DEVELOPMENTAL HISTORY OF ENTOMOSTRACA, CIRRIPEDES, AND RHIZOCEPHALA.
The section of the Branchiopoda includes two groups differing even in
their development,—the Phyllopoda and the Cladocera. The latter minute
animals, provided with six pairs of foliaceous feet, which chiefly
belong to the fresh waters, and are diffused under similar forms over
the whole world, quit the egg with their full number of limbs. The
Phyllopoda, on the contrary, in which the number of feet varies between
10 and 60 pairs, and some of which certainly live in the saturated lie
of salterns and natron-lakes, but of which only one rather divergent
genus (_Nebalia_) is found in the sea,[1] have to undergo a
metamorphosis. Mecznikow has recently observed the development of
_Nebalia,_ and concludes from his observations “that _Nebalia,_ during
its embryonal life, passes through the Nauplius- and Zoëa-stages, which
in the Decapoda occur partly (in _Penëus_) in the free state.”
“Therefore,” says he, “I regard Nebalia as a Phyllopodiform Decapod.”
The youngest larvæ [of the Phyllopoda] are Nauplii, which we have
already met with exceptionally in some Prawns, and which we shall now
find reproduced almost without exception. The body-segments and feet,
which are sometimes so numerous, are formed gradually from before
backwards, without the indication of any sharply-discriminated regions
of the body either by the time of their appearance or by their form.
All the feet are essentially constructed in the same manner and
resemble the maxillæ of the higher Crustacea.[2] We might regard the
Phyllopoda as Zoëæ which have not arrived at the formation of a
peculiarly endowed abdomen or thorax, and instead of these have
repeatedly reproduced the appendages which first follow the
Nauplius-limbs.
Of the Copepoda—some of which, living in a free state, people the fresh
waters, and in far more multifarious forms the sea, whilst others, as
parasites, infest animals of the most various classes and often become
wonderfully deformed—the developmental history, like their entire
natural history, was, until lately, in a very unsatisfactory state. It
is true, that we long ago knew that the _Cyclopes_ of our fresh waters
were excluded in the Nauplius-form, and that we were acquainted with
some others of their young states; we had learnt, through Nordmann,
that the same earliest form belonged to several parasitic Crustacea,
which had previously passed, almost universally, as worms; but the
connecting intermediate forms which would have permitted us to refer
the regions of the body and the limbs of the larvæ to those of the
adult animal, were wanting. The comprehensive and careful
investigations of Claus have filled up this deficiency in our
knowledge, and rendered the section of the Copepoda one of the best
known in the whole class. The following statements are derived from the
works of this able naturalist. From the abundance of valuable materials
which they contain I select only those which are indispensable for the
comprehension of the development of the Crustacea in general, because,
in what relates to the Copepoda in particular, the facts have already
been placed in the proper light by the representation of their most
recent investigator, and must appear to any one whose eyes are open, as
important evidence in favour of the Darwinian theory.[3]
All the larvæ of the free Copepoda investigated by Claus, have, at the
earliest period, three pairs of limbs (the future antennæ and
mandibles), the anterior with a single, and the two following ones with
a double series of joints, or branchiæ. The unpaired eye, labrum, and
mouth, already occupy their permanent positions. The posterior portion,
which is usually short and destitute of limbs, bears two terminal setæ,
between which the anus is situated. The form in this Nauplius-brood is
extremely various,—it is sometimes compressed laterally, sometimes
flat,—sometimes elongated, sometimes oval, sometimes round or even
broader than long, and so forth. The changes which the first larval
stages undergo during the progress of growth, consist essentially in an
extension of the body and the sprouting forth of new limbs. “The
following stage already displays a fourth pair of extremities, the
future maxillæ.” Then follow at once three new pairs of limbs (the
maxillipedes and the two anterior pairs of natatory feet). The larva
still continues like a Nauplius, as the three anterior pairs of limbs
