CRUSTACEA (_CONTINUED_): MALACOSTRACA: LEPTOSTRACA—PHYLLOCARIDA:
EUMALACOSTRACA: SYNCARIDA—ANASPIDACEA: PERACARIDA—MYSIDACEA—CUMACEA—
ISOPODA—AMPHIPODA: HOPLOCARIDA—STOMATOPODA
SUB-CLASS II.—MALACOSTRACA.
The Malacostraca are generally large Crustacea, and they are
characterised by the presence of a definite and constant number of
segments composing the body. In addition to the paired eyes we can
distinguish two pairs of antennae, a mandibular segment, and two
maxillary segments composing the head region proper; there then follow
eight thoracic segments, the limbs belonging to the anterior thoracic
segments being often turned forwards towards the mouth, and modified in
structure to act as maxillipedes, while at any rate the last four are
used in locomotion and are termed “pereiopods.”[84] The abdomen is
composed of six segments, which typically carry as many pairs of
biramous “pleopods,” and the body terminates in a telson. Not counting
the paired eyes or the telson, there are present nineteen segments. The
excretory organs in the adult open at the bases of the second antennae,
and are known as “green glands,” but in the larva maxillary glands may
be present homologous to those which persist in the adult Entomostraca.
This is the typical arrangement, but sometimes the maxillary glands
persist in adult Malacostraca, _e.g._ _Nebalia_, _Anaspides_, and some
Isopods.
The hepato-pancreatic diverticula are directed posteriorly, and not
anteriorly as in most Entomostraca, and the stomach is often furnished
with chitinous teeth and ridges forming an elaborate gastric mill,
especially in the larger Decapods.
SERIES 1. LEPTOSTRACA.
=Division. Phyllocarida.=
[Illustration:
FIG. 76.—_Nebalia geoffroyi_, ♀, × 20. _A.1_, _A.2_, 1st and 2nd
antennae; _Ab.1_, _Ab.6_, 1st and 6th abdominal appendages; _A.G._
antennary gland; _C_, half of caudal fork; _E_, eye; _G_, ventral
ganglionic chain; _H_, heart; _I_, intestine; _L_, upper
liver-diverticulum; _M_, adductor muscle of halves of carapace;
_MX_, palp of 1st maxilla; _O_, ovary; _R_, rostrum. (After Claus.)
]
The small shrimp-like Crustacean _Nebalia_, which is found burrowing in
the superficial layers of sand in the littoral and sometimes the deeper
regions of most seas, has been regarded, ever since its anatomy was made
out by Claus,[85] as a connecting link between Entomostraca and
Malacostraca, and has been placed in a separate group Leptostraca.
The segmentation of the body is Malacostracan, save that two extra
segments are present in the abdomen, and the paired compound eyes are
borne upon stalks. The eight thoracic limbs are all very similar; they
are built on the typical biramous plan, and each carries a bract; they
have been compared, owing to their flattened, expanded shape, to the
foliaceous limbs of the Phyllopods. The abdominal appendages are also
biramous. The heart is greatly elongated, stretching through thorax and
abdomen; there are present both the antennary excretory glands
characteristic of adult Malacostraca and the maxillary glands
characteristic of adult Entomostraca, and both the posterior and
anterior livers characteristic of the two Orders respectively are
present. This combination of characters justifies the belief that
_Nebalia_ represents a primitive form, standing to some extent in an
intermediate position between Entomostraca and Malacostraca, but it may
be doubted if the special relationship to the Phyllopoda, claimed on the
strength of the foliaceous appearance of the thoracic limbs, can be
legitimately pressed.
_Nebalia_ shows the clearest signs of relationship to the other
primitive Malacostraca, and especially to the Mysidae, which it
resembles not only in general form and in the essentially biramous
character of its appendages, but also in many embryological points and
in the similarity in development of the brood-pouch.[86]
A large number of very ancient palaeozoic fossils are known which are
placed provisionally with _Nebalia_ in the Division Phyllocarida, and
some of these are no doubt closely related to the existing isolated
genus. _Hymenocaris_ from the Cambrian.
SERIES 2. EUMALACOSTRACA.
Before entering on a description of the members of this Series it is
necessary to introduce and justify a new scheme of classification which
has been proposed by Dr. W. T. Calman. This scheme necessitates the
abandonment of the old Order Schizopoda, and also ignores the
distinction which used to be considered fundamental between the
sessile-eyed Crustacea (Edriophthalmata) and the stalk-eyed forms
(Podophthalmata).
The old group of Schizopoda, to which _Nebalia_ and the isolated form
_Anaspides_, to be considered later, are undoubtedly related, represent
very clearly the stem-forms from which the various branches of the
Malacostracan stock diverge. No doubt they are themselves specialised in
many directions, since they are a dominant group in present day seas,
but their organisation is fundamentally of a primitive type. We see this
especially in the comparative absence of fusion or reduction of the
segments of the body externally and of the nervous system internally,
and in the simple undifferentiated character of the trunk-limbs, all of
which conform to the primitive biramous type. The most anterior thoracic
limbs of the Schizopods are of particular interest. In the higher
Malacostraca three of these limbs are usually turned forwards towards
the mouth to act as maxillipedes, and the most anterior of all, the
first maxillipede, is apt, especially in the Decapoda, to take on a
flattened foliaceous form owing to the expansion of the basal segments
to act as gnathobases (see Fig. 1, A, p. 10). Now this appendage in the
Schizopods preserves its typical biramous character, and resembles the
succeeding thoracic limbs, but in many of the species the basal joints
show a tendency to be produced into biting blades (Fig. 1, E, p. 10),
thus indicating the first step in the evolution of the foliaceous first
maxillipede of the Decapoda. The primitive character of the Schizopods
is also indicated by the fact that most of the Decapoda with uniramous
limbs on the five hinder thoracic segments pass through what is known as
the “Mysis stage” in development, when these limbs are biramous, the
exopodites being subsequently lost in most cases.
The “Schizopoda” include a very large number of pelagic Crustacea of
moderate size, which superficially appear to resemble one another very
closely. The slender, elongated body, the presence of biramous limbs on
all the thoracic and abdominal segments, and the possession of a single
row of gills at the bases of the thoracic limbs, are, generally
speaking, typical of the families Mysidae, Lophogastridae, Eucopiidae,
and Euphausiidae, which go to make up the old Order Schizopoda.
It has, however, been pointed out first by Boas,[87] and subsequently by
Hansen and Calman,[88] that the Euphausiidae are in many respects
distinct from the other three families, and agree with the Decapoda,
while the Eucopiidae, Lophogastridae, and Mysidae agree with the
Cumacea, Isopoda, and Amphipoda.
