Facts and Arguments for Darwin, Fritz Muller [ebook reader computer .TXT] 📗

be compared to a Nauplius; in Ligia it appears like a maggot quite destitute of appendages, but produced into a long simple tail (Figure 37). The egg-membrane is retained longer than in Mysis; it bursts only when the limbs of the young Slater are already partially developed in their full number. The dorsal surface of the Slater is united to the larval skin a little behind the head. At this point, when the union has been dissolved a little before the change of skin, there is a foliaceous appendage, which exists only for a short time, and disappears before the young Slater quits the brood-pouch of the mother.

(FIGURE 37. Maggot-like larva of Ligia, magnified 15 diam. R remains of the egg-membrane. We see on the lower surface, from before backwards:—the anterior and posterior antennae, the mandibles, the anterior and posterior maxillae, maxillipedes, six ambulatory feet, the last segment of the middle-body destitute of appendages, five abdominal feet, and the caudal feet.)

The young animal, when it begins to take care of itself, resembles the old ones in almost all parts, except one important difference; it possesses only six, instead of seven pairs of ambulatory feet; and the last segment of the middle-body is but slightly developed and destitute of appendages. It need hardly be mentioned that the sexual peculiarities are not yet developed, and that in the males the hand-like enlargements of the anterior ambulatory feet and the copulatory appendages are still deficient.

(FIGURE 38. Embryo of a Philoscia in the egg, magnified 25 diam.)

To the question, how far the development of Ligia is repeated in the other Isopoda, I can only give an unsatisfactory answer. The curvature of the embryo upwards instead of downwards was met with by me as well as by Rathke in Idothea, and likewise in Cassidina, Philoscia, Tanais, and the Bopyridae,—indeed, I failed to find it in none of the Isopoda examined for this purpose. In Cassidina also the first larval skin without appendages is easily detected; it is destitute of the long tail, but is strongly bent in the egg, as in Ligia, and consequently cannot be mistaken for an “inner egg-membrane.” This, however, might happen in Philoscia, in which the larval skin is closely applied to the egg-membrane (Figure 38), and is only to be explained as the larval skin by a reference to Ligia and Cassidina. The foliaceous appendage on the back has long been known in the young of the common Water Slater (Asellus). ( Leydig has compared this foliaceous appendage of the Water Slaters with the “green gland” or “shell-gland” of other crustacea, assuming that the green gland has no efferent duct and appealing to the fact that the two organs occur “in the same place.” This interpretation is by no means a happy one. In the first place we may easily ascertain in Leucifer, as was also found to be the case by Claus, that the “green gland” really opens at the end of the process described by Milne-Edwards as a “tubercule auditif” and by Spence Bate as an “olfactory denticle.” And, secondly, the position is about as different as it can well be. In the one case a paired gland, opening at the base of the posterior antennae, and therefore on the lower surface of the SECOND segment; in the other an unpaired structure rising in the median line of the back BEHIND THE SEVENTH SEGMENT, (“behind the boundary line of the first thoracic segment,” Leydig).) That the last pair of feet of the thorax is wanting in the young of the Wood-lice (Porcellionides, M.-Edw.) and Fish-lice (Cymothoadiens, M.-Edw.) has already been noticed by Milne-Edwards. This applies also to the Box-Slaters (Idothea), to the viviparous Globe-Slaters (Sphaeroma) and Shield-Slaters (Cassidina), to the Bopyridae (Bopyrus, Entoniscus, Cryptoniscus, n.g.), and to the Cheliferous Slaters (Tanais), and therefore probably to the great majority of the Isopoda. All the other limbs are usually well developed in the young Isopoda. In Tanais alone, all the abdominal feet are wanting (but not those of the tail); they are developed simultaneously with the last pair of feet of the thorax.

(FIGURE 39. Embryo of Cryptoniscus planarioides, magnified 90 diam.

FIGURE 40. Last foot of the middle-body of the larva of Entoniscus Porcellanae, magnified 180 diam.)

The last pair of feet on the middle-body of the larva, consequently the penultimate pair in the adult animal, is almost always similar in structure to the preceding pair. A remarkable exception is, however, presented in this respect by Cryptoniscus and Entoniscus,—remarkable as a confirmation of Darwin’s proposition that “parts developed in an unusual manner are very variable,” for in the peculiarly-formed pair of feet there exists the greatest possible difference between the three species hitherto observed. In Cryptoniscus (Figure 39) this last foot is thin and rod-like; in Entoniscus Cancrorum remarkably long and furnished with a strongly thickened hand and a peculiarly constructed chela; in Entoniscus Porcellanae very short, imperfectly jointed, and with a large ovate terminal joint (Figure 40).

Some Isopods undergo a considerable change immediately before the attainment of sexual maturity. This is the case with the males of Tanais which have already been noticed, and, according to Hesse, with the Pranizae, in which both sexes are said to pass into the form known as Anceus. But Spence Bate, a careful observer, states that he has seen females of the form of Praniza laden with eggs far advanced in their development.

(FIGURE 41. Entoniscus Cancrorum, female, magnified 3 times.

FIGURE 42. Cryptoniscus planarioides, female, magnified 3 times.

FIGURE 43. Embryo of a Corophium, magnified 90 diam.)

