STRUCTURE OF THE HEART IN THE EDRIOPHTHALMA.
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. 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.
(Figs. 19 to 23. Tails of the Zoëæ of various Crabs. Fig. 19.
Pinnotheres. Fig. 20. Sesarma. Fig. 21. Xantho. Fig. 22 and 23 of
unknown origin. Figs. 19 to 23. Tails of the Zoëæ of various Crabs.
Fig. 19. Pinnotheres. Fig. 20. Sesarma. Fig. 21. Xantho. Figs. 22 and
23 of unknown origin.
It is further to be remarked that the Zoëæ of the Crabs, as also of the
_Porcellanæ,_ 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 setæ of the feet, and the antennæ, and
that they cast this in a few hours. In _Achæus_ I have observed that
the tail of this earliest larval skin resembles that of the larvæ of
Shrimps and Prawns, and the same appears to be the case in _Maia_ (see
Bell, ‘Brit. Stalk-eyed Crust.’ p. 44). Widely as they seem to differ
from them at the first glance, the Zoëæ of the _ Porcellanæ_ (Fig. 24)
approach those of the true Crabs very closely. The antennæ, organs of
the mouth, and natatory feet, exhibit the same structure. But the tail
bears _five_ pairs of setæ, 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.
Fig. 24. Zoëa of Porcellana stellicola, F. Müll. Magnified. Fig. 25.
Zoëa of the Tatuira (Hippa emerita), magnified. Fig. 26. Zoëa of a
small Hermit Crab, magnified . Fig. 24. Zoëa of _Porcellana
stellicola,_ F. Müll., magnified.
Fig. 25. Zoëa of the Tatuira (_Hippa emerita_), magnified.
Fig. 26. Zoëa of a small Hermit Crab, magnified.
The Zoëa of the Tatuira (Fig. 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
setæ.
The Zoëa of the Hermit Crabs (Fig. 26) possesses the simple inner
antennæ of the Zoëa of the true Crabs; the outer antennæ bear upon the
outside on a short stalk a lamella of considerable size analogous to
the scale of the antennæ 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 setæ. 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 setæ. The tail bears five pairs of
setæ. 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 Zoëæ of the
Shrimps and Prawns (Fig. 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 antennæ 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 antennæ
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 setæ on its
hinder margin.
Fig. 27. Zoëa of a Palæmon residing upon Rhizostoma cruciatum, Less.,
magnified. Fig. 27. Zoëa of a _Palæmon_ residing upon _Rhizostoma
cruciatum,_ Less., magnified.
The development of the Zoëa-brood to the sexually mature animal was
traced by Spence Bate in _Carcinus mænas._ 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 Zoëæ, 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.[4]
The development of the Palinuridiæ 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 antennæ 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 larvæ 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 _Penëus_ 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. Müller 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!
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.
The youngest Nauplius (Fig. 28) is immediately followed by forms in
which a fold of skin runs across the back behind the third pair of
feet, and four pairs of stout processes (rudiments of new limbs) sprout
forth on the ventral surface. Within the third pair of feet, powerful
mandibles are developed.
Fig. 28. Nauplius of a Prawn, magnified. Fig. 29. Young Zoëa of the
same Prawn, magnified. Fig. 28. Nauplius of a Prawn, magnified.
Fig. 29. Young Zoëa of the same Prawn, magnified.
In a subsequent moult the new limbs (maxillæ, and anterior and
intermediate maxillipedes) come into action, and in this way the
Nauplius becomes a Zoëa (Fig. 29), agreeing perfectly with the Zoëa of
the Crabs in the number of the appendages of the body, although very
different in form and mode of locomotion and even in many particulars
of internal structure. The chief organs of motion are still the two
anterior pairs of feet, which are slender and furnished with long setæ;
the third pair of feet loses its branches, and becomes converted into
mandibles destitute of palpi. The labrum acquires a spine directed
forward and of considerable size, which occurs in all the Zoëæ of
allied species. The biramose maxillipedes appear to assist but slightly
in locomotion. The forked tail reminds us rather of the forms occurring
in the lower Crustacea, especially the Copepoda, than of the
spatuliform caudal plate which characterises the Zoëæ of _Alpheus,
Palæmon, Hippolyte,_ and other Prawns, of the Hermit Crabs, the Tatuira
and the _ Porcellanæ._ The heart possesses only one pair of fissures,
and has no muscles traversing its interior like trabeculæ, whilst in
other Zoëæ two pairs of fissures and an interior apparatus of trabeculæ
are always distinctly recognisable.
Fig. 30. Older Zoëa of the same Prawn, magnified. Fig. 31. Mysis-form
of the same Prawn, magnified. Fig. 30. Older Zoëa of the same Prawn,
magnified.
Fig. 31. _Mysis_-form of the same Prawn, magnified.