It has, therefore, been suggested by these authors that the
classification of the Malacostraca should be revised, and Calman (_loc.
cit._) has brought forward the following scheme:—
The division PERACARIDA, including the Eucopiidae, Lophogastridae, and
Mysidae (= Mysidacea), the Cumacea, Isopoda, and Amphipoda, is
characterised by the fact that when a carapace is present it leaves at
least four of the thoracic segments free and uncoalesced: by the
presence of a brood-pouch formed from the oostegites on the thoracic
limbs of the female: by the elongated heart: by the few and simple
hepatic caeca: by the filiform spermatozoa: and by the direct method of
development without a complicated larval metamorphosis. The biting face
of the mandible has a movable joint, the “lacinia mobilis.”[89]
The division EUCARIDA, on the other hand, including the Euphausiidae and
the Decapoda, shows the converse of these characters. The carapace
coalesces with all the thoracic segments, there is never a brood-pouch
formed from oostegites, the hepatic caeca are much ramified, the heart
is short, the spermatozoa are spherical with radiating pseudopodia, the
development is indirect with a complicated metamorphosis, and the
mandible is without a lacinia mobilis.
Corresponding divisions are made by Calman to receive the other
Malacostraca, namely, the PHYLLOCARIDA for _Nebalia_, the SYNCARIDA for
_Anaspides_, and the HOPLOCARIDA for the Stomatopoda or Squillidae.
The important array of characters which separates the Euphausiidae from
the other Schizopods and unites them with the Decapoda can no longer be
neglected, and the consideration of _Anaspides_ and its allies will
further emphasise the extreme difficulty of retaining the Schizopoda as
a natural group. In the sequel Calman’s proposed scheme will be adopted.
DIVISION 1. SYNCARIDA.
There is no carapace, and all the eight thoracic segments may be free
and distinct. Eyes may be pedunculate or sessile. The mandible is
without a lacinia mobilis. There is no brood-pouch, the eggs being
deposited and hidden after fertilisation. The spermatozoa are filiform,
the hepatic caeca very numerous, and the heart tubular and elongated,
with ostia only in one place in the anterior thoracic region. The
auditory organ is at the base of the first antennae.
=Order. Anaspidacea.=
=Fam. 1. Anaspididae.=—The mountain-shrimp of Tasmania, _Anaspides
tasmaniae_, was first described by Thomson[90] in 1893 from specimens
taken in a little pool near the summit of Mount Wellington; it was
redescribed by Calman,[91] who drew attention to its remarkable
resemblance to certain Carboniferous fossils of Europe and N. America
(_Gampsonyx_, _Palaeocaris_, etc.).
The creature appears to be confined to the deep pools of the rivers and
tarns on the mountains of the southern and western portions of
Tasmania.[92] The waters in which it occurs are always cold and
absolutely clear, and there is no record of its living at altitudes much
below 2000 feet, while it frequently occurs at 4000 feet. The body may
attain upwards of two inches in length; it is deeply pigmented with
black chromatophores, and it is held perfectly horizontal without any
flexure. The animal rarely swims unless disturbed, usually walking about
on stones and water-plants at the bottom of deep pools. In walking the
endopodites of the thoracic limbs are chiefly instrumental, but they are
assisted by the exopodites of the abdominal limbs.
When frightened the shrimp can dart rapidly forwards or sideways by the
strokes of its powerful tail-fan, but it never jumps backwards as do the
other Malacostraca. It appears to browse upon the algal slime covering
the rocks and on the submerged liver-worts and mosses, but it does not
refuse animal food, even feeding on the dead bodies of members of its
own species. The thoracic limbs, which are all biramous except the last
pair, carry a double series of remarkable plate-like gills on their
coxopodites. The slender and setose exopodites of the thoracic limbs are
respiratory in function, being kept in continual motion even when the
animal is at rest, and serving to keep up a current of fresh water round
the gills.
_Anaspides_ shows a remarkable combination of structural characters,
some of which are peculiar, while others are possessed in common with
the Peracarida or Eucarida. The chief peculiar characters are the entire
absence of a carapace, and the freedom of the eight thoracic segments,
with eight free thoracic ganglia in the nerve-cord; the peculiar double
series of plate-like gills; the structure of the alimentary canal; and
the fact that the eggs, instead of being carried in a brood-pouch, or
affixed to the abdominal limbs, are deposited under stones and among
water-plants.[93]
[Illustration:
FIG. 77.—_Anaspides tasmaniae_ in natural position for walking, × 1.
The last two pereiopods point backwards and are overlapped by the
first two pleopods.
]
The Peracaridan features, uniting it especially with the Mysidacea, are
the structure of the elongated heart, the filiform spermatozoa, and the
fact that no complicated metamorphosis is passed through, the young
hatching out in a condition similar to, though possibly not identical
with, the adult form.
The Eucaridan, especially Decapodan, features are the presence of an
auditory sac on the basal joint of the antennules, and the modification
of the endopodites of the first two abdominal appendages in the male to
form a copulatory organ.
A type of a new genus of this family was found by me in the littoral
zone of the Great Lake of Tasmania at an elevation of 3700 feet, and
named _Paranaspides lacustris_.
This little shrimp (Fig. 78), which does not appear to grow to more than
an inch in length, is totally different in appearance from _Anaspides_,
being pale green and transparent, with a very marked dorsal hump as in
_Mysis_, to which it bears a very striking superficial resemblance. It
leads a more active swimming life than _Anaspides_, and with this habit
is correlated the flexure of the body and the greater size of the
tail-fan and the scale of the second antenna. The mandible is peculiar
in being furnished with a four-jointed biramous palp, while that of
_Anaspides_ is three-jointed and uniramous, and the first thoracic
appendage is provided with a setose biting lobe on the antepenultimate
joint, thus more resembling a maxillipede. In other respects it agrees
essentially in structure with _Anaspides_.
[Illustration:
FIG. 78.—_Paranaspides lacustris_, × 4. _a^1_, _a^2_, First and second
antennae; _Ab.1_, first abdominal segment; _ep_, epipodites or gills
on the thoracic legs; _md_, mandible; _Pl.1_, first pleopod; _T_,
telson; _Th.8_, eighth free thoracic segment; _U_, uropod, or sixth
pleopod.
]
=Fam. 2. Koonungidae.=—The sole representative of this family, _Koonunga
cursor_, has been recently described by Mr. O. A. Sayce,[94] of
Melbourne University, from a small stream some miles to the west of
Melbourne. Although plainly belonging to the Anaspidacea, this
interesting little animal, which only measures a few millimetres in
length, and follows a similar habit to _Anaspides_, running about with
its body unflexed, differs from all the other members of the Division in
possessing sessile instead of stalked eyes, in the first thoracic
segment being fixed to the head, and in a number of minor anatomical
points.
It is impossible at present to assign the Carboniferous forms
(_Gampsonyx_, _Palaeocaris_, etc.) to their exact position in the
Division, but it seems that they agreed more closely with _Anaspides_
than with the other two genera. From the position in which the fossils
are preserved, it would appear that they followed a similar walking
habit to _Anaspides_, and that the body was unflexed.
DIVISION 2. PERACARIDA.
The carapace, when present, leaves at least four of the thoracic somites
distinct; the first thoracic segment is always fused with the head. The
eyes are pedunculate or sessile.
The mandible possesses a lacinia mobilis. A brood-pouch is formed in the
female from oostegites attached to the thoracic limbs. The hepatic caeca
are few and simple; the heart is elongated and tubular; the spermatozoa
are filiform, and development takes place without a complicated
metamorphosis.
