DEVELOPMENTAL HISTORY OF EDRIOPHTHALMA.
Less varied than that of the Stalk-eyed Crustacea is the mode of
development of the Isopoda and Amphipoda, which Leach united in the
section Edriophthalma, or Crustacea with sessile eyes.
The Rock-Slaters (_Ligia_) may serve as an example of the development
of the Isopoda. In these, as in _Mysis,_ the caudal portion of the
embryo is bent not downwards, but upwards; as in _Mysis_ also, a larval
membrane is first of all formed, within which the Slater is developed.
In _Mysis_ this first larval skin may be compared to a Nauplius; in
_Ligia_ it appears like a maggot quite destitute of appendages, but
produced into a long simple tail (Fig. 37). The egg-membrane is
retained longer than in _Mysis_; it bursts only when the limbs of the
young Slater are already partially developed in their full number. The
dorsal surface of the Slater is united to the larval skin a little
behind the head. At this point, when the union has been dissolved a
little before the change of skin, there is a foliaceous appendage,
which exists only for a short time, and disappears before the young
Slater quits the brood-pouch of the mother.
Fig. 36. Embryo of Ligia in the egg, magnified. D. yelk; L. liver. Fig.
37. Maggot-like larva of Ligia, magnified. R remains of the
egg-membrane. We see on the lower surface, from before backwards:--the
anterior and posterior antennæ, the mandibles, the anterior and
posterior maxillæ, maxillipedes, six ambulatory feet, the last segment
of the middle-body destitute of appendages, five abdominal feet, and
the caudal feet. Fig. 36. Embryo of _Ligia_ in the egg, magnified. _D._
yelk; _L._ liver. Fig. 37. Maggot-like larva of _Ligia,_ magnified. _R_
remains of the egg-membrane. We see on the lower surface, from before
backwards:—the anterior and posterior antennæ, the mandibles, the
anterior and posterior maxillæ, maxillipedes, six ambulatory feet, the
last segment of the middle-body destitute of appendages, five abdominal
feet, and the caudal feet.
The young animal, when it begins to take care of itself, resembles the
old ones in almost all parts, except one important difference; it
possesses only six, instead of seven pairs of ambulatory feet; and the
last segment of the middle-body is but slightly developed and destitute
of appendages. It need hardly be mentioned that the sexual
peculiarities are not yet developed, and that in the males the
hand-like enlargements of the anterior ambulatory feet and the
copulatory appendages are still deficient.
To the question, how far the development of _Ligia_ is repeated in the
other Isopoda, I can only give an unsatisfactory answer. The curvature
of the embryo upwards instead of downwards was met with by me as well
as by Rathke in _Idothea,_ and likewise in _Cassidina, Philoscia,
Tanais,_ and the Bopyridæ,—indeed, I failed to find it in none of the
Isopoda examined for this purpose. In _Cassidina_ also the first larval
skin without appendages is easily detected; it is destitute of the long
tail, but is strongly bent in the egg, as in _Ligia,_ and consequently
cannot be mistaken for an “inner egg-membrane.” This, however, might
happen in _Philoscia,_ in which the larval skin is closely applied to
the egg-membrane (Fig. 38), and is only to be explained as the larval
skin by a reference to _Ligia_ and _Cassidina._ The foliaceous
appendage on the back has long been known in the young of the common
Water Slater (_Asellus_).[1] That the last pair of feet of the thorax
is wanting in the young of the Wood-lice (_Porcellionides,_ M.-Edw.)
and Fish-lice (_Cymothoadiens,_ M.-Edw.) has already been noticed by
Milne-Edwards. This applies also to the Box-Slaters (_Idothea_), to the
viviparous Globe-Slaters (_Sphæroma_) and Shield-Slaters (_Cassidina_),
to the Bopyridæ (_Bopyrus, Entoniscus, Cryptoniscus,_ n.g.), and to the
Cheliferous Slaters (_Tanais_), and therefore probably to the great
majority of the Isopoda. All the other limbs are usually well developed
in the young Isopoda. In _ Tanais_ alone, all the abdominal feet are
wanting (but not those of the tail); they are developed simultaneously
with the last pair of feet of the thorax.
Fig. 38. Embryo of a Philoscia in the egg, magnified. Fig. 39. Embryo
of Cryptoniscus planarioides, magnified. Fig. 40. Last foot of the
middle-body of the larva of Entoniscus Porcellanæ, magnified. Fig. 38.
