'Lecons sur la Physiol. et l'Anat. comp.' 3 page 197, the statement that, according to Frey and Leuckart, the heart of Caprella linearis possesses FIVE pairs of fissures. I have examined perfectly transparent young Caprellae (probably the young of Caprella attenuata, Dana, with which they occurred), but can only find the usual three pairs.)

Considering this uniformity presented by the heart in the entire order of the Amphipoda, it cannot but seem very remarkable, that in the very next order of the Isopoda, we find it to be one of the most changeable organs.

In the cheliferous Isopods (Tanais) the heart resembles that of the Amphipoda in its elongated tubular form, as well as in the number and position of the fissures, but with this difference, that the two fissures of each pair do not lie directly opposite each other.

(FIGURE 14. Heart of a young Ca.s.sidina.

FIGURE 15. Heart of a young Anilocra.

FIGURE 16. Abdomen of the male of Entoniscus Cancrorum. h. Heart. l.

Liver.)

In all other Isopoda the heart is removed towards the abdomen. In the wonderfully deformed parasitic Isopods of the Porcellanae (Entoniscus porcellanae), the spherical heart of the female is confined to a short s.p.a.ce of the elongated first abdominal segment, and seems to possess only a single pair of fissures. In the male of Entoniscus Cancrorum (n.

sp.), the heart (Figure 16) is situated in the third abdominal segment.

In the Ca.s.sidinae, the heart (Figure 14) is likewise short and furnished with two pairs of fissures, situated in the last segment of the thorax and the first segment of the abdomen. Lastly, in a young Anilocra, I find the heart (Figure 15) extending through the whole length of the abdomen and furnished with four (or five?) fissures, which are not placed in pairs but alternately to the right and left in successive segments. In other animals of this order, which I have as yet only cursorily examined, further differences will no doubt occur. But why, in two orders so nearly allied to each other, should we find in the one such a constancy, in the other such a variability, of the same highly important organ? From the schoolmen we need expect no explanation, they will either decline the discussion of the "wherefore" as foreign to their province, as lying beyond the boundaries of Natural History, or seek to put down the importunate question by means of a sounding paraphrase of the facts, abundantly sprinkled with Greek words. As I have unfortunately forgotten my Greek, the second way out of the difficulty is closed to me; but as I luckily reckon myself not amongst the incorporated masters, but, to use Baron von Liebig's expression, amongst the "promenaders on the outskirts of Natural History," this affected hesitation of the schoolmen cannot dissuade me from seeking an answer, which indeed presents itself most naturally from Darwin's point of view.

As not only the Tanaides (which reasons elsewhere stated (vide supra) justify us in regarding as particularly nearly related to the primitive Isopod) and the Amphipoda, but also the Decapod Crustacea, possess a heart with three pairs of fissures essentially in the same position; and as the same position of the heart recurs (vide infra) even in the embryos of the Mantis-Shrimps (Squilla), in which the heart of the adult animal, and even, as I have elsewhere shown, that of the larvae when still far from maturity, extends in the form of a long tube with numerous openings far into the abdomen, we must unhesitatingly regard the heart of the Amphipoda as the primitive form of that organ in the Edriophthalma. As, moreover, in these animals the blood flows from the respiratory organs to the heart without vessels, it is very easy to see how advantageous it must be to them to have these organs as much approximated as possible. We have reason to regard as the primitive mode of respiration, that occurring in Tanais (vide supra). Now, where, as in the majority of the Isopoda, branchiae were developed upon the abdomen, the position and structure of the heart underwent a change, as it approached them more nearly, but without the reproduction of a common plan for these earlier modes of structure, either because this transformation of the heart took place only after the division of the primary form into subordinate groups, or because, at least at the time of this division, the varying heart had not yet become fixed in any new form. Where, on the contrary, respiration remained with the anterior part of the body,--whether in the primitive fashion of Zoea, as in the Tanaides, or by the development of branchiae on the thorax, as in the Amphipoda,--the primitive form of the heart was inherited unchanged, because any variations which might make their appearance were rather injurious than advantageous, and disappeared again immediately.