represent rowing feet; at the next moult it is converted into the
youngest _Cyclops_-like state, when it resembles the adult animal in
the structure of the antennæ and buccal organs, although the number of
limbs and body segments is still much less, for only the rudiments of
the third and fourth pairs of natatory feet have made their appearance
in the form of cushions fringed with setæ, and the body consists of the
oval cephalothorax, the second, third, and fourth thoracic segments,
and an elongated terminal joint. In the Cyclopidæ the posterior antennæ
have lost their secondary branch, and the mandibles have completely
thrown off the previously existing natatory feet, whilst in the other
families these appendages persist, more or less altered. “Beyond this
stage of free development, many forms of the parasitic Copepoda, such
as _Lernanthropus_ and _Chondracanthus,_ do not pass, as they do not
acquire the third and fourth pairs of limbs, nor does a separation of
the fifth thoracic segment from the abdomen take place; others
(_Achtheres_) even fall to a lower grade by the subsequent loss of the
two pairs of natatory feet. But all free Copepoda, and most of the
parasitic Crustacea, pass through a longer or shorter series of stages
of development, in which the limbs acquire a higher degree of division
into joints in continuous sequence, the posterior pairs of feet are
developed, and the last thoracic segment and the different abdominal
segments are successively separated from the common terminal portion.”
Figs. 53 and 54. Nauplii of Copepoda, the former magnified, the latter
magnified 2x. Fig. 55. Nauplius of Tetraclita porosa after the first
moult, magnified 90 diam. The brain is seen surrounding the eye, and
from it the olfactory filaments issue; behind it are some delicate
muscles passing to the buccal hood. Fig. 53 and 54. Nauplii of
Copepoda, the former magnified, the latter magnified 2x.
Fig. 55. Nauplius of _Tetraclita porosa_ after the first moult,
magnified. The brain is seen surrounding the eye, and from it the
olfactory filaments issue; behind it are some delicate muscles passing
to the buccal hood.
There is only one thing more to be indicated in the developmental
history of the parasitic Crustacea, namely that some of them, such as
_Achtheres percarum,_ certainly quit the egg like the rest in a
Nauplius-like form, inasmuch as the plump, oval, astomatous body bears
two pairs of simple rowing feet, and behind these, as traces of the
third pair, two inflations furnished each with a long seta, but that
beneath this Nauplius-skin a very different larva lies ready prepared,
which in a few hours bursts its clumsy envelope and then makes its
appearance in a form “which agrees in the segmentation of the body and
in the development of the extremities with the first _Cyclops-stage_”
(Claus). The entire series of Nauplius-stages which are passed through
by the free Copepoda, are in this case completely over-leapt.
A final and very peculiar section of the Crustacea is formed by the two
orders of the Cirripedia and Rhizocephala.[4]
In these also the brood bursts out in the Nauplius-form, and speedily
strips off its earliest larva-skin which is distinguished by no
peculiarities worth noticing. Here also we find again the same pyriform
shape of the unsegmented body, the same number and structure of the
feet, the same position of the median eye (which, however, is wanting
in _Sacculina purpurea,_ and according to Darwin in some species of
_Lepas_), and the same position of the “buccal hood,” as in the Nauplii
of the Prawns and Copepoda. From the latter the Nauplii of the
Cirripedia and Rhizocephala are distinguished by the possession of a
dorsal shield or carapace, which sometimes (_Sacculina purpurea_)
projects far beyond the body all round; and they are distinguished not
only from other Nauplii, but as far as I know from all other Crustacea,
by the circumstance that structures which are elsewhere combined with
the two anterior limbs (antennæ), here occur separated from them.
The anterior antennæ of the Copepoda, Cladocera, Phyllopoda (Leydig,
Claus), Ostracoda (at least the Cypridinæ), Diastylidæ, Edriophthalma,
and Podophthalma, with few exceptions relating to terrestrial animals
or parasites, bear peculiar filaments which I have already repeatedly
mentioned as “olfactory filaments.” A pair of similar filaments spring,
in the larvæ of the Cirripedia and Rhizocephala, directly from the
brain.
