CRUSTACEA (_CONTINUED_): CIRRIPEDIA—PHENOMENA OF GROWTH AND SEX—
OSTRACODA
=Order III. Cirripedia.=
The Cirripedes are medium-sized Crustacea, with the body consisting of
few segments, and enveloped in a mantle formed as a fold of the external
integument, which may be strongly protected by calcified plates. The
abdomen is greatly reduced. The larva, after hatching out as a Nauplius,
and passing through a Cypris stage, when it resembles an Ostracod, fixes
itself to a foreign object by means of the first antennae, and becomes a
pupa, which after profound changes gives rise to the adult.
All the Cirripedes, when adult, live either a fixed or parasitic
existence, and as so frequently happens with animals of this kind, they
have departed widely from the ordinary structure of the class to which
they belong. Their anomalous appearance and the mystery surrounding
their propagation gave rise, probably, to the old legend that the
Barnacles (Lepadidae), which live attached to pieces of floating timber,
hatched out into Barnacle geese[63]; and even so late as 1678, in the
Royal Society’s _Transactions_, Sir Robert Moray describes what he takes
to be little birds enclosed in Barnacle shells, washed ashore on the
coast of Scotland: “The little Bill like that of a Goose, the Eyes
marked, the Head, Neck, Breast, Wings, Tail, and Feet formed, the
Feathers everywhere perfectly shaped and blackish coloured, and the Feet
like those of other Water-fowl, to my best remembrance.” Cuvier in his
classification of the animal kingdom included them in the Mollusca; and
it was not until 1830 that J. V. Thompson described their larval stages,
and showed conclusively that they belonged to the Crustacea. Since the
work of this naturalist a number of observers have securely founded our
knowledge of the group, but we may especially mention the epoch-making
works of Darwin,[64] Hoek,[65] and latterly of Gruvel.[66]
The young Cirripede is hatched out from the maternal mantle-cavity as a
free-swimming Nauplius, a larval form common to most of the Entomostraca
and to some Malacostraca; the Cirripede Nauplius (Fig. 49) is
characterised by the presence of well-developed frontal horns, and
usually by the long spiny processes which spring from various parts of
the body. As an introduction to the study of the group, it will be well
to follow the transformations of this larva in _Lepas_ up to the period
when it begins its sessile existence. The liberated Nauplii swim freely
near the surface of the sea, and remaining in this condition for several
days are dispersed widely from their birthplace; they are then
transformed by the process of moulting into the second larval stage,
known as the Cypris (Fig. 50), from its resemblance to a bivalve
Ostracod. The Cypris larva continues to swim about by means of the six
pairs of biramous thoracic legs until it finds a suitable place on which
to fix; in the case of _Lepas_ fixation usually takes place on loose
floating logs; the Cypris fixes itself by means of the first antennae,
at the bases of which a large cement-gland secretes an adhesive
substance. The biramous swimming legs are cast off, and six pairs of
biramous cirri characteristic of the adult take their place; at this
stage the body has the appearance shown in Fig. 51. The region of the
head at the base of the antennae now becomes greatly swollen and
elongated to form the peduncle or stalk of the adult; the larval bivalve
carapace is cast off and on the external surface of the mantle the
calcifications begin which will give rise to the exoskeletal plates of
the adult. This region is known as the “capitulum” of the adult, as
opposed to the “peduncle.” The young Cirripede is now known as a pupa,
and from this stage the adult form is reached by a gradual transition.
[Illustration:
FIG. 49.—Nauplius larva of _Lepas fascicularis_, × 12. _A_{1}_,
_A_{2}_, 1st and 2nd antennae; _B_, brain; _E_, eye; _H_,
fronto-lateral horn; _M_, mandible; _S_, stomach. (After Groom.)
]
[Illustration:
FIG. 50.—Cypris stage in the development of _Lepas australis_, × 15.
_A_, Peduncle; _A.M_, adductor muscle; _C_, caecum of oesophagus;
_C.g_, cement-glands; _Cr_, cirri (thoracic appendages); _E_,
compound eye; _E^1_, simple eye; _G_, ventral ganglia; _I_,
intestine; _M_, mouth; _M.C_, mantle-cavity; _O_, ovary; _S_,
stomach. (After Hoek.)
