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

(FIGURES 53 AND 54. Nauplii of Copepoda, the former magnified 90, the latter 180 diam.)

All the larvae of the free Copepoda investigated by Claus, have, at the earliest period, three pairs of limbs (the future antennae and mandibles), the anterior with a single, and the two following ones with a double series of joints, or branchiae. The unpaired eye, labrum, and mouth, already occupy their permanent positions. The posterior portion, which is usually short and destitute of limbs, bears two terminal setae, between which the anus is situated. The form in this Nauplius-brood is extremely various,—it is sometimes compressed laterally, sometimes flat,—sometimes elongated, sometimes oval, sometimes round or even broader than long, and so forth. The changes which the first larval stages undergo during the progress of growth, consist essentially in an extension of the body and the sprouting forth of new limbs. “The following stage already displays a fourth pair of extremities, the future maxillae.” Then follow at once three new pairs of limbs (the maxillipedes and the two anterior pairs of natatory feet). The larva still continues like a Nauplius, as the three anterior pairs of limbs represent rowing feet; at the next moult it is converted into the youngest Cyclops-like state, when it resembles the adult animal in the structure of the antennae and buccal organs, although the number of limbs and body segments is still much less, for only the rudiments of the third and fourth pairs of natatory feet have made their appearance in the form of cushions fringed with setae, and the body consists of the oval cephalothorax, the second, third, and fourth thoracic segments, and an elongated terminal joint. In the Cyclopidae the posterior antennae have lost their secondary branch, and the mandibles have completely thrown off the previously existing natatory feet, whilst in the other families these appendages persist, more or less altered. “Beyond this stage of free development, many forms of the parasitic Copepoda, such as Lernanthropus and Chondracanthus, do not pass, as they do not acquire the third and fourth pairs of limbs, nor does a separation of the fifth thoracic segment from the abdomen take place; others (Achtheres) even fall to a lower grade by the subsequent loss of the two pairs of natatory feet. But all free Copepoda, and most of the parasitic Crustacea, pass through a longer or shorter series of stages of development, in which the limbs acquire a higher degree of division into joints in continuous sequence, the posterior pairs of feet are developed, and the last thoracic segment and the different abdominal segments are successively separated from the common terminal portion.”

(FIGURE 55. Nauplius of Tetraclita porosa after the first moult, magnified 90 diam. The brain is seen surrounding the eye, and from it the olfactory filaments issue; behind it are some delicate muscles passing to the buccal hood.)

There is only one thing more to be indicated in the developmental history of the parasitic Crustacea, namely that some of them, such as Achtheres percarum, certainly quit the egg like the rest in a Nauplius-like form, inasmuch as the plump, oval, astomatous body bears two pairs of simple rowing feet, and behind these, as traces of the third pair, two inflations furnished each with a long seta, but that beneath this Nauplius-skin a very different larva lies ready prepared, which in a few hours bursts its clumsy envelope and then makes its appearance in a form “which agrees in the segmentation of the body and in the development of the extremities with the first Cyclops-stage” (Claus). The entire series of Nauplius-stages which are passed through by the free Copepoda, are in this case completely over-leapt.

A final and very peculiar section of the Crustacea is formed by the two orders of the Cirripedia and Rhizocephala. ( The most various opinions prevail as to the position of the Cirripedia. Some ascribe to them a very subordinate position among the Copepoda; as Milne-Edwards (1852). In direct opposition to this notion of his father’s, Alph. Milne-Edwards places them (as Basinotes) opposite to all the other Crustacea (Eleutheronotes). Darwin regards them as forming a peculiar sub-class equivalent to the Podophthalma, Edriophthalma, etc. This appears to me to be most convenient. I would not combine the Rhizocephala with the Cirripedia, as Liljeborg has done, but place them in opposition as equivalent, like the Amphipoda and Isopoda. The near relationship of the Cirripedia to the Ostracoda is also spoken of, but the similarity of the so-called “Cypris-like larvae,” or Cirriped-pupae as Darwin calls them, to Cypris is so purely external, even as regards the shell, that the relationship appears to me to be scarcely greater than that of Peltogaster socialis (Figure 59) with the family of the sausages.)

In these also the brood bursts out in the Nauplius-form, and speedily strips off its earliest larva-skin which is distinguished by no peculiarities worth noticing. Here also we find again the same pyriform shape of the unsegmented body, the same number and structure of the feet, the same position of the median eye (which, however, is wanting in Sacculina purpurea, and according to Darwin in some species of Lepas), and the same position of the “buccal hood,” as in the Nauplii of the Prawns and Copepoda. From the latter the Nauplii of the Cirripedia and Rhizocephala are distinguished by the possession of a dorsal shield or carapace, which sometimes (Sacculina purpurea) projects far beyond the body all round; and they are distinguished not only from other Nauplii, but as far as I know from all other Crustacea, by the circumstance that structures which are elsewhere combined with the two anterior limbs (antennae), here occur separated from them.