=Order I. Mysidacea.=
The Mysidacea, although pelagic, are not very often met with in the true
plankton on the surface; they generally swim some way below the surface,
going down in many cases into the abysses. For this reason they thrive
excellently in aquaria, and the common _Mysis vulgaris_ is often present
in such numbers in the tanks at the Zoological station at Naples as to
damage the other inmates by the mere press of numbers. The Mysidacea,
like the majority of the Peracarida, undergo a direct development, and
hatch out with the structure of the adult fully formed.
Many of the Mysidacea bear auditory sacs upon the sixth pair of
pleopods, a characteristic not found in the Euphausiacea.
=Fam. 1. Eucopiidae.=—The curious form _Eucopia australis_ (Fig. 79)
described by Sars,[95] may be chosen as an example of the Mysidacea.
The peculiarity of this form consists chiefly in the immense elongation
of the endopodites of the fifth, sixth, and seventh thoracic appendages.
Characteristic of the Mysidacea is the freedom of the hinder thoracic
segments from fusion with the carapace, otherwise this animal is seen
closely to resemble the _Euphausia_ figured (Fig. 102). _Eucopia
australis_, like so many of the Mysidacea, is a deep-sea animal, being
brought up with the dredge from over 1000 fathoms; it is very widely
distributed over the Atlantic Ocean.
[Illustration:
FIG. 79.—_Eucopia australis_, young female, × 3. _A_, 1st antenna;
_Ab.1_, 1st abdominal segment; _Ab.6_, 6th abdominal appendage; _E_,
eye; _T_, telson; _Th_, 5th thoracic appendage. (After Sars.)
]
=Fam. 2. Lophogastridae.=—The members of this family (_Lophogaster_,
_Gnathophausia_) agree with the Eucopiidae in the possession of branched
gills on some of the thoracic limbs, in the absence of auditory sacs on
the sixth pair of pleopods, in the presence of normally developed
pleopods in both the male and female, and in the brood-lamellae being
developed on all seven of the thoracic limbs. The endopodites of the
posterior thoracic limbs are, however, of a normal size.
[Illustration:
FIG. 80.—Dorsal view of male _Diastylis stygia_, × 12. _A_, 2nd
antenna; _Ab.6_, 6th abdominal appendage. (After Sars.)
]
=Fam. 3. Mysidae.=—These differ from both the foregoing families in the
absence of gills, in the presence of an auditory sac on the sixth
pleopods, in the reduction of the other pleopods in the female, and in
the brood-lamellae being developed only on the more posterior pairs of
thoracic limbs. A number of closely related genera compose this family,
of which _Mysis_, _Boreomysis_, and _Siriella_ may be mentioned. _Mysis
oculata_, _var. relicta_, is a fresh-water form from the lakes of
northern and central Europe.
=Order II. Cumacea.=[96]
The Cumacea are a group of small marine animals rarely attaining an inch
in length, which agree with the Mysidacea in the characters noted above
as diagnostic of the Division Peracarida; they possess, however, in
addition a number of peculiar properties, and Sars believes them to be
of a primitive nature showing relationship to _Nebalia_, and possibly to
an ancestral Zoaea-like form. They follow a habit similar to that of the
Mysidacea, being caught either in the surface-plankton or in great
depths, many of the deep-sea forms being blind. They are, however, not
true plankton forms, and they appear to attain a greater development
both in point of variety and size in the seas of the northern
hemisphere. The thoracic limbs may be biramous, but there is a tendency
among many of the genera to lose the exopodites of some of the thoracic
legs, an exopodite never being present on the last few thoracic limbs of
the female and on the last in the male. In the Cumidae the four
posterior pairs in both sexes have no exopodites. The first three
thoracic appendages following the maxillae are distinguished as
maxillipedes; they are uniramous, and the first pair carries an
epipodite and a large gill upon the basal joints. Pleopods are only
developed in the male sex.
The flagellum of the second antennae in the male may be enormously
elongated, as in the Atlantic deep-sea species shown in Fig. 80, so as
to exceed in length the rest of the body.
=Fam. 1. Cumidae.=—No sharp demarcation between thorax and abdomen. Four
posterior pairs of legs in both sexes without exopodites. Male with five
well-developed pleopods in addition to the uropods. Telson wanting.
_Cuma_, _Cyclaspis_, etc.
=Fam. 2. Lampropidae.=—Body-form resembles that of Cumidae. All the
thoracic limbs except the last have exopodites. The male has three pairs
of pleopods. Telson present. _Lamprops_, _Platyaspis_, etc.
=Fam. 3. Leuconidae.=—Body-form similar to above. Male has only two
pairs of pleopods. Mouth-parts peculiar, much less setose than in other
families. Telson absent. _Leucon_, _Eudorella_.
=Fam. 4. Diastylidae.=—Anterior part of thorax sharply marked off from
posterior part. Male has two pairs of pleopods. Telson present.
_Diastylis_ (Fig. 80). _D. goodsiri_ from the Arctic ocean measures over
an inch in length.
=Fam. 5. Pseudocumidae.=—Rather similar to Diastylidae, but differ in
reduced size of telson and presence of exopodites on third and fourth
thoracic legs of female. This family is represented by three very
similar marine forms of the genus _Pseudocuma_; but, as Sars has
shown,[97] the Caspian Sea contains thirteen peculiar species, only one
of which can be referred to the genus _Pseudocuma_, while the rest may
be partitioned among four genera, _Pterocuma_, _Stenocuma_,
_Caspiocuma_, _Schizorhynchus_.
=Order III. Isopoda.=
The Isopoda and the Amphipoda are frequently classed together as
Arthrostraca or Edriophthalmata, owing to a number of features which
they share in common, as, for instance, the sessile eyes which
distinguish them from the podophthalmatous Schizopoda and Decapoda, the
absence of a carapace, and the thoracic limbs which are uniramous
throughout their whole existence. For the rest, in the presence of
brood-plates and the other diagnostic characters, they are plainly
allied to the other Peracarida, and an easy transition is effected from
the Mysidacea to the Isopoda through the Chelifera or Anisopoda. Only
one thoracic segment is usually fused with the head, the appendage of
this segment being the maxillipede; in the Chelifera among Isopoda, and
the Caprellidae among Amphipoda, two thoracic segments are fused with
the head.
The Isopoda are distinguished from the Amphipoda by the dorso-ventral
flattening of the body, as opposed to the lateral flattening in the
Amphipoda, by the posterior position of the heart, and by the branchial
organs being situated on the abdominal instead of on the thoracic limbs.
The Isopoda, following Sars’[98] classification, fall into six
sub-orders—the Chelifera, Flabellifera, Valvifera, Asellota, Oniscoida,
and Epicarida,—to which must be added the Phreatoicidea.
=Sub-Order 1. Chelifera.=
The Chelifera, including the families (1) =Apseudidae= and (2)
=Tanaidae=, are interesting in that they afford a transition between the
ordinary Isopods and the Mysidacea. The important features in which they
resemble the Mysidacea are, first, the fusion of the first two thoracic
segments with the head, with the coincident formation of a kind of
carapace in which the respiratory functions are discharged by a pair of
branchial lamellae attached to the maxillipedes; and, second, the
presence of very small exopodites on the first two thoracic appendages
of the Apseudidae.