]
[Illustration:
FIG. 51.—Pupa of _Lepas pectinata_, × 8. _A_, Antenna; _C_, carina;
_M_, adductor muscle; _S_, scutum; _T_, tergum. (After Gruvel.)
]
The body of the adult _Lepas_ is retracted into the mantle, and lies
free in the mantle-cavity, but is continuous anteriorly with the tissues
of the peduncle, into which the mantle does not extend. The thorax, with
its six pairs of legs, can be protruded from the mantle-cavity through
the slit-like opening which separates the two valves of the mantle along
the ventral middle line; and when the animal is feeding, the thoracic
legs are so protruded, and by their concerted waving action they drive
the food-particles in the water along the channel between them, until
the particles reach the oral cone, where they are masticated by the
mandibles and two pairs of maxillae, and so passed into the alimentary
canal. When the animal is disturbed it rapidly retracts its limbs, the
valves of the mantle are closed by means of a strong adductor muscle in
the head, and the animal is protected from all external influences. In
the acorn-barnacles (Operculata), which live in great numbers attached
to rocks and other objects between tide-marks, the body is constructed
on a similar plan, save that there is no stalk, and the body is
completely enclosed in a hard calcareous box formed from the mantle,
which, when the valves are closed, as they always are during low tide,
completely protect the animal inside from desiccation or danger of any
kind. Besides the cement-glands situated in the peduncle, we can
distinguish the generative organs, consisting of a pair of ovaries and
testes, the majority of Cirripedes being hermaphrodite. The testes open
at the end of an elongated median penis behind the thoracic limbs, while
the ovaries, situated in the peduncle, have paired openings into the
mantle-cavity on either side of the head. A pair of maxillary glands or
kidneys are present, and the alimentary canal is provided with various
digestive glands. Special branchial organs are not present in the
Pedunculate Cirripedes, but in the Operculate genera two branchiae are
formed from the plications of the internal surface of the mantle. There
is no contractile heart, and the circulatory system is poorly developed.
The Cirripedes are badly furnished with sensory organs; the remains of a
simple Nauplius eye may persist, situated on the upper part of the
stomach, but the chief sense-organs are the sensory hairs upon the
limbs.
[Illustration:
FIG. 52.—=A=, Dwarf male of _Scalpellum vulgare_, × 27; =B=, diagram
of Stalked Barnacle. _a_, Peduncle; _al_, alimentary canal; _b_,
brain; _c_, carina; _e_, remains of Nauplius eye; _gl_,
cement-gland; _m_, mantle-cavity; _o_, its opening; _ov_, ovary;
_p_, penis; _s_, scutum; _t_, testis; _tm_, tergum, seen in =A= as
the shaded body above the reference-line of _e_ and to the right of
the carina, on the left of the figure.
]
The recent Cirripedes fall into six clearly defined Sub-orders.
=Sub-Order 1. Pedunculata.=
In this division, sometimes combined with the Operculata as THORACICA,
owing to the extremely reduced state of the abdomen, the body is borne
on a distinct stalk, and the bivalve arrangement of the mantle is
clearly retained. The mantle is protected externally by a number of
calcareous plates, the arrangement of which is typical of the various
genera. It appears that in the most primitive and geologically oldest
Cirripedes, the probable ancestors of the Pedunculate and Operculate
sub-orders, the arrangement of the plates was somewhat irregular, and
they were far more numerous than in the modern forms, so that passing
from these older types to modern times we witness a reduction in the
number and a greater precision in the arrangement of the skeletal parts.
[Illustration:
FIG. 53.—=A=, _Turrilepas wrightianus_ (Silurian), × 1; =B=,
_Archaeolepas redtenbacheri_ (Jurassic), × 1. _C_, carina; _R_,
rostrum; _S_, scutum; _T_, tergum. (After Zittel.)
]
One of the most ancient Cirripedes known is _Turrilepas_, which occurs
in the Silurian deposits of England, but it is also known from earlier
deposits, while undoubted Cirripedes have been found in the Cambrian of
North America. The body of _Turrilepas_ is enclosed in imbricating
plates, as shown in Fig. 53, A.
