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

(* Bell (‘Brit. Stalk-eyed Crust.’ page 45) considers himself justified in “eliminating” Thompson’s observation at once, because he could only have examined ovigerous females preserved in alcohol. But any one who had paid so much attention as Thompson to the development of these animals, must have been well able to decide with certainty upon eggs, if not too far from maturity or badly preserved, whether a Zoea would be produced from them. Moreover, the mode of life of the Land-Crabs is in favour of Thompson. “Once in the year,” says Troschel’s ‘Handbuch der Zoologie,’ “they migrate in great crowds to the sea in order to deposit their eggs, and afterwards return much exhausted towards their dwelling places, which are reached only by a few.” For what purpose would be these destructive migrations in species whose young quit the egg and the mother as terrestrial animals?) All the Anomura seem likewise to commence their lives as Zoeae: witness the Porcellanae, the Tatuira (Hippa emerita) and the Hermit Crabs. Among the Macrura we are acquainted with the same earliest form principally in several Shrimps and Prawns, such as Crangon (Du Cane), Caridina (Joly), Hippolyte, Palaemon, Alpheus, etc. Lastly, it is not improbable, that the youngest brood of the Mantis-Shrimps (Squilla) is also in the same case.

The most important peculiarities which distinguish this Zoea-brood from the adult animal, are as follows:—

The middle-body with its appendages, those five pairs of feet to which these animals owe their name of Decapoda, is either entirely wanting, or scarcely indicated; the abdomen and tail are destitute of appendages, and the latter consists of a single piece. The mandibles, as in the Insecta, have no palpi. The maxillipedes, of which the third pair is often still wanting, are not yet brought into the service of the mouth, but appear in the form of biramose natatory feet. Branchiae are wanting, or where their first rudiments may be detected as small verruciform prominences, these are dense cell-masses, through which the blood does not yet flow, and which therefore have nothing to do with respiration. An interchange of the gases of the water and blood may occur all over the thin-skinned surface of the body; but the lateral parts of the carapace may unhesitatingly be indicated as the chief seat of respiration. They consist, exactly as described by Leydig in the Daphniae, of an outer and inner lamina, the space between which is traversed by numerous transverse partitions dilated at their ends; the spaces between these partitions are penetrated by a more abundant flow of blood than occurs anywhere else in the body of the Zoea. To this may be added that a constant current of fresh water passes beneath the carapace in a direction from behind forwards, maintained as in the adult animal, by a foliaceous or linguiform appendage of the second pair of maxillae (Figure 18). The addition of fine coloured particles to the water allows this current of water to be easily detected even in small Zoeae.

(FIGURE 17. Zoea of a Marsh Crab (Cyclograpsus ?), magnified 45 diam.

FIGURE 18. Maxilla of the second pair in the same species, magnified 180 diam.)

The Zoeae of the Crabs (Figure 17) are usually distinguished by long, spiniform processes of the carapace. One of these projects upwards from the middle of the back, a second downwards from the forehead, and frequently there is a shorter one on each side near the posterior inferior angles of the carapace. All these processes are, however, wanting in Maia according to Couch, and in Eurynome according to Kinahan; and in a third species of the same group of the Oxyrhynchi (belonging or nearly allied to the genus Achaeus) I also find only an inconsiderable dorsal spine, whilst the forehead and sides are unarmed. This is another example warning us to be cautious in deductions from analogy. Nothing seemed more probable than to refer back the beak-like formation of the forehead in the Oxyrhynchi to the frontal process of the Zoea, and now it appears that the young of the Oxyrhynchi are really quite destitute of any such process. The following are more important peculiarities of the Zoeae of the Crabs, although less striking than these processes of the carapace which, in combination with the large eyes, often give them so singular an appearance:—the anterior (inner) antennae are simple, not jointed, and furnished at the extremity with from two to three olfactory filaments; the posterior (outer) antennae frequently run out into a remarkably long spine-like process (“styliform process,” Spence Bate), and bear, on the outside, an appendage, which is sometimes very minute (“squamiform process” of Spence Bate), corresponding with the antennal scale of the Prawns, ( In a memoir on the metamorphoses of the Porcellanae I have erroneously described this appendage as the “flagellum.”) and the first rudiment of the future flagellum is often already recognisable. Of natatory feet (afterwards maxillipeds) only two pairs are present; the third (not, as Spence Bate thinks, the first) is entirely wanting, or, like the five following pairs of feet, present only as a minute bud. The tail, of very variable form, always bears THREE pairs of setae at its hinder margin. The Zoeae of the Crabs usually maintain themselves in the water in such a manner that the dorsal spine stands upwards, the abdomen is bent forwards, the inner branch of the natatory feet is directed forwards, and the outer one outwards and upwards.

(FIGURES 19 TO 23. Tails of the Zoeae of various Crabs.

FIGURE 19. Pinnotheres.

FIGURE 20. Sesarma.

FIGURE 21. Xantho.

FIGURES 22 AND 23 of unknown origin.)

It is further to be remarked that the Zoeae of the Crabs, as also of the Porcellanae, of the Tatuira and of the Shrimps and Prawns, are enveloped, on escaping from the egg, by a membrane veiling the spinous processes of the carapace, the setae of the feet, and the antennae, and that they cast this in a few hours. In Achaeus I have observed that the tail of this earliest larval skin resembles that of the larvae of Shrimps and Prawns, and the same appears to be the case in Maia (see Bell, ‘Brit. Stalk-eyed Crust.’ page 44).