The second pair of thoracic limbs, _i.e._ the pair behind the
maxillipedes, are developed both in the Apseudidae and Tanaidae into a
pair of powerful chelae, and these frequently show marked sexual
differences, being much more highly developed in the males than in the
females. The biramous and flattened pleopods are purely natatory in
function, and the uropods or pleopods of the sixth pair are terminal in
position and slender.
[Illustration:
FIG. 81.—_Apseudes spinosus_, ♂, × 15. _A_, 1st antenna; _Ab_, 6th
abdominal appendage; _T_, 2nd thoracic appendage. (After Sars.)
]
Both families, of which the Apseudidae contain the larger forms,
sometimes attaining to an inch in length, are littoral in habit, or
occur in sand and ooze at considerable depths, many of the genera being
blind. Many Tanaids (_e.g._ _Leptochelia_, _Tanais_, _Heterotanais_,
etc.) live in the algal growths of the littoral zone, and being highly
heliotropic they are easy to collect if a basinful of algae is placed in
a strong light. The females carry the eggs about with them in a
brood-pouch formed, as is usual in the Peracarida, by lamellae produced
from the bases of the thoracic limbs. The males on coming to maturity do
not appear to grow any more, or to take food, their mouth-parts
frequently degenerating and the alimentary canal being devoid of food.
They are thus in the position of insects which do not moult after coming
to maturity; and, as in Insects, the males are apt to show a kind of
high and low dimorphism—certain of the males being small with secondary
sexual characters little different from those of the females, while
others are large with these characters highly developed. Fritz Müller,
in his _Facts for Darwin_, observes that in a Brazilian species of
_Leptochelia_, apparently identical with the European _L. dubia_, the
males occur under two totally distinct forms—one in which the chelae are
greatly developed, and another in which the chelae resemble those of the
female, but the antennae in this form are provided with far longer and
more numerous sensory hairs than in the first form. Müller suggested
that these two varieties were produced by natural selection, the
characters of the one form compensating for the absence of the
characters of the other. A general consideration of the sexual
dimorphism in the Tanaidae[99] lends some support to this view, since
the smaller species with feeble chelae do appear to be compensated by a
greater development of sensory hairs on the antennae, but the specific
differences are so difficult to appreciate in the Tanaidae that it is
possible that the two forms of the male in Müller’s supposed single
species really belonged to two separate species.
=Sub-Order 2. Flabellifera.=
The Flabellifera include a number of rather heterogeneous families which
resemble one another, however, in the uropods being lateral and not
terminal, and being expanded together with the telson to form a caudal
fan for swimming. The pleopods are sometimes natatory and sometimes
branchial in function. Some of the families are parasitic or
semi-parasitic in habit.
=Fam. 1. Anthuridae.=—These are elongated cylindrical creatures found in
mud and among weeds upon the sea-bottom; their mouth-parts are evidently
intended for piercing and sucking, but whether they are parasitic at
certain periods on other animals is not exactly known. _Anthura_,
_Paranthura_, _Cruregens_.
[Illustration:
FIG. 82.—_Gnathia maxillaris_. =A=, Segmented larva, × 10; =B=,
Praniza larva, × 5; =C=, gravid female, × 5; =D=, male, × 5.
]
=Fam. 2. Gnathiidae.=[100]—These forms appear to be related to the
Anthuridae; they are ectoparasitic on various kinds of fish during
larval life, but on assuming the adult state they do not feed any more,
subsisting merely on the nourishment amassed during the larval periods.
The larvae themselves are continually leaving their hosts, and can be
taken in great numbers living freely among weeds on the sea-bottom. The
larvae, together with the adults of _Gnathia maxillaris_, are extremely
abundant among the roots of the sea-weed _Poseidonia cavolinii_ in the
Bay of Naples. The young larvae hatch out from the body of the female in
the state shown in Fig. 82, A. This minute larva fixes upon a fish, and
after a time it is transformed into the so-called Praniza larva (B), in
which the gut is so distended with the fluid sucked from the host that
the segmentation in the hind part of the thorax is entirely lost. When
this larva moults it may, however, reacquire temporarily its
segmentation. After a certain period of this parasitic mode of life the
Praniza finally abandons its host, and becomes transformed into the
adult male or female. This may take place at very different stages in
the growth of the larva, the range of variation in size of the adults
being 1–8 mm., and it must be remembered that when once the adult
condition is assumed growth entirely ceases. What it is that determines
the stage of growth in each individual when it shall be transformed into
the adult is not known. The males and females differ from one another so
extraordinarily that it was for long denied that they were both derived
from the Praniza larvae. This is nevertheless the case. The change from
the Praniza to the female (Fig. 82, C) is not very great. The ovary
absorbs all the nourishment in the gut and comes to occupy the whole of
the body, all the other organs degenerating, including the alimentary
canal and mouth-parts. Indeed, only the limbs with their muscles and the
nervous system remain. The change to the male (D) is more radical. The
food is here stored in the liver, which increases in the male just as
the ovary does in the female. The segmentation is reacquired, and the
massive square head is formed from the hinder part of the head in the
Praniza, the anterior portion with its stylet-like appendages being
thrown away. The powerful nippers of the male are not formed inside the
cases of the old styliform mandibles, but are independent and possibly
not homologous organs. The meaning of the marked sexual dimorphism and
the use of the males’ nippers are not in the least known, though the
animals are easy to keep under observation. In captivity the males never
take the slightest notice of either larval or adult females.
=Fam. 3. Cymothoidae.=[101]—This is a group of parasites more completely
parasitic than the foregoing, but their outer organisation does not
differ greatly from an ordinary Isopodan form. A great many very similar
species are known which infest the gill-chambers, mouths, and skin of
various fishes. The chief interest that attaches to them is found in the
fact that a number of them, and perhaps all, are hermaphrodite, each
individual acting as a male when free-swimming and young, and then
subsequently settling down and becoming female. This condition is
exactly the same as that occurring universally in the great group of
parasitic Isopoda, the Epicarida, to be considered later. There is no
evidence that the Cymothoidae are phyletically related to the Epicarida,
so that the similar sexual organisation appears to be due to convergence
resulting from similar conditions of life. The general question of
hermaphroditism in the Crustacea has been shortly discussed on pp.
105–106. _Cymothoa._
=Fam. 4. Cirolanidae.=—In this family is placed the largest Isopod
known—the deep-sea _Bathynomus giganteus_, found in the Gulf of Mexico
and the Indian Ocean, sometimes measuring a foot long by four inches
broad. A common small littoral form is _Cirolana_.
=Fam. 5. Serolidae.=[102]—The genus _Serolis_ comprises flattened forms
bearing a curious resemblance to Trilobites, which Milne Edwards
considered more than superficial. The genus is confined to the littoral
and deep waters of the southern hemisphere.