In this order we meet for the first time with an extensive retrograde metamorphosis as a consequence of a parasitic mode of life. Even in some Fish-lice (Cymothoa) the young are lively swimmers, and the adults stiff, stupid, heavy fellows, whose short clinging feet are capable of but little movement. In the Bopyridae (Bopyrus, Phryxus, Kepone, etc., which might have been conveniently left in a single genus), which are parasitic on Crabs, Lobsters, etc., taking up their abode chiefly in the branchial cavity, the adult females are usually quite destitute of eyes; the antennae are rudimentary; the broad body is frequently unsymmetrically developed in consequence of the confined space; its segments are more or less amalgamated with each other; the feet are stunted, and the appendages of the abdomen transformed from natatory feet with long setae into foliaceous or tongue-shaped and sometimes ramified branchiae. In the dwarfish males the eyes, antennae, and feet, are usually better preserved than in the females; but on the other hand all the appendages of the abdomen have not unfrequently disappeared, and sometimes every trace of segmentation. In the females of Entoniscus, which are found in the body-cavity of Crabs and Porcellanae, the eyes, antennae, and buccal organs, the segmentation of the vermiform body, and in one species (Figure 41) the whole of the limbs, disappear almost without leaving a trace; and Cryptoniscus planarioides would almost be regarded as a Flatworm rather than an Isopod, if its eggs and young did not betray its Crustacean nature. Among the males of these various Bopyridae, that of Entoniscus Porcellanae occupies the lowest place; it is confined all its life to six pairs of feet, which are reduced to shapeless rounded lumps.

The Amphipoda are distinguishable from the Isopoda at an early period in the egg by the different position of the embryo, the hinder extremity of which is bent downwards. In all the animals of this order which have been examined for it, ( In the genera Orchestoidea, Orchestia, Allorchestes, Montagua, Batea n.g., Amphilochus, Atylus, Microdeutopus, Leucothoe, Melita, Gammarus (according to Meissner and La Valette), Amphithoe, Cerapus, Cyrtophium, Corophium, Dulichia, Protella and Caprella.) a peculiar structure makes its appearance very early on the anterior part of the back, by which the embryo is attached to the “inner egg-membrane,” and which has been called the “micropylar apparatus,” but improperly as it seems to me. ( Little as a name may actually affect the facts, we ought certainly to confine the name “micropyle” to canals of the egg-membrane, which serve for the entrance of the semen. But the outer egg-membrane passes over the “micropylar apparatus” of the Amphipoda without any perforation, according to Meissner’s and La Valette’s own statements; it appears never to be present before fecundation, attains its greatest development at a subsequent period of the ovular life, and the delicate canals which penetrate it do not even seem to be always present, indeed it seems to belong to the embryo rather than to the egg-membrane. I have never been able to convince myself that the so-called “inner egg-membrane” is really of this nature, and not perhaps the earliest larva skin, not formed until after impregnation, as might be supposed with reference to Ligia, Cassidina and Philoscia.) It will remind us of the union of the young Isopoda with the larval membrane and of the unpaired “adherent organ” on the nape of the Cladocera, which is remarkably developed in Evadne and persists throughout life; but in Daphnia pulex, according to Leydig, although present in the young animals, disappears without leaving a trace in the adults.

The young animal, whilst still in the egg, acquires the full number of its segments and limbs. In cases where segments are amalgamated together, such as the last two segments of the thorax in Dulichia, the last abdominal segments and the tail in Gammarus ambulans and Corophium dentatum, n. sp., and the last abdominal segments and the tail in Brachyscelus,* or where one or more segments are deficient, as in Dulichia and the Caprellae, we find the same fusion and the same deficiencies in young animals taken out of the brood-pouch of their mother. (* According to Spence Bate, in Brachyscelus crusculum the fifth abdominal segment is not amalgamated with the sixth (the tail) but with the fourth, which I should be inclined to doubt, considering the close agreement which this species otherwise shows with the two species that I have investigated.) Even peculiarities in the structure of the limbs, so far as they are common to both sexes, are usually well-marked in the newly hatched young, so that the latter generally differ from their parents only by their stouter form, the smaller number of the antennal joints and olfactory filaments, and also of the setae and teeth with which the body or feet are armed, and perhaps by the comparatively larger size of the secondary flagellum. An exception to this rule is presented by the Hyperinae which usually live upon Acalephae. In these the young and adults often have a remarkably different appearance; but even in these there is no new formation of body-segments and limbs, but only a gradual transformation of these parts.*

(* In the young of Hyperia galba Spence Bate did not find any of the abdominal feet, or the last two pairs of thoracic feet, but this very remarkable statement required confirmation the more because he examined these minute animals only in the dried state. Subsequently I had the wished-for opportunity of tracing the development of a Hyperia which is not uncommon upon Ctenophora, especially Beroe gilva, Eschsch. The youngest larva from the brood-pouch of the mother already possess THE WHOLE of the thoracic feet; on the other hand, like Spence Bate, I cannot find those of the abdomen. At first simple enough, all these feet soon become converted, like the anterior feet, into richly denticulated prehensile feet, and indeed of three different forms, the anterior feet (Figure 44) the two following pairs (Figure 45) and finally the three last pairs (Figure 46) being similarly constructed and different from the rest. In this form the feet remain for a very long time, whilst the abdominal appendages grow into powerful natatory organs, and the eyes, which at first seemed to me to be wanting, into large hemispheres. In the transition to the form of the adult animal the last three pairs of feet (Figure 49)