Embryo of a _Philoscia_ in the egg, magnified.
Fig. 39. Embryo of _Cryptoniscus planarioides,_ magnified.
Fig. 40. Last foot of the middle-body of the larva of _Entoniscus
Porcellanæ,_ magnified.
The last pair of feet on the middle-body of the larva, consequently the
penultimate pair in the adult animal, is almost always similar in
structure to the preceding pair. A remarkable exception is, however,
presented in this respect by _ Cryptoniscus_ and
_Entoniscus,_—remarkable as a confirmation of Darwin’s proposition that
“parts developed in an unusual manner are very variable,” for in the
peculiarly-formed pair of feet there exists the greatest possible
difference between the three species hitherto observed. In _
Cryptoniscus_ (Fig. 39) this last foot is thin and rod-like; in
_Entoniscus Cancrorum_ remarkably long and furnished with a strongly
thickened hand and a peculiarly constructed chela; in _ Entoniscus
Porcellanæ_ very short, imperfectly jointed, and with a large ovate
terminal joint (Fig. 40).
Some Isopods undergo a considerable change immediately before the
attainment of sexual maturity. This is the case with the males of
_Tanais_ which have already been noticed, and, according to Hesse, with
the _Pranizæ,_ in which both sexes are said to pass into the form known
as _Anceus._ But Spence Bate, a careful observer, states that he has
seen females of the form of _Praniza_ laden with eggs far advanced in
their development.
Fig. 41. Entoniscus Cancrorum, female, magnified. Fig. 42. Cryptoniscus
planarioides, female, magnified. Fig. 43. Embryo of a Corophium,
magnified. Fig. 41. _Entoniscus Cancrorum,_ female, magnified.
Fig. 42. _Cryptoniscus planarioides,_ female, magnified.
Fig. 43. Embryo of a _Corophium,_ magnified.
In this order we meet for the first time with an extensive retrograde
metamorphosis as a consequence of a parasitic mode of life. Even in
some Fish-lice (_Cymothoa_) the young are lively swimmers, and the
adults stiff, stupid, heavy fellows, whose short clinging feet are
capable of but little movement. In the Bopyridæ (_Bopyrus, Phryxus,
Kepone,_ etc., which might have been conveniently left in a single
genus), which are parasitic on Crabs, Lobsters, etc., taking up their
abode chiefly in the branchial cavity, the adult females are usually
quite destitute of eyes; the antennæ are rudimentary; the broad body is
frequently unsymmetrically developed in consequence of the confined
space; its segments are more or less amalgamated with each other; the
feet are stunted, and the appendages of the abdomen transformed from
natatory feet with long setæ into foliaceous or tongue-shaped and
sometimes ramified branchiæ. In the dwarfish males the eyes, antennæ,
and feet, are usually better preserved than in the females; but on the
other hand all the appendages of the abdomen have not unfrequently
disappeared, and sometimes every trace of segmentation. In the females
of _ Entoniscus,_ which are found in the body-cavity of Crabs and _
Porcellanæ,_ the eyes, antennæ, and buccal organs, the segmentation of
the vermiform body, and in one species (Fig. 41) the whole of the
limbs, disappear almost without leaving a trace; and _Cryptoniscus
planarioides_ would almost be regarded as a Flatworm rather than an
Isopod, if its eggs and young did not betray its Crustacean nature.
Among the males of these various Bopyridæ, that of _Entoniscus
Porcellanæ_ occupies the lowest place; it is confined all its life to
six pairs of feet, which are reduced to shapeless rounded lumps.
The Amphipoda are distinguishable from the Isopoda at an early period
in the egg by the different position of the embryo, the hinder
extremity of which is bent downwards. In all the animals of this order
which have been examined for it,[2] a peculiar structure makes its
appearance very early on the anterior part of the back, by which the
embryo is attached to the “inner egg-membrane,” and which has been
called the “micropylar apparatus,” but improperly as it seems to me.[3]
It will remind us of the union of the young Isopoda with the larval
membrane and of the unpaired “adherent organ” on the nape of the
Cladocera, which is remarkably developed in _Evadne_ and persists
throughout life; but in _Daphnia pulex,_ according to Leydig, although
present in the young animals, disappears without leaving a trace in the
adults.