During this Zoëal period the paired eyes, the segments of the
middle-body and abdomen, the posterior maxillipedes, the lateral caudal
appendages and the stump-like rudiments of the feet of the middle-body
are formed (Fig. 30). The caudal appendages sprout forth like other
limbs freely on the ventral surface, whilst in other Prawns, the
_Porcellanæ,_ etc., they are produced in the interior of the
spatuliform caudal plate.
As the feet of the middle-body come into action, simultaneously with
other profound changes, the Zoëa passes into the _ Mysis-_ or
Schizopod-form (Fig. 31). The antennæ cease to serve for locomotion,
their place is taken by the thoracic feet, furnished with long setæ,
and by the long abdomen which just before was laboriously dragged along
as a useless burden, but now, with its powerful muscles, jerks the
animal through the water in a series of lively jumps. The anterior
antennæ have lost their long setæ, and by the side of the last (fourth)
joint, endowed with olfactory filaments, there appears a second branch,
which is at first of a single joint. The previously multi-articulate
outer branch of the posterior antennæ has become a simple lamella, the
antennal scale of the Prawn; beside this appears the stump-like
rudiment of the flagellum, probably as a new formation, the inner
branch disappearing entirely. The five new pairs of feet are biramose,
the inner branch short and simple, the outer one longer, annulated at
the end, furnished with long setæ, and kept, as in _Mysis,_ in constant
whirling motion. The heart acquires new fissures, and interior muscular
trabeculæ.
During the _Mysis_-period, the auditory organs in the basal joint of
the anterior antennæ are formed; the inner branches of the first three
pairs of feet are developed into chelæ and the two hinder pairs into
ambulatory feet; palpi sprout from the mandibles, branchiæ on the
thorax, and natatory feet on the abdomen. The spine on the labrum
becomes reduced in size. In this way the animal gradually approaches
the Prawn-form, in which the median eye has become indistinct, the
spine of the labrum, and the outer branches of the cheliferous and
ambulatory feet have been lost, the mandibular palpi and the abdominal
feet have acquired distinct joints and setæ, and the branchiæ come into
action.
In another Prawn, the various larval states of which may be easily
recognised as belonging to the same series by the presence of a
dark-yellow, sharply-defined spot surrounding the median eye, the
youngest Zoëa (Fig. 32), probably produced from the Nauplius, agrees in
all essential particulars with the species just described; its further
development is, however, very different, especially in that neither the
feet of the middle, nor those of the hind-body are formed
simultaneously, and that a stage of development comparable to Mysis in
the number and structure of the limbs does not occur.
Fig. 32. Youngest (observed) Zoëa of another Prawn. The minute buds of
the third pair of maxillipedes are visible. The formation of the
abdominal segments has commenced. Paired eyes still wanting. Magnified.
Fig. 33. Older larva produced from the Zoëa represented in Fig. 32. The
last segment and the last two pairs of feet of the middle-body are
wanting. Magnified. Fig. 32. Youngest (observed) Zoëa of another Prawn.
The minute buds of the third pair of maxillipedes are visible. The
formation of the abdominal segments has commenced. Paired eyes still
wanting. Magnified.
Fig. 33. Older larva produced from the Zoëa represented in Fig. 32. The
last segment and the last two pairs of feet of the middle-body are
wanting. Magnified.
Traces of the outer maxillipedes make their appearance betimes. Then
feet appear upon four segments of the middle-body, and these are
biramose on the three anterior segments, and simple, the inner branch
being deficient, on the fourth segment. On the inner branches the chelæ
are developed; the outer branches are lost before an inner branch has
made its appearance on the fourth segment (Fig. 32). The latter again
becomes destitute of appendages, so that in this case at an early
period four, and at a later only three, segments of the middle-body
bear limbs. The fifth segment is still entirely wanting, whilst all the
abdominal segments have also acquired limbs, and this one after the
other, from before backwards. The adult animal, as shown by the three
pairs of chelæ, will certainly be very nearly allied to the preceding
species.[5]
The youngest larva of the Schizopod genus Euphausia observed by Claus,
stands very near the youngest Zoëa of our Prawns; but whilst its
anterior antennæ are already biramose, and it therefore appears to be
more advanced, it still wants the middle maxillipedes. In it also Claus
found the heart furnished with only a single pair of fissures. Do not
Nauplius-like states in this case also precede the Zoëa?
The developmental history of _Mysis,_ the near relationship of which
with the Shrimps and Prawns has recently again been generally
recognised, has been described in detail by Van Beneden. So far as I
have tested them I can only confirm his statements. The development of
the embryo commences with the formation of the tail! This makes its
appearance as a simple lobe, the dorsal surface of which is turned
towards and closely applied to that of the embryo. (The young of other
Stalk-eyed Crustacea are, as is well known, bent in the egg in such a
manner that the ventral surfaces of the anterior and posterior halves
of the body are turned towards each other,—in these, therefore, the
dorsal, and in Mysis the ventral surface appears convex.) The tail soon
acquires the furcate form with which we made acquaintance in the last
Prawn-Zoëa described. Then two pairs of thick ensiform appendages make
their appearance at the opposite end of the body, and behind these a
pair of tubercles which are easily overlooked. These are the antennæ
and mandibles. The egg-membrane now bursts, before any internal organ,
or even any tissue, except the cells of the cutaneous layer, is formed.