(Fig. 56. Nauplius of Sacculina purpurea, shortly before the second
moult, magnified 180 diam. We may recognise in the first pair of feet
the future adherent feet, and in the abdomen six pairs of natatory feet
with long setæ. Fig. 57. Pupa of a Balanide (Chthamalus ?), magnified.
The adherent feet are retracted within the rather opaque anterior part
of the shell. Fig. 58. Pupa of Sacculina purpurea, magnified. The
filaments on the adherent feet may be the commencements of the future
roots. Fig. 56. Nauplius of _Sacculina purpurea,_ shortly before the
second moult, magnified. We may recognise in the first pair of feet the
future adherent feet, and in the abdomen six pairs of natatory feet
with long setæ.
Fig. 57. Pupa of a Balanide (_Chthamalus ?_), magnified. The adherent
feet are retracted within the rather opaque anterior part of the shell.
Fig. 58. Pupa of _Sacculina purpurea,_ magnified. The filaments on the
adherent feet may be the commencements of the future roots.
At the base of the inferior antennæ in the Decapoda the so-called
“green-gland” has its opening; in the Macrura at the end of a conical
process. A similar conical process with an efferent duct traversing it
is very striking in most of the Amphipoda. In the Ostracoda, Zenker
describes a gland situated in the base of the inferior antennæ, and
opening at the extremity of an extraordinarily long “spine.” In the
Nauplii of _Cyclops_ and _Cyclopsine,_ Claus finds pale “shell-glands,”
which commence in the intermediate pair of limbs (the posterior
antennæ). On the other hand in the Nauplii of the Cirripedia and
Rhizocephala the “shell-glands” open at the ends of conical processes,
sometimes of most remarkable length, which spring from the angles of
the broad frontal margin, and have been interpreted sometimes as
antennæ (Burmeister, Darwin) and sometimes as mere “horns of the
carapace” (Krohn). The connexion of the “shell-glands” with the frontal
horns has been recognised unmistakably in the larvæ of _Lepas,_ and
indeed the resemblance of the frontal horns with the conical processes
on the inferior antennæ of the Amphipoda, is complete throughout.[5]
Notwithstanding their agreement in this important peculiarity, the
Nauplii of these two orders present material differences in many other
particulars. The abdomen of the young Cirripede is produced beneath the
anus into a long tail-like appendage which is furcate at the extremity,
and over the anus there is a second long, spine-like process; the
abdomen in the Rhizocephala terminates in two short points,—in a
“moveable caudal fork, as in the Rotatoria,” (O. Schmidt). The young
Cirripedes have a mouth, stomach, intestine, and anus, and their two
posterior pairs of limbs are beset with multifarious teeth, setæ, and
hooks, which certainly assist in the inception of nourishment. All this
is wanting in the young Rhizocephala. The Nauplii of the Cirripedia
have to undergo several moults whilst in that form; the Nauplii of the
Rhizocephala, being astomatous, cannot of course live long as Nauplii,
and in the course of only a few days they become transformed into
equally astomatous “pupæ,” as Darwin calls them.
The carapace folds itself together, so that the little animal acquires
the aspect of a bivalve shell, the foremost limbs become transformed
into very peculiar adherent feet (“prehensile antennæ,” Darwin), and
the two following pairs are cast off; like the frontal horns. On the
abdomen six pairs of powerful biramose natatory feet with long setæ
have been formed beneath the Nauplius-skin, and behind these are two
short, setigerous caudal appendages (Fig. 58).
The pupæ of the Cirripedia (Fig. 57), which are likewise astomatous,
agree completely in all these parts with those of the Rhizocephala,
even to the minutest details of the segmentation and bristling of the
natatory feet;[6] they are especially distinguished from them by the
possession of a pair of composite eyes. Sometimes also traces of the
frontal horns seem to persist.[7]
As the Cirripedia and Rhizocephala now in general resemble each other
far more than in their Nauplius-state, this is also the case with the
individual members of each of the two orders.
