may be recognised within the anterior ones. From this ”egg-like larva”--(Darwin says of it, ”I hardly know what to call it”)--the pupa is directly produced. Its carapace is but slightly compressed laterally and hairy, as in Sacculina purpurea; the adherent feet are of considerable size, and the natatory feet are wanting, as, in the adult animal, are the corresponding cirri.
As I learn from Mr. Spence Bate, the Nauplius-stage appears to be overleaped and the larvae to leave the egg in the pupa-form, in the case of a Rhizocephalon (Peltogaster ?) found by Dr. Powell in the Mauritius.
(FIGURES 61 TO 63. Eggs of Tetrac.l.i.ta porosa in segmentation, magnified 90 diam. The larger of the two first-formed spheres of segmentation is always turned towards the pointed end of the egg.
FIGURE 64. Egg of Lernaeodiscus Porcellanae, in segmentation, magnified 90 diam.)
I will conclude this general view with a few words upon the earliest processes in the development of the Crustacea. Until recently it was regarded as a general rule that, by the partial segmentation of the vitellus a germinal disc was formed, and in this, corresponding to the ventral surface of the embryo, a primitive band. We now know that in the Copepoda (Claus), in the Rhizocephala (Figure 64), and, as I can add, in the Cirripedia (Figures 61 to 63) the segmentation is complete, and the embryos are sketched out in their complete form without any preceding primitive band. Probably the latter will always be the case where the young are hatched as true Nauplii (and not merely with a Nauplius-skin, as in Achtheres). The two modes of development may occur in very closely allied animals, as is proved by Achtheres among the Copepoda.* (* I have not mentioned the Pycnogonidae, because I do not regard them as Crustacea; nor the Xiphosura and Trilobites, because, having never investigated them myself, I knew too little about them, and especially because I am unacquainted with the details of the explanations given by Barrande of the development of the latter. According to Mr. Spence Bate ”the young of Trilobites are of the Nauplius-form.”)
CHAPTER 10. ON THE PRINCIPLES OF CLa.s.sIFICATION.
Perhaps some one else, more fortunate than myself, may be able, even without Darwin, to find the guiding clue through the confusion of developmental forms, now so totally different in the nearest allies, now so surprisingly similar in members of the most distant groups, which we have just cursorily reviewed. Perhaps a sharper eye may be able, with Aga.s.siz, to make out ”the plan established from the beginning by the Creator,”* (* ”A plan fully matured in the beginning and undeviatingly pursued;” or ”In the beginning His plan was formed and from it He has never swerved in any particular” (Aga.s.siz and Gould, 'Principles of Zoology').) who may have written here, as a Portuguese proverb says ”straight in crooked lines.”* (* ”Deos escrive direito em linhas tortas.” To read this remarkable writing we need the spectacles of Faith, which seldom suit eyes accustomed to the Microscope.) I cannot but think that we can scarcely speak of a general plan, or typical mode of development of the Crustacea, differentiated according to the separate Sections, Orders, and Families, when, for example, among the Macrura, the River Crayfish leaves the egg in its permanent form; the Lobster with Schizopodal feet; Palaemon, like the Crabs, as a Zoea; and Peneus, like the Cirripedes, as a Nauplius,--and when, still, within this same sub-order Macrura, Palinurus, Mysis and Euphausia again present different young forms,--when new limbs sometimes sprout forth as free rudiments on the ventral surface, and are sometimes formed beneath the skin which pa.s.ses smoothly over them, and both modes of development are found in different limbs of the same animal and in the same pair of limbs in different animals,--when in the Podophthalma the limbs of the thorax and abdomen make their appearance sometimes simultaneously, or sometimes the former and sometimes the latter first, and when further in each of the two groups the pairs sometimes all appear together, and sometimes one after the other,--when, among the Hyperina, a simple foot becomes a chela in Phronima and a chela a simple foot in Brachyscelus, etc.
And yet, according to the teaching of the school, it is precisely in youth, precisely in the course of development, that the ”Type” is mostly openly displayed. But let us hear what the Old School has to tell us as to the significance of developmental history, and its relation to comparative anatomy and systematic zoology.
Let two of its most approved masters speak.
”Whilst comparative anatomy,” said Johannes Muller, in 1844, in his lectures upon this science (and the opinions of my memorable teacher were for many years my own), ”whilst comparative anatomy shows us the infinitely multifarious formation of the same organ in the Animal Kingdom, it furnishes us at the same time with the means, by the comparison of these various forms, of recognising the truly essential, the type of these organs, and separating therefrom everything unessential. In this, developmental history serves it as a check or test. Thus, as the idea of development is not that of mere increase of size, but that of progress from what is not yet distinguished, but which potentially contains the distinction in itself, to the actually distinct,--it is clear, that the less an organ is developed, so much the more does it approach the type, and that, during its development, it more and more acquires peculiarities. The types discovered by comparative anatomy and developmental history must therefore agree.”
Then, after Johannes Muller has combated the idea of a graduated scale of animals, and of the pa.s.sage through several animal grades during development, he continues:--”What is true in this idea is, that every embryo at first bears only the type of its section, from which the type of the Cla.s.s, Order, etc., is only afterwards developed.”