Widely as they seem to differ from them at the first glance, the Zoeae of the Porcellanae (Figure 24) approach those of the true Crabs very closely. The antennae, organs of the mouth, and natatory feet, exhibit the same structure. But the tail bears FIVE pairs of setae, and the dorsal spine is wanting, whilst, on the contrary, the frontal process and the lateral spines are of extraordinary length, and directed straight forward and backward.

(FIGURE 24. Zoea of Porcellana stellicola, F. Mull. Magnified 15 diam.

FIGURE 25. Zoea of the Tatuira (Hippa emerita), magnified 45 diam.

FIGURE 26. Zoea of a small Hermit Crab, magnified 45 diam.)

The Zoea of the Tatuira (Figure 25) also appears to differ but little from those of the true Crabs, which it likewise resembles in its mode of locomotion. The carapace possesses only a short, broad frontal process; the posterior margin of the tail is edged with numerous short setae.

The Zoea of the Hermit Crabs (Figure 26) possesses the simple inner antennae of the Zoea of the true Crabs; the outer antennae bear upon the outside on a short stalk a lamella of considerable size analogous to the scale of the antennae of the Prawns; on the inside, a short, spine-like process; and between the two the flagellum, still short, but already furnished with two apical setae. As in the Crabs, there are only two pairs of well-developed natatory feet (maxillipedes), but the third pair is also present in the form of a two-jointed stump of considerable size, although still destitute of setae. The tail bears five pairs of setae. The little animal usually holds itself extended straight in the water, with the head directed downwards.

This is also the position in which we usually see the Zoeae of the Shrimps and Prawns (Figure 27), which agree in their general appearance with those of the Hermit Crabs. Between the large compound eyes there is in them a small median eye. The inner antennae bear, at the end of a basal joint sometimes of considerable length, on the inside a plumose seta, which also occurs in the Hermit Crabs, and on the outside a short terminal joint with one or more olfactory filaments. The outer antennae exhibit a well-developed and sometimes distinctly articulated scale, and within this usually a spiniform process; the flagellum appears generally to be still wanting. The third pair of maxillipedes seems to be always present, at least in the form of considerable rudiments. The spatuliform caudal lamina bears from five to six pairs of setae on its hinder margin.

The development of the Zoea-brood to the sexually mature animal was traced by Spence Bate in Carcinus maenas. He proved that the metamorphosis is a perfectly gradual one, and that no sharply separated stages of development, like the caterpillar and pupa of the Lepidoptera, could be defined in it. Unfortunately we possess only this single complete series of observations, and its results cannot be regarded at once as universally applicable; thus the young Hermit Crabs retain the general aspect and mode of locomotion of Zoeae, whilst the rudiments of the thoracic and abdominal feet are growing, and then, when these come into action, appear at once in a perfectly new form, which differs from that of the adult animal chiefly by the complete symmetry of the body and by the presence of four pairs of well-developed natatory feet on the abdomen. ( Glaucothoe Peronii, M.-Edw., may be a young and still symmetrical Pagurus of this kind.)

(FIGURE 27. Zoea of a Palaemon residing upon Rhizostoma cruciatum, Less., magnified 45 diam.)

The development of the Palinuridiae seems to be very peculiar. Claus found in the ova of the Spiny Lobster (Palinurus), embryos with a completely segmented body, but wanting the appendages of the tail, abdomen, and last two segments of the middle-body; they possess a single median and considerably compound eye; the anterior antennae are simple, the posterior furnished with a small secondary branch; the mandibles have no palpi; the maxillipedes of the third pair, like the two following pairs of feet, are divided into two branches of nearly equal length; whilst the last of the existing pairs of feet and the second pair of maxillipedes bear only an inconsiderable secondary branch. Coste, as is well known, asserts that he has bred young Phyllosomata from the ova of this lobster—a statement that requires further proof, especially as the more recent investigations of Claus upon Phyllosoma by no means appear to be in its favour.

The large compound eyes, which usually soon become moveable, and sometimes stand upon long stalks even in the earliest period, as well as the carapace, which covers the entire fore-body, indicate at once that the position of the larvae hitherto considered, notwithstanding all their differences, is under the Podophthalma. But not a single characteristic of this section is retained by the brood of some Prawns belonging to the genus Peneus or in its vicinity. These quit the egg with an unsegmented ovate body, a median frontal eye, and three pairs of natatory feet, of which the anterior are simple, and the other two biramose—in fact, in the larval form, so common among the lower Crustacea, to which O.F. Muller gave the name of Nauplius. No trace of a carapace! no trace of the paired eyes! no trace of masticating organs near the mouth which is overarched by a helmet-like hood!

(FIGURE 28. Nauplius of a Prawn, magnified 45 diam.

FIGURE 29. Young Zoea of the same Prawn, magnified 45 diam.

FIGURE 30. Older Zoea of the same Prawn, magnified 45 diam.

FIGURE 31. Mysis-form of the same Prawn, magnified 45 diam.)

In the case of one of these species the intermediate forms which lead from the Nauplius to the Prawn,