=Fam. 6. Sphaeromidae.=[103]—These are flattened, broad-bodied forms,
most commonly met with in the Mediterranean and warmer seas. Without
being actually parasitic, they are frequently found as scavengers in
decaying material, and they show some relationship to the parasitic
Cymothoidae. In some of the genera, _e.g._ _Cymodoce_, the ovigerous
female shows a degenerate condition of the mouth-parts, while the
maxillipedes undergo an enlargement, and are used for causing a current
through the brood-chamber.
=Sub-Order 3. Valvifera.=
[Illustration:
FIG. 83.—_Munnopsis typica_ (Munnopsidae), ♂, × 2. _A_, 2nd antenna;
_Ab_, abdomen; _T_, 5th thoracic appendage or 4th leg. (After Sars.)
]
The Valvifera, illustrated by the =Idotheidae= and =Arcturidae=, are
characterised by the uropods being turned back and expanded to form
folding doors covering up the delicate pleopods, which are mostly
respiratory in function, though the anterior pairs may serve as swimming
organs. _Arcturus_ is a typically deep sea genus, many species,
remarkably furnished with spiny processes, having been taken by the
_Challenger_ in the southern hemisphere. The Idotheidae are more
littoral forms, several species of _Idothea_ being commonly met with off
the British coasts, occasionally penetrating into brackish or even fresh
water.
=Sub-Order 4. Asellota.=
In this group the abdominal segments are fused dorsally to form a
shield-like caudal region; the pleopods are respiratory in function and
reduced in numbers, the first pair being often expanded and produced
backwards to form an operculum covering the rest. Several of the
Asellota are fresh-water, _Asellus aquaticus_ (=Asellidae=) being
extremely abundant all over Europe in weed-grown ditches, the mud of
slowly-moving streams, and even on the shores of large lakes. They are
mostly sluggish in habit, but the marine =Munnopsidae= (Fig. 83,
_Munnopsis_) are expert swimmers, the swimming organs being fashioned by
the expansion and elongation of the thoracic legs.
=Sub-Order 5. Oniscoida.=
The Oniscoida[104] are terrestrial forms in which the abdomen is fully
segmented, the pleopods are respiratory, their endopodites being
delicate branchiae, while their exopodites are plate-like and form
protective opercula for the gills, and the uropods are biramous and not
expanded. The epimera of the segments are greatly produced. The
terrestrial Isopods, although air-breathers,[105] are dependent on
moisture, and are only found in damp situations. It seems probable that
they have been derived from marine Isopods, since the more generalised
of them, _e.g._, _Ligia_ (Fig. 84), common on the English coasts, are
only found in damp caves and crannies in the rocks.
[Illustration:
FIG. 84.—_Ligia oceanica_, ventral and dorsal views, × 1. (From
original drawings prepared for Professor Weldon.)
]
The related _Ligidium_ is found far inland, but always in the
neighbourhood of water. These two genera may be distinguished by the
numerous joints in the flagellum of the second antennae, the flagellum
being in all cases the portion of the antenna succeeding the long fifth
joint. _Philoscia muscorum_ occurs usually near the coast, but it is
also found inland in England under trees in damp moss. This genus and
the common _Oniscus_, found in woods, are distinguished by the presence
of three joints in the flagellum of the second antenna. _Philoscia_ can
be distinguished from _Oniscus_ by its narrower body and the pretty
marbled appearance of its back. The genus _Trichoniscus_ has four joints
in the flagellum; various species are found in woods. In _Porcellio_ and
_Armadillidium_ there are only two joints in the flagellum, while
_Armadillidium_, the common garden wood-louse, can be distinguished from
all others by the flattened shape of the uropods, and the habit of
rolling up into a ball like an Armadillo.
There is also a very peculiar species, _Platyarthrus hoffmannseggii_,
which occurs in England and Northern Europe, and always lives in ants’
nests. It is supposed that they serve as scavengers for the ants, which
tend them carefully, and evidently treat them as domestic animals of
some kind. The small creature is quite white and blind, and has
exceedingly short antennae.
=Sub-Order 6. Epicarida.=
The Epicarida include an immense number of Isopods, parasitic upon other
Crustacea. In the adult state they become greatly deformed, and offer
very few characters of classificatory value, but they all pass through
certain highly characteristic larval stages which are essentially
similar in the different families. All the species are protandric
hermaphrodites, each individual being male while in a larval state, and
then losing its male organisation and becoming female as the parasitic
habit is assumed.
Two series of families are recognised according to the larval stages
passed through, the =Cryptoniscina=, in which the adult male
organisation is assumed in the Cryptoniscus stage, and the female
condition is imposed directly upon this form, and the =Bopyrina=, in
which the Cryptoniscus passes into a further larval stage, the Bopyrus,
which performs the function of the male, and upon which the female
organisation is imposed as the parasitic habit is assumed.
The following is a list of the Epicarida with the Crustacea which serve
as their hosts[106]:—
Cryptoniscina │Microniscidae │on Copepoda.
„ │Cryptoniscidae │on Ostracoda.
„ │Liriopsidae │on Rhizocephala.
„ │Hemioniscidae │on Cirripedia.
„ │Cabiropsidae │on Isopoda.
„ │Podasconidae │on Amphipoda.
„ │Asconiscidae │on Schizopoda.
Bopyrina │Dajidae │on Decapoda
„ │Phryxidae │ „
„ │Bopyridae │ „
„ │Entoniscidae │ „
[Illustration:
FIG. 85.—Epicaridian larva, probably belonging to one of the
Cryptoniscina. _A_, 2nd antenna; _Ab_, abdominal appendages; _T_,
thoracic appendages. (From Bonnier, after Hansen.)
]
In all cases the first larval form which hatches out from the maternal
brood-pouch is called the Epicaridian larva (Fig. 85).
This little larva has two pairs of antennae, a pair of curious frontal
processes, and a pair of mandibles. The other mouth-parts are missing;
there are only six thoracic limbs, but the full complement of six
biramous pleopods are present, and at the end of the body there may be a
long tube of unknown function.
As a type of the =Cryptoniscina= we may take the =Liriopsidae=,[107]
parasitic on the Rhizocephala, which are, of course, themselves
parasitic on the Decapoda, the whole association forming a very
remarkable study in Carcinology.
Almost every species of the Rhizocephala is subject to the attacks of
Liriopsids, the latter fixing either on the Rhizocephala themselves, or
else on the Decapod host at a point near the fixation of the
Rhizocephalous parasite. An exceedingly common Liriopsid is _Danalia
curvata_, parasitic on _Sacculina neglecta_, which is itself parasitic
on the spider-crab, _Inachus mauritanicus_, at Naples. The adult
_Danalia_ is a mere curved bag full of eggs or developing embryos, and
without any other recognisable organs except two pairs of spermathecae
upon the ventral surface where the spermatozoa derived from the larval
males are stored.
[Illustration:
FIG. 86.—_Inachus mauritanicus_, ♀, × 1, carrying two _Sacculina
neglecta_ (_a_, _b_), and a _Danalia curvata_ (_c_), the latter
bearing two dwarf males.