In _Archaeolepas_ of the Upper Jurassic (Lithographic slates of Bavaria)
the arrangement of scutes typical of the Lepadidae is foreshadowed, but
the whole of the peduncle is protected by rows of plates (Fig. 53, B),
as in _Turrilepas_.
The above-mentioned genera did not survive into the Cretaceous period,
their places being taken by the genera _Pollicipes_ and _Scalpellum_,
which first appeared in the Silurian and persist to the present time,
the older and more primitive _Pollicipes_ being represented by about
half a dozen living species, while the species of _Scalpellum_ are
exceedingly numerous.
=Fam. 1. Polyaspidae.=—This family includes the three genera,
_Pollicipes_, _Scalpellum_, and _Lithotrya_.
[Illustration:
FIG. 54.—_Pollicipes mitella_, × 1. (After Darwin.)
]
_Pollicipes_ is not only very ancient geologically (being found from the
Ordovician upward), but it preserves the primitive characteristic of
numerous skeletal plates, the peduncle being frequently covered with
small calcareous pieces, which graduate into the larger more regularly
placed scutes on the capitulum (Fig. 54). The species of this genus,
many of which are among the largest Cirripedes, are widely distributed
in the temperate and tropical seas, living for the most part attached to
rocks and often in deep water. _P. cornucopia_ occurs off the English
and Scottish coasts.
The members of the genus _Scalpellum_, which is represented by
exceedingly numerous species in the Cretaceous period, also possess a
large number of plates on the capitulum, and often on the peduncle as
well, but never so many as in _Pollicipes_. Although the arrangement of
the plates varies much in the different species, we may describe a
fairly typical case, that of the common _Scalpellum vulgare_ (Fig. 55,
B).
The valves of the capitulum are held together by the median dorsal piece
called the “carina”; the other unpaired skeletal piece is the “rostrum,”
in front, just below the place where the valves gape to allow the
protrusion of the limbs. The paired pieces receive the names “scutum,”
“tergum,” and “laterals,” and the peduncle is covered with rows of small
plates.
The genus _Scalpellum_ is a very large one, and is widely distributed,
though at the time at which Darwin wrote only six species were known.
The reason for this is to be found in the fact that the great majority
of the species live at great depths, so that they remained unknown until
the expeditions of the _Challenger_ and other deep-sea expeditions
brought them to light. They may affix themselves, generally in
considerable numbers together, on branching organisms, such as Corals,
Polyzoa, and Hydroids, but often also on empty shells, rocks, and other
foreign bodies. The body is colourless or of a pale flesh colour, but a
colony of these animals, expanded and drooping in various attitudes from
a piece of coral, gives the appearance of some graceful exotic flower.
[Illustration:
FIG. 55.—=A=, Complemental male of _Scalpellum peronii_, × 20; =B=,
hermaphrodite individual of _S. vulgare_, × 2. _a_, Complemental
males, _in situ_; _b_, rostrum. (=A=, after Gruvel; =B=, after
Darwin.)
]
Perhaps the most interesting feature of the genus is the remarkable
variation in the sexual constitution of some of the species. The great
majority of the Pedunculata and all the Operculata are hermaphrodites,
which habitually cross-fertilise one another, and this they are well
fitted to do, since they all live gregariously and are provided with a
long exsertile penis for transferring the spermatozoa from one to the
other. In _Pollicipes_, however, the individuals of which often live
solitarily, it appears that self-fertilisation may occur. In
_Scalpellum_ three different kinds of sexual constitution may occur: (1)
According to Hoek in _S. balanoides_, taken by the _Challenger_, the
individuals are ordinary cross-fertilising hermaphrodites. (2) In the
great majority of species, including the common _S. vulgare_, as
originally described by Darwin, and since confirmed by Hoek and
Gruvel,[67] the individuals are hermaphrodite, but there are present
affixed to the adult hermaphrodites, just inside the opening of the
valves in a pocket of the mantle, a varying number of exceedingly minute
males, called by Darwin “complemental males.” These tiny organisms are
really little more than bags of spermatozoa, but they possess to varying
degrees the ordinary organs of the adult in a reduced condition. The
male of _S. peronii_ (Fig. 55, A) retains the shape and skeletal plates
of the ordinary form, and differs chiefly in its reduced size; but the
more common condition is exhibited by the male of _S. vulgare_ (Fig. 52,
A), where the scutes are reduced to vestiges round the mantle-opening,
and almost the whole of the body is occupied by the greatly developed
generative organs. (3) In a few species, e.g. _S. velutinum_ and _S.
ornatum_, the individuals are purely dioecious, being either females of
the ordinary structure resembling the hermaphrodites of the other
Lepadidae, or dwarfed males resembling closely the complemental males
described above for _S. vulgare_.