The pupæ in both orders attach themselves by means of the adherent
feet; those of the Cirripedes to rocks, shells, turtles, drift-wood,
ships, etc.,—those of the Rhizocephala to the abdomen of Crabs,
_Porcellanæ,_ and Hermit Crabs. The carapace of the Cirripedes becomes
converted, as is well-known, into a peculiar test, on account of which
they were formerly placed among the Mollusca, and the natatory feet
grow into long cirri, which whirl nourishment towards the mouth, which
is now open. The Rhizocephala remain astomatous; they lose all their
limbs completely, and appear as sausage-like, sack-shaped or discoidal
excrescences of their host, filled with ova (Figs. 59, 60); from the
point of attachment closed tubes, ramified like roots, sink into the
interior of the host, twisting round its intestine, or becoming
diffused among the sac-like tubes of its liver. The only manifestations
of life which persist in these _non plus ultras_ in the series of
retrogressively metamorphosed Crustacea, are powerful contractions of
the roots, and an alternate expansion and contraction of the body, in
consequence of which water flows into the brood-cavity and is again
expelled, through a wide orifice.[8]
Fig. 59. Young of Peltogaster socialis on the abdomen of a small Hermit
Crab; in one of them the fasciculately ramified roots in the liver of
the Crab are shown. Animal and roots deep yellow. Fig. 60. Young
Sacculina purpurea with its roots; the animal purple-red, the roots
dark grass-green. Magnified. Figs. 61 to 63. Eggs of Tetraclita porosa
in segmentation, magnified. The larger of the two first-formed spheres
of segmentation is always turned towards the pointed end of the egg.
Fig. 64. Egg of Lernæodiscus Porcellanæ, in segmentation, magnified 90
Fig. 59. Young of _Peltogaster socialis_ on the abdomen of a small
Hermit Crab; in one of them the fasciculately ramified roots in the
liver of the Crab are shown. Animal and roots deep yellow.
Fig. 60. Young _Sacculina purpurea_ with its roots; the animal
purple-red, the roots dark grass-green. Magnified.
Figs. 61–63. Eggs of _Tetraclita porosa_ in segmentation, magnified.
The larger of the two first-formed spheres of segmentation is always
turned towards the pointed end of the egg.
Fig. 64. Egg of _Lernæodiscus Porcellanæ,_ in segmentation, magnified.
Out of several Cirripedes, which are anomalous both in structure and
development, _Cryptophialus minutus_ must be mentioned here; Darwin
found it in great quantities together in the shell of _Concholepas
peruviana_ on the Chonos Islands. The egg, which is at first
elliptical, soon, according to Darwin, becomes broader at the anterior
extremity, and acquires three club-shaped horns, one at each anterior
angle and one behind; no internal parts can as yet be detected.
Subsequently the posterior horn disappears, and the adherent feet may
be recognised within the anterior ones. From this “egg-like
larva”—(Darwin says of it, “I hardly know what to call it”)—the pupa is
directly produced. Its carapace is but slightly compressed laterally
and hairy, as in _Sacculina purpurea_; the adherent feet are of
considerable size, and the natatory feet are wanting, as, in the adult
animal, are the corresponding cirri. As I learn from Mr. Spence Bate,
the Nauplius-stage appears to be overleaped and the larvæ to leave the
egg in the pupa-form, in the case of a Rhizocephalon (_Peltogaster ?_)
found by Dr. Powell in the Mauritius.
