Facts and Arguments for Darwin

by Fritz Müller

Let us first glance over the extant facts.

Among the Stalk-eyed Crustacea (Podophthalma) we know only a very few species which quit the egg in the form of their parents, with the full number of well-jointed appendages to the body. This is the case according to Rathke[^[1]] in the European fresh-water Crayfish, and according to Westwood in a West Indian Land Crab (Gecarcinus). Both exceptions therefore belong to the small number of Stalk-eyed Crustacea which live in fresh water or on the land, as indeed in many other cases fresh-water and terrestrial animals undergo no transformations, whilst their allies in the sea have a metamorphosis to undergo. I may refer to the Earthworms and Leeches among the Annelida, which chiefly belong to the land and to fresh water,—to the Planariæ of the fresh waters and the Tetrastemma of the sparingly saline Baltic among the Turbellaria,—to the Pulmonate Gasteropoda, and to the Branchiferous Gasteropoda of the fresh waters, the young of which (according to Troschel’s ‘Handb. der Zoologie’) have no ciliated buccal lobes, although such organs are possessed by the very similar Periwinkles (Littorina).

All the marine forms of this section appear to be subject to a more or less considerable metamorphosis. This appears to be only inconsiderable in the common Lobster, the young of which, according to Van Beneden, are distinguished from the adult animal, by having their feet furnished, like those of Mysis, with a swimming branch projecting freely outwards. From a figure given by Couch the appendages of the abdomen and tail also appear to be wanting.

Far more profound is the difference of the youngest brood from the sexually mature animal in by far the greater majority of the Podophthalma, which quit the egg in the form of Zoëa. This young form occurs, so far as our present observations go, in all the Crabs, with the sole exception of the single species investigated by Westwood. I say species, and not genus, for in the same genus, Gecarcinus, Vaughan Thompson found Zoëa-brood,[^[2]] which is also met with in other terrestrial Crabs (Ocypoda, Gelasimus, etc.). All the Anomura seem likewise to commence their lives as Zoëæ: witness the Porcellanæ, 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, Palæmon, 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 Zoëa-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. Branchiæ 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 Daphniæ, 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 Zoëa. 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 maxillæ (Fig. 18). The addition of fine coloured particles to the water allows this current of water to be easily detected even in small Zoëæ.

Fig. 17. Zoëa of a Marsh Crab (Cyclograpsus ?), magnified.
Fig. 18. Maxilla of the second pair in the same species, magnified.

The Zoëæ of the Crabs (Fig. 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 Achæus) 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 Zoëa, 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 Zoëæ 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) antennæ are simple, not jointed, and furnished at the extremity with from two to three olfactory filaments; the posterior (outer) antennæ 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,[^[3]] 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 setæ at its hinder margin. The Zoëæ 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.