I close this series of isolated examples with an observation which indeed only half belongs to the province of the Crustacea to which these pages ought to be confined, and which also has no further connexion with the preceding circ.u.mstances than that of being an "intelligible and intelligence-bringing fact" only from the point of view of Darwin's theory. To-day as I was opening a specimen of Lepas anatifera in order to compare the animal with the description in Darwin's 'Monograph on the Subcla.s.s Cirripedia,' I found in the sh.e.l.l of this Cirripede, a blood-red Annelide, with a short, flat body, about half an inch long and two lines in breadth, with twenty-five body-segments, and without projecting setigerous tubercles or jointed cirri. The small cephalic lobe bore four eyes and five tentacles; each body-segment had on each side at the margin a tuft of simple setae directed obliquely upwards, and at some distance from this, upon the ventral surface, a group of thicker setae with a strongly uncinate bidentate apex. There was above EACH of the lateral tufts of bristles a branchia, simple on a few of the foremost segments, and then strongly arborescent to the end of the body.

The animal, a female filled with ova, evidently, from these characters, belongs to the family of the Amphinomidae; the only family the members of which, being excellent swimmers, live in the open sea.

That this animal had not strayed accidentally into the Lepas, but appertained to it as a regular and permanent guest, is evidenced by its considerable size in proportion to the narrow entrance of the test of the Lepas, by the complete absence of the iridescence which usually distinguishes the skin of free Annelides and especially of the Amphinomidae, by the formation and position of the inferior setae, etc.

But that a worm belonging to this particular family Amphinomidae living in the high sea, occurs as a guest in the Lepas, which also floats in the sea attached to wood, etc., is at once intelligible from the stand-point of the Darwinian theory, whilst the relationship of this parasite to the free-living worms of the open sea remains perfectly unintelligible under the supposition that it was independently created for dwelling in the Lepas.

But however favourable the examples. .h.i.therto referred to may be for Darwin, the objection may be raised against them, and that with perfect justice, that they are only isolated facts, which, when the considerations founded upon them are carried far beyond what is immediately given, may only too easily lead us from the right path, with the deceptive glimmer of an ignis fatuus. The higher the structure to be raised, the wider must be the a.s.suring base of well-sifted facts.

Let us turn then to a wider field, that of the developmental history of the Crustacea, upon which science has already brought together a varied abundance of remarkable facts, which, however, have remained a barren acc.u.mulation of unmanageable raw-material, and let us see how, under Darwin's hand, these scattered stones unite to form a well-jointed structure, in which everything, bearing and being borne, finds its significant place. Under Darwin's hand! for I shall have nothing to do except just to place the building stones in the position which his theory indicates for them. "When kings build, the carters have to work."

CHAPTER 7. DEVELOPMENTAL HISTORY OF PODOPHTHALMA.

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* in the European fresh-water Crayfish, and according to Westwood in a West Indian Land Crab (Gecarcinus). (* Authorities are cited only for facts which I have had no opportunity of confirming.) 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 Planariae 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 s.e.xually mature animal in by far the greater majority of the Podophthalma, which quit the egg in the form of Zoea. 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 Zoea-brood,* which is also met with in other terrestrial Crabs (Ocypoda, Gelasimus, etc.). (* 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 princ.i.p.ally 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 dest.i.tute 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-ma.s.ses, 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 s.p.a.ce between which is traversed by numerous transverse part.i.tions dilated at their ends; the s.p.a.ces between these part.i.tions 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 pa.s.ses 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 a.n.a.logy. 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 dest.i.tute 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 a.n.a.logous 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 dest.i.tute 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 s.e.xually 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, a.s.serts 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, have been discovered in a nearly continuous series.