I will conclude this general view with a few words upon the earliest
processes in the development of the Crustacea. Until recently it was
regarded as a general rule that, by the partial segmentation of the
vitellus a germinal disc was formed, and in this, corresponding to the
ventral surface of the embryo, a primitive band. We now know that in
the Copepoda (Claus), in the Rhizocephala (Fig. 64), and, as I can add,
in the Cirripedia (Figs. 61–63) the segmentation is complete, and the
embryos are sketched out in their complete form without any preceding
primitive band. Probably the latter will always be the case where the
young are hatched as true _Nauplii_ (and not merely with a
Nauplius-skin, as in _Achtheres_). The two modes of development may
occur in very closely allied animals, as is proved by _Achtheres_ among
the Copepoda.[9]
[1] If the Phyllopoda may be regarded as the nearest allies of the
Trilobites, they would furnish, with _Lepidosteus_ and _Polypterus,
Lepidosiren_ and _Protopterus,_ a further example of the preservation
in fresh waters of forms long since extinguished in the sea. The
occurrence of the _Artemiæ_ in supersaline water would at the same
time show that they do not escape destruction by means of the fresh
water, but in consequence of the less amount of competition in it.
[2] “The maxilla of the Decapod-larva (Krebslarve) is a sort of
Phyllopodal foot” (Claus).
[3] I am still unacquainted with Claus’ latest and larger work, but no
doubt the same may be said of it.
[4] The most various opinions prevail as to the position of the
Cirripedia. Some ascribe to them a very subordinate position among the
Copepoda; as Milne-Edwards (1852). In direct opposition to this notion
of his father’s, Alph. Milne-Edwards places them (as _Basinotes_)
opposite to all the other Crustacea (_Eleuthéronotes_). Darwin regards
them as forming a peculiar sub-class equivalent to the Podophthalma,
Edriophthalma, etc. This appears to me to be most convenient. I would
not combine the Rhizocephala with the Cirripedia, as Liljeborg has
done, but place them in opposition as equivalent, like the Amphipoda
and Isopoda. The near relationship of the Cirripedia to the Ostracoda
is also spoken of, but the similarity of the so-called “_Cypris_-like
larvæ,” or Cirriped-pupæ as Darwin calls them, to _Cypris_ is so
purely external, even as regards the shell, that the relationship
appears to me to be scarcely greater than that of _Peltogaster
socialis_ (Fig. 59) with the family of the sausages.
[5] In connexion with this it may be mentioned that, in the females of
Brachyscelus, in which the posterior antennæ are deficient, the
conical processes with the canal permeating them are nevertheless
retained.
[6] Compare the figure given by Darwin (Balanidæ Pl. xxx fig. 5) of
the first natatory foot of the pupa of _Lepas australis,_ with that of
_Lernæodiscus Porcellanæ_ published in the ‘Archiv für
Naturgeschichte’ (1863, Taf. iii, fig. 5). The sole distinction, that
in the latter there are only 3 setæ at the end of the outer branch,
whilst in the Cirripedia there are 4 on the first and 5 on the
following natatory feet, may be due to an error on my part.
[7] Darwin describes as “acoustic orifices” small apertures in the
shell of the pupæ of the Cirripedia, which, frequently surrounded by a
border, are situated, in _Lepas pectinata,_ upon short, horn-like
processes. I feel scarcely any hesitation in regarding the apertures
as those of the “shell-glands,” and the horn-like processes as remains
of the frontal horns.
[8] The roots of _Sacculina purpurea_ (Fig. 60) which is parasitic
upon a small Hermit Crab, are made use of by two parasitic Isopods,
namely a Bopyrus and the before mentioned _Cryptoniscus planarioides_
(Fig. 42). These take up their abode beneath the _Sacculina_ and cause
it to die away by intercepting the nourishment conveyed by the roots;
the roots, however, continue to grow, even without the _Sacculina,_
and frequently attain an extraordinary extension, especially when a
_Bopyrus_ obtains its nourishment from them.
[9] I have not mentioned the Pycnogonidæ, because I do not regard them
as Crustacea; nor the Xiphosura and Trilobites, because, having never
investigated them myself, I knew too little about them, and especially
because I am unacquainted with the details of the explanations given
by Barrande of the development of the latter. According to Mr. Spence
Bate “the young of Trilobites are of the Nauplius-form.”)