]
In Fig. 86 is represented a female of _Inachus mauritanicus_ which
carried upon it two Sacculinae and a _Danalia curvata_, and upon the
latter are seen two minute larval males in the act of fertilising the
adult _Danalia_. The eggs develop into the Epicaridian stage, after
which the larva passes into the Cryptoniscus stage (Fig. 87). In this
larval form the segments are clearly delimited; the only mouth-parts
present are the mandibles, but there are seven pairs of thoracic limbs
and the full number of pleopods. This Cryptoniscus stage is found in all
the Epicarida, and only differs in detail in the various families.
[Illustration:
FIG. 87.—Ventral view of Cryptoniscus larva of _Danalia curvata_, ♂, ×
25.
]
In the Cryptoniscina the Cryptoniscus larva is the male, and at this
stage possesses a pair of large testes in the thorax. The ovaries are
also present at this stage as very small bodies applied to the anterior
ends of the testes. The larval males in this state seek out adult fixed
Danaliae and fertilise them; and, when this is accomplished, they
themselves become fixed to the host and begin to develop into the adult
female condition. The limbs are all lost, and out of the mouth grows a
long proboscis (Fig. 88, _P_), which penetrates the tissues of the host.
The ovaries begin to grow, and a remarkable process of absorption in the
testes takes place. These organs, when fixation occurs, are never empty
of spermatozoa, and are frequently crammed with them. After fixation
some large cells at the interior borders of the testes begin to feed
upon the remains of these organs and to grow enormously in size and to
multiply by amitosis. These phagocytes, as they really are, attain an
enormous size, but they are doomed to degeneration, the chromatin
becoming dispersed through the cytoplasm, and the nuclei dividing first
by amitosis and then breaking up and disappearing. As the parasite
grows, the heart at the posterior end of the body ceases to beat; the
ovaries increase enormously at the expense of the alimentary canal, and
on the ventral surface two pairs of spermathecae are invaginated ready
to receive the spermatozoa of a larval male. In the adult condition,
after fertilisation has taken place and the ovaries occupy almost the
whole of the body, the remains of the phagocytic cells can be seen on
the dorsal surface in a degenerate state. They evidently are not used as
food, and their sole function is to make away with the male organisation
when it has become useless.[108]
[Illustration:
FIG. 88.—Side view of _Danalia curvata_, × 15, shortly after fixation
and loss of larval appendages. _A_, Alimentary canal; _E_, eye; _H_,
heart; _N_, phagocytic cells; _O_, ovary; _P_, proboscis.
]
[Illustration:
FIG. 89.—Optical section (dorsal view) of _Danalia curvata_, in the
same stage as Fig. 88. _A_, Alimentary canal; _Ec_, ectoderm; _H_,
heart; _N_, phagocytic cells; _O_, ovaries; _P_, proboscis.
]
In the series =Bopyrina=, after the free-living Epicaridian and
Cryptoniscus stages, a further larval state is assumed, called the
Bopyrus, which is the functional male, and, after performing this
function, passes on to the adult female condition.
The family =Bopyridae= is parasitic in the branchial chamber of
Decapoda, especially Macrura and Anomura. When one of these Decapods is
infested with an adult Bopyrid the gill-chamber in which it is situated
is greatly swollen, as shown in Fig. 90. A very common Bopyrid is
_Bopyrus fougerouxi_, parasitic in the gill-chambers of _Palaemon
serratus_. The _Bopyrus_ larva or functional male has the appearance
shown in Fig. 91. It differs from the Cryptoniscus stage in possessing a
rudimentary pair of anterior thoracic limbs and seven pairs normally
developed, while the abdominal limbs are plate-like and branchial in
function. The male can often be found attached to the female beneath the
last pair of incubatory lamellae.
[Illustration:
FIG. 90.—_Galathea intermedia_, with a _Pleurocrypta microbranchiata_
under its left branchiostegite (_B_), × 1. (After Sars.)
]
[Illustration:
FIG. 91.—Ventral view of male _Bopyrus fougerouxi_, × 30. _A_, 1st and
2nd antennae; _T_, 8th (last) thoracic appendage. (After Bonnier.)
]
The adult female condition, which is assumed after the Bopyrid stage is
passed through, is illustrated in Fig. 92. The body acquires a
remarkable asymmetry, due to the unequal pressure exerted by the walls
of the gill-chamber. The antennae and mandibles (Fig. 92, B) are
entirely covered up by the largely expanded maxillipedes; maxillae are,
as usual, entirely absent. Very large lamellae grow out from the bases
of the thoracic limbs to form a brood-pouch, and in this manner the
adult condition is attained.
The final complication in the life-histories of these Isopoda is reached
by the family =Entoniscidae=, which are parasitic when adult inside the
thoracic cavity of Brachyura and Paguridae. The cephalothorax of a
_Carcinus maenas_, which contains an adult _Portunion maenadis_ (_P_),
is shown in Fig. 93. The parasite is of a reddish colour when alive.
[Illustration:
FIG. 92.—_Bopyrus fougerouxi._ =A=, Ventral view of female carrying a
male (_M_) between her abdominal appendages, × 8; =B=, ventral view
of part of head of female, the maxillipedes and the left mandible
having been removed. _A.1_, _A.2_, 1st and 2nd antennae; _M_, male;
_Mn_, right mandible; _Mx_, left maxillipede; _O_, oostegite; _T_,
left 4th thoracic appendage or 3rd leg. (After Bonnier.)
]
[Illustration:
FIG. 93.—Cephalothorax of _Carcinus maenas_, seen from the ventral
side, containing a parasitic _Portunion maenadis_ (_P_), × ½. (After
Bonnier.)
]
The Entoniscidae pass through a free living Epicaridian and Cryptoniscus
stage, and become adult males in the Bopyrus stage. It is stated,
however, by Giard and Bonnier[109] that these individuals, which
actually function as males, never grow up into adult females, though all
the adult females have passed through a male stage in which the male
genital ducts are not formed. The hermaphroditism, therefore, in these
animals at any rate is absolutely useless from a reproductive point of
view, and this justifies our looking for some other explanation of it,
such as was suggested on p. 105.
[Illustration:
FIG. 94.—_Portunion maenadis_, ♀:—=A=, Young, × 10; =B=, older, × 5;
=C=, adult, before the eggs are laid, × 3. _A_, 2nd antenna; _Ab_,
abdomen; _B_, anterior lobe of brood-pouch; _B′_, its lateral lobe;
_H_, head; 1, 2, 1st and 2nd incubatory lamellae (oostegites).
(After Giard and Bonnier.)
]
The Bopyrus fixes in the gill-chamber of the host and becomes converted
into the adult female by a series of transformations. As these changes
take place it invaginates the wall of the gill-chamber and pushes its
way into the thoracic cavity of the crab, though it lies all the time
enveloped in the invaginated wall of the gill-chamber, and not free in
the body-cavity of the crab. The transformations which it undergoes are
shown in Fig. 94. The body first assumes a grub-like appearance (A), and
two pairs of incubatory lamellae (1, 2) grow out from the first and
second thoracic segments. In the next stage (B) these lamellae assume
gigantic proportions, and four pairs of branchiae grow out from the
abdominal segments (_Ab_). In the final stage (C) the incubatory
lamellae have further increased in size, and constitute the main bulk of
the body; the enormous mass of eggs is passed into the incubatory pouch,
and all that remains of the rest of the body is the small head (H) and
the abdomen (_Ab_), furnished with its branchiae. Communication with the
external world is kept up through an aperture which leads from the
brood-pouch into the gill-chamber of the host, and through this aperture
the young are hatched out when they are developed sufficiently.