[Illustration:
FIG. 56.—_Lithotrya dorsalis_, x 1. _B_, Basal calcareous cup; _C_,
carina; _R_, rostrum; _S_, scutum; _T_, tergum. (After Darwin.)
]
The nature and derivation of these various conditions will be discussed
when the parallel cases found in _Ibla_ and among the Rhizocephala have
been described.
The remaining genus of the Polyaspidae, also characterised by the
presence of numerous skeletal plates on the capitulum, is _Lithotrya_
(Fig. 56), which bores into rocks and shells, and is an inhabitant of
the warm and tropical seas.
The peduncle of the full-grown animal is completely imbedded in the rock
or shell to which it is attached, and at the basal end of the peduncle
is situated a cup composed of large irregular calcified pieces. This cup
is, however, not formed until the animal has ceased to burrow. The
excavation of the substratum is effected by means of a number of small
rasping plates which cover the peduncle, the whole being set in motion
by the peduncular muscles.
[Illustration:
FIG. 57.—_Conchoderma virgata_, × 1. _C_, Carina; _S_, scutum; _T_,
tergum. (After Darwin.)
]
=Fam. 2. Pentaspidae.=—In this family are placed a number of genera, and
among them the common _Lepas_, the species of which possess typically
five skeletal plates, viz., a carina and a pair of scuta and of terga,
the peduncle being naked. These forms are a later development of
Cirripede evolution, and did not come into existence till Tertiary
times. Some of them, e.g. _Oxynaspis_, live at considerable depths
attached to corals, etc., but large numbers float on the surface of the
sea, fixed often on logs and wreckage of various kinds. _Dichelaspis_ is
found attached to the shells of large Crustacea.
_Conchoderma_ is an interesting genus, the species of which live affixed
to various floating objects, the keels of ships, etc.; the mantle is
often brilliantly coloured, as in _C. virgata_, and the skeletal plates
are reduced to the merest vestiges, leaving the greater part of the body
fleshy.
[Illustration:
FIG. 58.—_Ibla cumingii_, ♀, × 1. _S_, Scutum; _T_, tergum. (After
Darwin.)
]
[Illustration:
FIG. 59.—_Ibla cumingii_, dwarf male, × 32. _A_, Antennae; _B_, part
of male imbedded in the female, to which the torn membrane _M_
belongs; _E_, eye; _Th_, thoracic appendages or cirri. (After
Darwin.)
]
=Fam. 3. Tetraspidae.=—This family includes the single genus _Ibla_
(Fig. 58), which possesses only four skeletal plates, a pair of terga
and of scuta, coloured blue, while the peduncle is covered with brown
spines. There are only two very similar species known, _I. cumingii_,
which is found attached to the peduncle of _Pollicipes mitella_, and _I.
quadrivalvis_, living on masses of the Siphonophore _Galeolaria
decumbens_. These two species are quite different in the partition of
the sexes. In _I. cumingii_ the large individuals of normal structure
are females, inside the mantle-cavities of which are attached dwarf
males of the form shown in Fig. 59.
These organisms have the peduncle buried completely in the substance of
the female’s mantle, inside which they live; they exhibit a degenerate
structure, but still retain two pairs of cirri. The large individuals of
_I. quadrivalvis_, on the other hand, are hermaphrodites, but they
harbour within their mantles minute complemental males similar to those
of _I. cumingii_, though they are rather larger.
[Illustration:
FIG. 60.—Diagram of the shell of an Operculate Cirripede. _a_ “Ala,”
or overlapped portion of a “compartment”; _B_, basis; _C_, carina;
_C.L_, carino-lateral; _L_, lateral; _R_, rostrum; _r_, “radius,” or
overlapping portion of a compartment; _R.L_, rostro-lateral. (After
Darwin.)