The young animal, whilst still in the egg, acquires the full number of
its segments and limbs. In cases where segments are amalgamated
together, such as the last two segments of the thorax in _Dulichia,_
the last abdominal segments and the tail in _ Gammarus ambulans_ and
_Corophium dentatum,_ n. sp., and the last abdominal segments and the
tail in _Brachyscelus,_[4] or where one or more segments are deficient,
as in _Dulichia_ and the _Caprellæ,_ we find the same fusion and the
same deficiencies in young animals taken out of the brood-pouch of
their mother. Even peculiarities in the structure of the limbs, so far
as they are common to both sexes, are usually well-marked in the newly
hatched young, so that the latter generally differ from their parents
only by their stouter form, the smaller number of the antennal joints
and olfactory filaments, and also of the setæ and teeth with which the
body or feet are armed, and perhaps by the comparatively larger size of
the secondary flagellum. An exception to this rule is presented by the
Hyperinæ which usually live upon Acalephæ. In these the young and
adults often have a remarkably different appearance; but even in these
there is no new formation of body-segments and limbs, but only a
gradual transformation of these parts.[5]
Figs. 44 to 46. Feet of a half-grown Hyperia Martinezii, n. sp. (Named
after my valued friend the amiable Spanish zoologist, M. Francisco de
Paula Martinez y Saes, at present on a voyage round the world.) Figs.
47 to 49. Feet of a nearly adult male of the same species; 44 and 47
from the first pair of anterior feet (gnathopoda); 44 and 48 from the
first, and 46 and 49 from the last pair of thoracic feet. Magnified.
Figs. 44–46. Feet of a half-grown Hyperia Martinezii, n. sp. (Named
after my valued friend the amiable Spanish zoologist, M. Francisco de
Paula Martinez y Saes, at present on a voyage round the world.)
Figs. 47–49. Feet of a nearly adult male of the same species; 44 and 47
from the first pair of anterior feet (gnathopoda); 44 and 48 from the
first, and 46 and 49 from the last pair of thoracic feet. Magnified.
Thus, in order to give a few examples, the powerful chelæ of the
antepenultimate pair of feet, of _Phromina sedentaria,_ are produced,
according to Pagenstecher, from simple feet of ordinary structure; and
_vice versà,_ the chelæ on the penultimate pair of feet of the young
_Brachyscelus,_ become converted into simple feet. In the young of the
last-mentioned genus the long head is drawn out into a conical point
and bears remarkably small eyes; in course of growth, the latter, as in
most of the Hyperinæ, attain an enormous size, and almost entirely
occupy the head, which then appears spherical, etc.
The difference of the sexes which, in the Gammarinæ is usually
expressed chiefly in the structure of the anterior feet (gnathopoda,
Sp. Bate) and in the Hyperinæ in the structure of the antennæ, is often
so great that males and females have been described as distinct
species, and even repeatedly placed in different genera (_Orchestia_
and _Talitrus, Cerapus_ and _Dercothoë, Lestrigonus_ and _Hyperia_) or
even families (_Hypérines anormales_ and _Hypérines ordinaires_).
Nevertheless it is only developed when the animals are nearly
full-grown. Up to this period the young resemble the females in a
general way, even in some cases in which these differ more widely than
the males from the “Type” of the order. Thus in the male Shore-hoppers
(_Orchestia_) the second pair of the anterior feet is provided with a
powerful hand, as in the majority of the Amphipoda, but very
differently constructed in the females. The young, nevertheless,
resemble the female. Thus also,—and this is an extremely rare
case,[6]—the females of _Brachyscelus_ are destitute of the posterior
(or inferior) antennæ; the male possesses them like other Amphipodæ; in
the young I, like Spence Bate, can find no trace of them.
It is, however, to be particularly remarked, that the development of
the sexual peculiarities does not stand still on the attainment of
sexual maturity.
For example, the younger sexually mature males of _Orchestia
Tucurauna,_ n. sp., have slender inferior antennæ, with the joints of
the flagellum not fused together, the clasping margin (“palm,” Sp.
Bate) of the hand in the second pair of feet is uniformly convex, the
last pair of feet is slender and similar to the preceding. Subsequently
the antennæ become thickened, two, three, or four of the first joints
of the flagellum are fused together, the palm of the hand acquires a
deep emargination near its inferior angle, and the intermediate joints
of the last pair of feet become swelled into a considerable
incrassation. No museum-zoologist would hesitate about fabricating two
distinct species, if the oldest and youngest sexually mature males were
sent to him without the uniting intermediate forms. In the younger
males of _Orchestia Tucuratinga,_ although the microscopic examination
of their testes showed that they were already sexually mature, the
emargination of the clasping margin of the hand (represented in Fig.