The presence of these parasites, although they are never in actual
contact with the internal organs of the crab, calls forth the same
phenomenon of parasitic castration as was observed in the Rhizocephala.
A remarkable association is also found to exist between the Entoniscidae
and Rhizocephala, of such a kind that, on the whole, a crab infested
with a Rhizocephalan is more likely to harbour an Entoniscid than one
without. The explanation of this association is probably that a crab
with a _Sacculina_ inside it is prevented from moulting as often as an
uninfected crab, and, in consequence, the larval stages of the
Entoniscid in the crab’s gill-chamber are more safely passed through.
=Sub-Order 7. Phreatoicidea.=[110]
The members of this sub-order, although agreeing with the Isopoda in the
essentials of their anatomy, resemble the Amphipoda in being rather
laterally compressed, and in having the hand of the first free thoracic
limb enlarged and subchelate. The abdomen is greatly produced laterally
by expansions of the segments. In fact, the shape of the body and of the
limbs is very Amphipodan.—_Phreatoicus_ from New Zealand, Southern
Australia, and Tasmania. _Phreatoicopsis_,[111] a very large form from
Gippsland, Victoria. Only one family exists, =Phreatoicidae=.
=Order IV. Amphipoda.=
In this order the body is flattened laterally, the heart is anterior in
position, and the branchial organs are attached to the thoracic limbs.
There are three well defined sub-orders, (i.) the Crevettina, including
a vast assemblage of very similar animals, of which the common
_Gammarus_ and _Orchestia_ may serve as examples; (ii.) the Laemodipoda
or Caprellids, and (iii.) the Hyperina.
We cannot do more than touch on the organisation of these sub-orders.
=Sub-Order 1. Crevettina.=
In this sub-order only one thoracic segment is fused with the head; the
basal joints of the thoracic limbs are expanded to form broad lateral
plates, and the abdomen is well developed, with six pairs of pleopods,
the last three pairs being always turned backwards, and stiffened to act
as uropods.
This group has numerous fresh-water representatives, _e.g._ _Gammarus_
of several species, the blind well-shrimp _Niphargus_, and the S.
American _Hyalella_; but the vast majority of the species are marine,
and are found especially in the littoral zone wherever the rocks are
covered with a rich growth of algae, Polyzoa, etc. The Talitridae or
“Sand-hoppers” have deserted the waters and live entirely in the sand
and under rocks on the shore, and one common European species,
_Orchestia gammarellus_, penetrates far inland, and may be found in
gardens where the soil is moist many miles from the sea.
The Rev. T. R. R. Stebbing, in his standard work[112] on this group,
recognises forty-one families, and more than 1000 species, so that we
can only mention a few of the families, many of which, indeed, differ
from one another in small characters.
=Fam. Lysianassidae.=—The first joint of the first antenna is short,
with an accessory flagellum. Mandible with a palp, and with an almost
smooth cutting edge. The third joint of the second gnathopod is
elongated. This family is entirely marine, comprising forty-eight
genera, with species distributed in all seas. One genus,
_Pseudalibrotus_, inhabits the brackish water of the Caspian Sea.
_Lysianassa_ has several common British and Mediterranean species.
=Fam. Haustoriidae.=—The members of this family are specially adapted
for burrowing, the joints of the hinder thoracic limbs being expanded,
and furnished with spines for digging. Some of the species are common on
the British coasts, _e.g._ _Haustorius arenarius_. _Pontoporeia_ has an
interesting distribution, one species, _P. femorata_, being entirely
marine, in the Arctic and North Atlantic, _P. affinis_ inhabiting the
Atlantic, and also fresh-water lakes in Europe and North America, _P.
microphthalma_ being confined to the Caspian Sea, and _P. loyi_ to Lakes
Superior and Michigan.
=Fam. Gammaridae.=—Includes fifty-two genera. The first antennae are
slender, with the accessory flagellum very variable. The mandibles have
a dentate cutting edge, spine-row, and molar surface, and a
three-jointed palp. The first two thoracic limbs are subchelate. This
family includes a few marine, but mostly brackish and fresh-water
species. _Crangonyx_ is entirely subterranean in habitat, as is
_Niphargus_, _N. forelii_ occurring, however, in the deep waters of Lake
Geneva. Both these genera are blind. _Gammarus_ has thirty species, _G.
locusta_ being the common species on the North Atlantic coasts, and _G.
pulex_ the common fresh-water species of streams and lakes in Europe. A
number of Gammaridae inhabit the Caspian Sea, _e.g._ _Boeckia_,
_Gmelina_, _Niphargoides_, etc., while the enormous Gammarid fauna of
Lake Baikal, constituting numerous genera, showing a great variety of
structure, some of them being blind, belong to this family, _e.g._
_Macrohectopus_ (_Constantia_), _Acanthogammarus_, _Heterogammarus_,
etc.
[Illustration:
FIG. 95.—_Gammarus locusta_, ♂ (above) and ♀ (below), × 4. _Abd.1_,
First abdominal segment; _T_, telson; _Th_, seventh free thoracic
segment (= 8th thoracic segment); _U_, third uropod. (After Della
Valle.)
]
=Fam. Talitridae.=—This family may be distinguished by the absence of a
palp on the mandible, and by one ramus of the uropods being very small
or wanting. The various kinds of “Sand-hoppers” belong here, familiar
creatures on every sandy coast between tide-marks. The genera _Talitrus_
and _Talorchestia_ always frequent sand, while _Orchestia_ is generally
found under stones and among weed. Some species of _Orchestia_, e.g. _O.
gammarellus_, live inland in moist places at some distance from the sea;
one species of _Talitrus_ (_T. sylvaticus_) occurs at great elevations
in forests in Southern Australia.
_Hyale_ is a coastal genus, and is also found on floating objects in the
Sargasso Sea. _Hyalella_ is confined to Lake Titicaca and the fresh
waters of South America. _Chiltonia_ from S. Australasia.
=Fam. Corophiidae.=—The members of this family have a rather flattened
body and small abdomen, and the side-plates on the thorax are small. The
uropods are also small and weak. Some species of the genus _Corophium_
are characteristic of the Caspian Sea.
=Sub-Order 2. Laemodipoda.=
[Illustration:
FIG. 96.—_Caprella grandimana_, × 4. _a_, Abdomen; _g_, gills; _t_,
3rd (first free) thoracic segment; _t′_, 8th thoracic segment.
(After P. Mayer.)