]
=Fam. 4. Anaspidae.=—This includes the remaining pedunculate genera,
characterised by the fleshy nature of the mantle and peduncle, which are
both entirely devoid of calcifications. The species of _Alepas_ live
upon Echinoderms and various other animals; _Chaetolepas_ upon
_Sertularia_, and _Gymnolepas_ upon Medusae. _Anelasma squalicola_ is an
interesting form, living parasitically upon the Elasmobranch fishes,
_Selache maxima_ and _Spinax niger_ in the North Sea. The peduncle is
deeply buried in the flesh of the host, so that only a portion of the
dark blue capitulum protrudes to the surface. From the whole surface of
the peduncle a system of branching processes is given off, which ramify
far into the tissues of the fish, and communicate inside the peduncle
with the lacunar tissue, which is packed round all the organs of the
Cirripede. There can be small doubt that the _Anelasma_ derives its
nutriment parasitically through this root-system, since the cirri are
mere fleshy lobes unadapted to securing food, and the alimentary canal
is always empty. This animal has a suggestive bearing on the
Rhizocephala, which, as will be shown, derive their nutriment from a
system of roots penetrating the host and growing out from what
corresponds morphologically to the peduncle.
=Sub-Order 2. Operculata.=
[Illustration:
FIG. 61.—_Balanus tintinnabulum_, with the right half of the shell and
of the operculum removed, seen from the right side. _A_, Antennae,
the size of which is exaggerated; _A.M_, adductor muscle; _B_,
basis; _C_, carina; _Cr_, cirri or thoracic appendages; _D_,
oviduct; _G_, ovary; _L_, lateral compartment; _Lb_, labrum or upper
lip; _M_, _M_, depressor muscles of scutum and tergum; _M.C_,
mantle-cavity; _O_, orifice of excretory organ; _O.M_, opercular
membrane; _R_, rostrum; _S_, scutum; _St_, region of stomach; _T_,
tergum. (After Darwin.)
]
The “acorn-barnacles” appear later in geological time than the earlier
stalked forms. _Verruca_ and _Chthamalus_ are found in the Chalk, and
survive down to the present day, but _Balanus_ does not occur until
middle Tertiary times. Representatives of the last-named genus are
familiar to every one, as the hard sharp objects which cover rocks and
piles near high-water mark on every sea-coast. If we examine the hard
skeleton of one of these animals, we find that, unlike the Pedunculata,
they possess no stalk, the capitulum being fused on to the surface of
attachment by a broad basal disc. Typically, there may be considered to
be eight skeletal pieces forming the outer ring which invests the soft
parts of the animal, an unpaired rostrum and carina, and laterally a
pair of rostro-lateral, lateral, and carino-lateral “compartments,” as
shown in Figs. 60, 63.
The skeletal ring is roofed over by a pair of terga at the carinal end
and a pair of scuta at the rostral end; these four plates make up the
operculum by which the animal can shut itself completely up in its
shell, or between the valves of which it can protrude its limbs for
obtaining food.
[Illustration:
FIG. 62.—Diagrammatic section of the growing shell of _Balanus
porcatus_. _C_, Canals; _Ct_, cuticle; _H_, hypodermis (=
epidermis); _H′_, part of shell secreted by the hypodermis; _Hl_,
hypodermal lamina; _M_, part of shell secreted by the mantle. (After
Gruvel.)
]
The relation of the animal to its shell is shown in Fig. 61. The shell
in the Operculata is not merely secreted as a dead structure on the
external surface of the epidermis, but represents a living calciferous
tissue interpenetrated by living laminae (Fig. 62, Hl) derived partly
from the external hypodermis and partly from the lining of the mantle.
The hard parts of the shell usually also contain spaces and canals (C).
[Illustration:
FIG. 63.—Diagrams of shells of Operculata. =A=, _Catophragmus_
(Octomeridae); =B=, _Balanus_, _Coronula_, etc. (Hexameridae); =C=,
_Tetraclita_ (Tetrameridae). _C_, carina; _C.L_, carino-lateral;
_L_, lateral; _R_, rostrum; _R.L_, rostro-lateral.
]
The various forms of Acorn-barnacle may be classified according to the
number of pieces that go to make up the skeleton; thus starting with the
typical number eight (Fig. 63, A), we find that in various degrees a
fusion between neighbouring pieces has taken place in the different
families.