50) and the corresponding process of the finger, are still entirely
wanting. The same may be observed in _Cerapus_ and _Caprella,_ and
probably in all cases where hereditary sexual differences occur.
Fig. 50. Foot of the second pair ('second pair of gnathopoda') of the
male of Orchestia Tucurauna, magnified. Fig. 51. Foot of the second
pair ('second pair of gnathopoda') of the female of Orchestia
Tucurauna, magnified. Fig. 52. Male of a Bodotria, magnified. Note the
long inferior antennæ, which are closely applied to the body, and of
which the apex is visible beneath the caudal appendages. Fig. 50. Foot
of the second pair (“second pair of gnathopoda”) of the male of
_Orchestia Tucurauna,_ magnified.
Fig. 51. Foot of the second pair (“second pair of gnathopoda”) of the
female of _Orchestia Tucurauna,_ magnified.
Fig. 52. Male of a _Bodotria,_ magnified. Note the long inferior
antennæ, which are closely applied to the body, and of which the apex
is visible beneath the caudal appendages.
Next to the extensive sections of the Stalk-eyed and Sessile-eyed
Crustacea, but more nearly allied to the former than to the latter,
comes the remarkable family of the _ Diastylidæ_ or _Cumacea._ The
young, which Kröyer took out of the brood-pouch of the female, and
which attained one-fourth of the length of their mother, resembled the
adult animals almost in all parts. Whether, as in _Mysis_ and _Ligia,_
a transformation occurs within the brood-pouch, which is constructed in
the same way as in _Mysis,_ is not known.[7] The caudal portion of the
embryo in the _Diastylidæ,_ as I have recently observed, is curved
upwards as in the Isopoda, and the last pair of feet of the thorax is
wanting.
Equally scanty is our knowledge of the developmental history of the
Ostracoda. We know scarcely anything except that their anterior limbs
are developed before the posterior one (Zenker). The development of
_Cypris_ has recently been observed by Claus:—“The youngest stages are
shell-bearing Nauplius-forms.”
[1] Leydig has compared this foliaceous appendage of the Water Slaters
with the “green gland” or “shell-gland” of other crustacea, assuming
that the green gland has no efferent duct and appealing to the fact
that the two organs occur “in the same place.” This interpretation is
by no means a happy one. In the first place we may easily ascertain in
_Leucifer,_ as was also found to be the case by Claus, that the “green
gland” really opens at the end of the process described by
Milne-Edwards as a “tubercule auditif” and by Spence Bate as an
“olfactory denticle.” And, secondly, the position is about as
different as it can well be. In the one case a paired gland, opening
at the base of the posterior antennæ, and therefore on the lower
surface of the _second_ segment; in the other an unpaired structure
rising in the median line of the back _behind the seventh segment,_
(“behind the boundary line of the first thoracic segment,” Leydig).
[2] nIn the genera _Orchestoidea, Orchestia, Allorchestes, Montagua,
Batea_ n.g., _Amphilochus, Atylus, Microdeutopus, Leucothoë, Melita,
Gammarus_ (according to Meissner and La Valette), _Amphithoë, Cerapus,
Cyrtophium, Corophium, Dulichia, Protella_ and _Caprella._ote
[3] Little as a name may actually affect the facts, we ought certainly
to confine the name “micropyle” to canals of the egg-membrane, which
serve for the entrance of the semen. But the outer egg-membrane passes
over the “micropylar apparatus” of the Amphipoda without any
perforation, according to Meissner’s and La Valette’s own statements;
it appears never to be present before fecundation, attains its
greatest development at a subsequent period of the ovular life, and
the delicate canals which penetrate it do not even seem to be always
present, indeed it seems to belong to the embryo rather than to the
egg-membrane. I have never been able to convince myself that the
so-called “inner egg-membrane” is really of this nature, and not
perhaps the earliest larva skin, not formed until after impregnation,
as might be supposed with reference to _Ligia, Cassidina_ and
_Philoscia._
[4] According to Spence Bate, in _Brachyscelus crusculum_ the fifth
abdominal segment is not amalgamated with the sixth (the tail) but
with the fourth, which I should be inclined to doubt, considering the
close agreement which this species otherwise shows with the two
species that I have investigated.