The anterior antennae of the Copepoda, Cladocera, Phyllopoda (Leydig, Claus), Ostracoda (at least the Cypridinae), Diastylidae, Edriophthalma, and Podophthalma, with few exceptions relating to terrestrial animals or parasites, bear peculiar filaments which I have already repeatedly mentioned as “olfactory filaments.” A pair of similar filaments spring, in the larvae of the Cirripedia and Rhizocephala, directly from the brain.

(FIGURE 56. Nauplius of Sacculina purpurea, shortly before the second moult, magnified 180 diam. We may recognise in the first pair of feet the future adherent feet, and in the abdomen six pairs of natatory feet with long setae.)

At the base of the inferior antennae in the Decapoda the so-called “green-gland” has its opening; in the Macrura at the end of a conical process. A similar conical process with an efferent duct traversing it is very striking in most of the Amphipoda. In the Ostracoda, Zenker describes a gland situated in the base of the inferior antennae, and opening at the extremity of an extraordinarily long “spine.” In the Nauplii of Cyclops and Cyclopsine, Claus finds pale “shell-glands,” which commence in the intermediate pair of limbs (the posterior antennae). On the other hand in the Nauplii of the Cirripedia and Rhizocephala the “shell-glands” open at the ends of conical processes, sometimes of most remarkable length, which spring from the angles of the broad frontal margin, and have been interpreted sometimes as antennae (Burmeister, Darwin) and sometimes as mere “horns of the carapace” (Krohn). The connexion of the “shell-glands” with the frontal horns has been recognised unmistakably in the larvae of Lepas, and indeed the resemblance of the frontal horns with the conical processes on the inferior antennae of the Amphipoda, is complete throughout. ( In connexion with this it may be mentioned that, in the females of Brachyscelus, in which the posterior antennae are deficient, the conical processes with the canal permeating them are nevertheless retained.)

(FIGURE 57. Pupa of a Balanide (Chthamalus ?), magnified 50 diam. The adherent feet are retracted within the rather opaque anterior part of the shell.

FIGURE 58. Pupa of Sacculina purpurea, magnified 180 diam. The filaments on the adherent feet may be the commencements of the future roots.)

Notwithstanding their agreement in this important peculiarity, the Nauplii of these two orders present material differences in many other particulars. The abdomen of the young Cirripede is produced beneath the anus into a long tail-like appendage which is furcate at the extremity, and over the anus there is a second long, spine-like process; the abdomen in the Rhizocephala terminates in two short points,—in a “moveable caudal fork, as in the Rotatoria,” (O. Schmidt). The young Cirripedes have a mouth, stomach, intestine, and anus, and their two posterior pairs of limbs are beset with multifarious teeth, setae, and hooks, which certainly assist in the inception of nourishment. All this is wanting in the young Rhizocephala. The Nauplii of the Cirripedia have to undergo several moults whilst in that form; the Nauplii of the Rhizocephala, being astomatous, cannot of course live long as Nauplii, and in the course of only a few days they become transformed into equally astomatous “pupae,” as Darwin calls them.

The carapace folds itself together, so that the little animal acquires the aspect of a bivalve shell, the foremost limbs become transformed into very peculiar adherent feet (“prehensile antennae,” Darwin), and the two following pairs are cast off; like the frontal horns. On the abdomen six pairs of powerful biramose natatory feet with long setae have been formed beneath the Nauplius-skin, and behind these are two short, setigerous caudal appendages (Figure 58).

The pupae of the Cirripedia (Figure 57), which are likewise astomatous, agree completely in all these parts with those of the Rhizocephala, even to the minutest details of the segmentation and bristling of the natatory feet;* they are especially distinguished from them by the possession of a pair of composite eyes. (* Compare the figure given by Darwin (Balanidae Plate 30 Figure 5) of the first natatory foot of the pupa of Lepas australis, with that of Lernaeodiscus Porcellanae published in the ‘Archiv fur Naturgeschichte’ (1863 Taf 3 Figure 5). The sole distinction, that in the latter there are only 3 setae at the end of the outer branch, whilst in the Cirripedia there are 4 on the first and 5 on the following natatory feet, may be due to an error on my part.) Sometimes also traces of the frontal horns seem to persist. ( Darwin describes as “acoustic orifices” small apertures in the shell 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 “shell-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, shells, turtles, drift-wood, ships, 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