]
=Fam. 1. Caprellidae=[113] are also chiefly littoral forms, swarming
among rocks covered by algae, though they are by no means so easy to
detect as the Gammaridae and Tanaidae which haunt similar situations. In
a basinful of algae or Polyzoa taken from the rocks fringing the Bay of
Naples, the latter are easily collected, the Tanaidae always crawling
out of the weeds in the direction of the light, while the Gammarids dart
about in all directions; but the Caprellidae, with their branching
stick-like forms, harmonise so well with their surroundings that it
requires an experienced eye to detect them. The body is elongated and
thin, resembling that of a stick-insect. The first two thoracic segments
are more or less completely fused with the head; the second and third
thoracic limbs end in claws; the two following thoracic limbs are normal
in the genus _Proto_, rudimentary in _Protella_, and absent in the
remaining genera, though their gills remain as conspicuous flabellate
structures. The three hind legs are normal, and the abdomen is reduced
to a tiny wart at the hind end of the greatly elongated thorax.
P. Mayer has described cases of external hermaphroditism as being fairly
common in certain species, _e.g._ _Caprella acutifrons_, and this is
interesting if we take into consideration the frequent partial
hermaphroditism exhibited by the gonad of _Orchestia_ at certain times
of year (see p. 104).
=Fam. 2. Cyamidae.=—These are closely related to the Caprellidae in the
form of the limbs and the reduced state of the abdomen. _Cyamus ceti_,
which lives ectoparasitically on the skin of whales, has the body
expanded laterally instead of being elongated, as in the Caprellids.
=Sub-Order 3. Hyperina.=
[Illustration:
FIG. 97.—_Phronima sedentaria_, ♀, in a _Pyrosoma_ colony, × 1. (After
Claus, from Gerstaecker and Ortmann.)
]
These are an equally distinct and curious group of Amphipods,
characterised by the large size of the head and the transparency of the
body. Instead of haunting the littoral zone they are pelagic in habit,
and many of them live inside transparent pelagic Molluscs, Tunicates, or
Jellyfish. A well known form is _Phronima sedentaria_, which inhabits
the glassy barrel-like cases of the Tunicate _Pyrosoma_ in the
Mediterranean. The female is often taken in the plankton together with
her brood in one of these curious glass houses; the zooids of the
_Pyrosoma_ colony are completely eaten away and the external surface of
the case, instead of being rough with the tentacles of the zooids, is
worn to a smooth, glass-like surface. It has been observed that the
female actively navigates her house upon the surface of the sea; she
clings on with her thoracic legs inside, while the abdomen is pushed out
through an opening of the _Pyrosoma_ case behind, and by its alternate
flexion and extension drives the boat forwards, the water being thus
made to enter at the front aperture and supply the female and her brood
with nourishment.
DIVISION 3. HOPLOCARIDA.
The carapace leaves at least four of the thoracic somites distinct. The
eyes are pedunculate. The mandibles are without a lacinia mobilis; there
are no oostegites, the eggs being carried in a chamber formed by the
maxillipedes. The hepatic caeca are much ramified, the heart is greatly
elongated, stretching through thorax and abdomen, with a pair of ostia
in each segment. The spermatozoa are spherical, and there is a
complicated and peculiar metamorphosis.
=Order. Stomatopoda.=
[Illustration:
FIG. 98.—Lateral view of _Squilla_ sp., × 1. _A.1_, _A.2_, 1st and 2nd
antennae; _Ab.1_, 1st abdominal segment; _Ab.6_, 6th abdominal
appendage; _C_, cephalothorax, consisting of the head fused with the
first five thoracic segments; _E_, eye; _M_, 2nd maxillipede; _T_,
telson. (After Gerstaecker and Ortmann.)
]
The Stomatopoda are rather large animals, occasionally reaching a foot
in length, all of which exhibit a very similar structure; _Squilla
mantis_ and _S. desmaresti_ are found on the south coast of England not
very frequently; but they are very common in the Mediterranean, living
in holes or in the sand within the littoral zone of shallow water. They
differ from all the other Malacostraca by a combination of characters,
and Calman proposes the term HOPLOCARIDA for a division equivalent to
the Peracarida, Eucarida, etc.
The abdomen is very broad and well developed, ending in a widely
expanded telson. There is a carapace which covers the four anterior
thoracic segments, leaving the four posterior segments free. The portion
of the head carrying the stalked eyes constitutes an apparently separate
segment articulated to the head. The antennae, mandibles, and maxillae
are normal; there then follow five pairs of uniramous thoracic limbs
turned forwards as maxillipedes and ending in claws; the second pair of
these is modified into a huge raptorial arm, exactly resembling that of
a Praying _Mantis_ (cf. vol. v. p. 242), by means of which the _Squilla_
seizes its prey. The last three thoracic limbs are small and biramous.
The pleopods are powerful, flattened, biramous swimming organs with
small hooks or “retinaculae” upon their endopodites, which link together
each member of a pair in the middle, and with large branching gills upon
the exopodites.
The internal anatomy exhibits several primitive features. The nervous
system is not at all concentrated, there being a separate ganglion for
each segment; and the heart stretches right through thorax and abdomen,
with a pair of ostia in each segment. There are also ten hepatic
diverticula given off segmentally from the intestine.
The female has the curious habit of carrying the developing eggs in a
chamber improvised by the apposition of the maxillipedes, so that it
looks rather as if she were in the act of devouring her own brood.
The metamorphosis of the larvae, despite the work of Claus[114] and
Brooks,[115] is not very accurately known, especially uncertain being
the identification of the different larvae with their adult forms. The
chief interest consists in the fact that certain of the anterior
thoracic limbs develop in their normal order and degenerate, to be
reformed later, just as in the Phyllosoma larva of the Loricata (see pp.
165, 166).
In one series of larvae, probably not of _Squilla_ itself, but of
related genera, the young hatch out as “Erichthoidina” (Fig. 99), with
the thoracic appendages developed as biramous organs as far as the fifth
pair, and with a single abdominal pair of limbs.
[Illustration:
FIG. 99.—Erichthoidina larva of a Stomatopod, with five pairs of
maxillipedes, and the first pair of abdominal appendages, × 10.
(From Balfour, after Claus.)
]
The abdominal series of limbs is next completed; the second thoracic
limb assumes its adult raptorial structure, but the succeeding three
limbs become greatly reduced and may entirely degenerate, leaving the
posterior six thoracic segments without limbs.
Usually the anterior three pairs are only reduced, and then redevelop
side by side with the small posterior limbs as they appear. This larva
is then termed the “Erichthus” (Fig. 100); but when they completely
disappear the larva is called a “Pseudozoaea,” owing to its resemblance
to the Zoaea stage of the Decapoda, which is also characterised by the
suppressed development of the thoracic segments.
[Illustration:
FIG. 100.—Older Erichthus larva, with six pairs of abdominal
appendages, × 15. (From Balfour, after Claus.)
]
The so-called “Alima” larva of _Squilla_ is also a Pseudozoaea, but it
is apparently arrived at directly without the previous formation and
degeneration of the anterior thoracic limbs, the larva hatching out from
the egg in the Pseudozoaeal stage.
=Fam. Squillidae.=—Of the six known genera none extend into the cold
subarctic seas; the majority are characteristic of the warm or tropical
seas (_Gonodactylus_), some of the species having very wide ranges,
_e.g._ _G. chiragra_, which is completely circumtropical, and appears to
have entered the Mediterranean at some period, though it is very rare
there.