[5] In the young of _Hyperia galba_ Spence Bate did not find any of
the abdominal feet, or the last two pairs of thoracic feet, but this
very remarkable statement required confirmation the more because he
examined these minute animals only in the dried state. Subsequently I
had the wished-for opportunity of tracing the development of a
_Hyperia_ which is not uncommon upon Ctenophora, especially _Beroë
gilva,_ Eschsch. The youngest larva from the brood-pouch of the mother
already possess _the whole_ of the thoracic feet; on the other hand,
like Spence Bate, I cannot find those of the abdomen. At first simple
enough, all these feet soon become converted, like the anterior feet,
into richly denticulated prehensile feet, and indeed of three
different forms, the anterior feet (Fig. 44) the two following pairs
(Fig. 45) and finally the three last pairs (Fig. 46) being similarly
constructed and different from the rest. In this form the feet remain
for a very long time, whilst the abdominal appendages grow into
powerful natatory organs, and the eyes, which at first seemed to me to
be wanting, into large hemispheres. In the transition to the form of
the adult animal the last three pairs of feet (Fig. 49) especially
undergo a considerable change. The difference between the two sexes is
considerable; the females are distinguished by a very broad thorax,
and the males (_Lestrigonus_) by very long antennæ, of which the
anterior bear an unusual abundance of olfactory filaments.
Their youngest larvæ of course cannot swim; they are helpless
little animals which firmly cling especially to the swimming laminæ
of their host; the adult _ Hyperiæ,_ which are not unfrequently met
with free in the sea, are, as is well known, the most admirable
swimmers in their order. (“Il nage avec une rapidité extrême,” says
Van Beneden of _H. Latreillii_ M.-Edw.)
The transformation of the _Hyperiæ_ is evidently to be regarded as
_acquired_ and not _ inherited,_ that is to say the late appearance
of the abdominal appendages and the peculiar structure of the feet
in the young are not to be brought into unison with the historical
development of the Amphipoda, but to be placed to the account of
the parasitic mode of life of the young.
As in _Brachyscelus,_ free locomotion has been continued to the
adult and not to the young, contrary to the usual method among
parasites. Still more remarkable is a similar circumstance in
_Caligus,_ among the parasitic Copepoda. The young animal,
described by Burmeister as a peculiar genus, _ Chalimus,_ lies at
anchor upon a fish by means of a cable springing from its forehead,
and having its extremity firmly seated in the skin of the fish.
When sexual maturity is attained, the cable is cut, and the adult
_Caligi,_ which are admirable swimmers, are not unfrequently
captured swimming freely in the sea. (See ‘Archiv. für
Naturgeschichte’ 1852, I. p. 91).
[6] “I know of no case in which the inferior (antennæ) are obsolete,
when the superior are developed,” Dana. (Darwin, ‘Monograph on the
Subclass Cirripedia, Lepadidæ’ p. 15.)
[7] A trustworthy English Naturalist, Goodsir, described the
brood-pouch and eggs of _Cuma_ as early as 1843. Kröyer, whose
painstaking care and conscientiousness is recognised with wonder by
every one who has met him on a common field of work, confirmed
Goodsir’s statements in 1846, and, as above mentioned, took out of the
brood-pouch embryos advanced in development and resembling their
parents. By this the question whether the Diastylidæ are full-grown
animals or larvæ, is completely and for ever set at rest, and only the
famous names of Agassiz, Dana and Milne-Edwards, who would recently
reduce them again to larvæ (see Van Beneden, ‘Rech. sur la Fauna
littor. de Belgique’ Crustacées, pp. 73, 74), induce me, on the basis
of numerous investigations of my own, to declare in Van Beneden’s
words; “Parmi toutes les formes embryonnaires de podophthalmes ou
d’édriophthalmes que nous avons observées sur nos côtes, nous n’en
avons pas vu une seule qui eût même la moindre resemblance avec un
_Cuma_ quelconque.” The _only thing_ that suits the larvæ of
_Hippolyte, Palæmon_ and _Alpheus,_ in the family character of the
Cumacea as given by Kröyer which occupies three pages (Kröyer,
‘Naturh. Tidsskrift, Ny Raekke,’ Bd. ii. pp. 203–206) is: “Duo
antennarum paria.” And this, as is well known, applies to nearly all
Crustacea. How well warranted are we therefore in identifying the
latter with the former. However, it is sufficient for any one to
glance at the larva of _Palæmon_ (Fig. 27) and the Cumacean (Fig. 52)
in order to be convinced of their extraordinary similarity!