The young animal might be called a Nauplius; but essentially there is
nothing but a rough copy of a Nauplius-skin, almost like a new
egg-membrane, within which the _ Mysis_ is developed. The ten pairs of
appendages of the fore- (maxillæ, maxillipedes) and middle-body make
their appearance simultaneously, as do the five pairs of abdominal feet
at a later period. Soon after the young _Mysis_ casts the
Nauplius-envelope it quits the brood-pouch of the mother.[6]
For some time, owing to an undue importance being ascribed to the want
of a particular branchial cavity, _Mysis, Leucifer,_ and _Phyllosoma_
were referred to the Stomapoda, which are now again limited, as
originally by Latreille, to the Mantis-shrimps (_Squilla_), the
Glass-shrimps (_Erichthus_) and their nearest allies. Of the
developmental history of these we have hitherto been acquainted with
only isolated fragments. The tracing of the development in the egg is
rendered difficult by the circumstance, that the Mantis-shrimps do not,
like the Decapoda, carry their spawn about with them, but deposit it in
the subterranean passages inhabited by them in the form of thin, round,
yellow plates. The spawn is consequently exceedingly difficult to
procure, and unfortunately it becomes spoilt in a day when it is
removed from its natural hatching place, whilst on the contrary the
progress of development may be followed for weeks together in the eggs
of a single Crab kept in confinement. The eggs of _ Squilla,_ like
those removed from the body of the Crab, die because they are deprived
of the rapid stream of fresh water which the mother drives through her
hole for the purpose of her own respiration.
The accompanying representation of the embryo of _Squilla_ shows that
it possesses a long, segmented abdomen without appendages, a bilobate
tail, six pairs of limbs, and a short heart; the latter only pulsates
weakly and slowly. If it acquires more limbs before exclusion, the
youngest larva must stand on the same level as the youngest larva of
_Euphausia_ observed by Claus.
Fig. 34. Embryo of a Squilla, magnified. a. heart. Fig. 35. Older larva
(Zoëa) of a Stomapod, magnified. Fig. 34. Embryo of a Squilla,
magnified. _a._ heart.
Fig. 35. Older larva (Zoëa) of a Stomapod, magnified.
Of the two larval forms at present known which are with certainty to be
ascribed, if not to _Squilla,_ at least to a Stomapod, I pass over the
younger one[7] as its limbs cannot be positively interpreted, and will
only mention that in it the last three abdominal segments are still
destitute of appendages. The older larva (Fig. 35), which resembles the
mature Squilla especially in the structure of the great raptorial feet
and of the preceding pair, still wants the six pairs of feet following
the raptorial feet. The corresponding body-segments are already well
developed, an unpaired eye is still present, the anterior antennæ are
already biramose, whilst the flagellum is wanting in the posterior, and
the mandibles are destitute of palpi; the four anterior abdominal
segments bear biramose natatory feet, without branchiæ; the fifth
abdominal segment has no appendages, and this is also the case with the
tail, which still appears as a simple lamina, fringed on the hinder
margin with numerous short teeth. It is evident that the larva stands
essentially in the grade of Zoëa.
[1] Authorities are cited only for facts which I have had no
opportunity of confirming.
[2] Bell (‘Brit. Stalk-eyed Crust.’ p. xlv) 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
Zoëa 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?
[3] In a memoir on the metamorphoses of the Porcellanæ I have
erroneously described this appendage as the “flagellum.”
[4] _Glaucothoë Peronii,_ M.-Edw., may be a young and still
symmetrical _Pagurus_ of this kind.
[5] The oldest observed larvæ (see Fig. 33) are characterised by the
extraordinary length of the flagella of the outer antennæ, and in this
respect resemble the larva of _Sergestes_ found by Claus near Messina
(Zeitschr. für Wiss. Zool. Bd. xiii. Taf. 27, Fig. 14). This unusual
length of the antennæ leads to the supposition that they belong to our
commonest Prawn, which is very frequently eaten, and is most nearly
allied to _Penëus setiferus_ of Florida. Claus’s _Acanthosoma_ (_l.
c._ Fig. 13) is like the younger _ Mysis_-form of the larva figured by
me in the ‘Archiv für Naturgeschichte,’ 1836, Taf. 2, Fig. 18, and
which I am inclined to refer to _Sicyonia carinata._
[6] Van Beneden, who regards the eye-peduncles as limbs, cannot
however avoid remarking upon _Mysis_: “Ce pédicule n’apparaît
aucunement comme les autres appendices, et paraît avoir une autre
valeur morphologique.”
[7] ‘Archiv für Naturgeschichte’ 1863. Taf. 1.