In 1856, in an elementary work,* (* 'Principles of Zoology' Part 1 Comparative Physiology. By Louis Aga.s.siz and A.A. Gould Revised Edition Boston 1856.) in which it is usual to admit only what are regarded as the a.s.sured acquisitions of science, Aga.s.siz expresses himself as follows:--
”The ovarian eggs of all animals are perfectly identical, small cells with a vitellus, germinal vesicle and germinal spot” (paragraph 278).
”The organs of the body are formed in the sequence of their organic importance; the most essential always appear first. Thus the organs of vegetative life, the intestine, etc., appear later than those of animal life, the nervous system, skeleton, etc., and these in turn are preceded by the more general phenomena belonging to the animal as such”
(paragraph 318). ”Thus, in Fishes, the first changes consist in the segmentation of the vitellus and the formation of a germ, processes which are common to all cla.s.ses of animals. Then the dorsal furrow, characteristic of the Vertebrate, appears--the brain, the organs of the senses; at a later period are formed the intestine, the limbs, and the permanent form of the respiratory organs, from which the cla.s.s is recognised with certainty. It is only after exclusion that the peculiarities of the structure of the teeth and fins indicate the genus and species” (paragraph 319). ”Hence the embryos of different animals resemble each other the more, the younger they are” (paragraph 320).
”Consequently the high importance of developmental history is indubitable. For, if the formation of the organs takes place in the order corresponding to their importance, this sequence must of itself be a criterion of their comparative value in cla.s.sification. The peculiarities which appear earlier should be considered of higher value than those which appear subsequently” (paragraph 321). ”A system, in order to be true and natural, must agree with the sequence of the organs in the development of the embryo” (paragraph 322).
I do not know whether any one at the present day will be inclined to subscribe to this proposition in its whole extent.* (* Aga.s.siz' own views have lately become essentially different, so far as can be made out from Rud. Wagner's notice of his 'Essay on Cla.s.sification.' Aga.s.siz himself does not attempt any criticism of the above cited older views, which, however, are still widely diffused. With his recent conception I am unfortunately acquainted only from R. Wagner's somewhat confused report, and have therefore thought it better not to attempt any critical remarks upon it.) It is certain, however, that views essentially similar are still to be met with everywhere in discussions on cla.s.sification, and that even within the last few years, the very sparingly successful attempts to employ developmental history as the foundation of cla.s.sification have been repeated.
But how do these propositions agree with our observations on the developmental history of the Crustacea? That these observations relate for the most part to their ”free metamorphosis” after their quitting the egg, cannot prejudice their application to the propositions enunciated especially with regard to ”embryonal development” in the egg; for Aga.s.siz himself points out (paragraph 391) that both kinds of change are of the same nature and of equal importance and that no ”radical distinction” is produced by the circ.u.mstance that the former take place before and the latter after birth.
”The ovarian eggs of all animals are identical, small cells with vitellus, germinal vesicle and germinal spot.” Yes, somewhat as all Insects are identical, small animals with head, thorax, and abdomen; that is to say if, only noticing what is common to them, we leave out of consideration the difference of their development, the presence or absence and the multifarious structure of the vitelline membrane, the varying composition of the vitellus, the different number and formation of the germinal spots, etc. Numerous examples, which might easily be augmented, of such profound differences, are furnished by Leydig's 'Lehrbuch der Histologie.' In the Crustacea the ovarian eggs actually sometimes furnish excellent characters for the discrimination of species of the same genus; thus, for example, in one Porcellana of this country they are blackish-green, in a second deep blood-red, and in a third dark yellow; and within the limits of the same order they present considerable differences in size, which, as Van Beneden and Claus have already pointed out, stands in intimate connexion with the subsequent mode of development.
”The organs of the body are formed in the sequence of their organic importance; the most essential always appear first.” This proposition might be characterised a priori as undemonstrable, since it is impossible either in general, or for any particular animal, to establish a sequence of importance amongst equally indispensable parts. Which is the more important, the lung or the heart--the liver or the kidney?--the artery or the vein? Instead of giving the preference, with Aga.s.siz, to the organs of animal life, we might with equal justice give it to those of vegetative life, as the latter are conceivable without the former, but not the former without the latter. We might urge that, according to this proposition, provisional organs as the first produced must exceed the later-formed permanent organs in importance.
But let us stick to the Crustacea. In Polyphemus Leydig finds the first traces of the intestinal tube even during segmentation. In Mysis a provisional tail is first formed, and in Ligia a maggot-like larva-skin.
The simple median eye appears earlier, and would therefore be more important than the compound paired eyes; the scale of the antennae in the Prawns would be more important than the flagellum; the maxillipedes of the Decapoda would be more important than the chelae and ambulatory feet, and the anterior six pairs of feet in the Isopoda, than the precisely similarly formed seventh pair; in the Amphipoda the most important of all organs would be the ”micropylar apparatus,” which disappears without leaving a trace soon after hatching; in Cyclops the setae of the tail would be more important than all the natatory feet; in the Cirripedia the posterior antennae, as to which we do not know what becomes of them, would be more important than the cirri, and so forth.
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