=Fam. 1. Verrucidae.=—The ancient genus _Verruca_, which is still widely
distributed in all seas, and is found fixed upon foreign objects on the
sea-bottom at various depths, is interesting on account of the asymmetry
of its shell, which bears a different aspect according to which side one
regards it from. This asymmetry is brought about by the skeletal pieces
(carina, rostrum, and paired terga and scuta) shifting their positions
after fixation has taken place.
=Fam. 2. Octomeridae.=—In this family the eight plates composing the
shell are separate and unfused (Fig. 63, A). The majority of the species
come from the Southern hemisphere, _e.g._ the members of the genera
_Catophragmus_ and _Octomeris_, but _Pachylasma giganteum_ occurs in
deep water in the Mediterranean, where it has been found fixed upon
Millepore corals.
=Fam. 3. Hexameridae.=—This family includes by far the greater number of
the Acorn-barnacles, in which only six plates are present, the laterals
having fused with the carino-laterals (Fig. 63, B). The very large genus
_Balanus_ belongs here, the common _B. tintinnabulum_ of our coasts
being found all over the world, and occurring under a number of
inconstant varietal forms. Especial interest attaches to certain other
genera, from their habit of living parasitically on soft-bodied animals,
whose flesh they penetrate.
_Coronula diadema_ and _Tubicinella trachealis_ live embedded in the
skin of whales, the shell of the first-named being of a highly
complicated structure with hollow triangular compartments into which the
mantle is drawn out.
_Xenobalanus globicipitis_ lives attached to various Cetacea, and is
remarkable for the rudimentary condition of its skeleton, the six plates
of which form a mere disc of attachment from which the greatly elongated
naked body rises, resembling one of the naked Stalked Barnacles.
=Fam. 4 Tetrameridae.=—In this family only four skeletal plates are
present (Fig. 63, C). This family is chiefly confined to tropical seas
or those of the Southern hemisphere. The chief genera are _Tetraclita_
and _Pyrgoma_, found in British seas.
=Sub-Order 3. Acrothoracica.=
Gruvel includes in this sub-order four genera (_Alcippe_,
_Cryptophialus_, _Kochlorine_, and _Lithoglyptes_) the species of which
live in cavities excavated in the shells of molluscs or in the hard
parts of corals.
[Illustration:
FIG. 64.—_Alcippe lampas._ =A=, ♀, × about 10, seen from the right
side, with part of the right half of the animal removed; =B=, dwarf
male, × about 30. _A.M_, adductor muscle; _An_, antenna; _C_, 1st
pair of cirri; _Cr_, posterior thoracic appendages; _E_, eye; _G_,
testis; _M.C_, mantle-cavity; _O_, ovary; _P_, penis; _T_,
penultimate thoracic segment; _V._ vesicula seminalis. (After
Darwin.)
]
Darwin discovered and described _Cryptophialus minutus_, and placed it
in a sub-order Abdominalia, believing that it was distinguished from all
the foregoing Cirripedes by the presence of a well-developed abdomen.
Since the discovery of other allied genera, it has been decided that the
abdomen is equally reduced in these forms, and that the terminal
appendages do not belong to this region, but to the thorax.
The sexes are separate. The body of the female (Fig. 64, A) is enclosed
in a chitinous mantle, armed with teeth by which the excavation is
effected, and is attached to the cavity in the host by means of a horny
disc. Upon this disc the dwarf males (B) are found.
_Alcippe lampas_ inhabits holes on the inner surface of dead _Fusus_ and
_Buccinum_ shells; _Cryptophialus minutus_ the shells of _Concholepas
peruviana_; _C. striatus_ [68] the plates of _Chiton_; _Kochlorine
hamata_ the shells of _Haliotis_; and _Lithoglyptes varians_ shells and
corals from the Indian Ocean.
=Sub-Order 4. Ascothoracica.=
These are small hermaphrodite animals completely enveloped in a soft
mantle, which live attached to and partly buried in various organisms,
such as the branching Black Corals (_Gerardia_). They retain the
thoracic appendages in a modified state, and the body is segmented into
a number of somites, the last of which probably represents an abdomen.
_Laura gerardiae_, described by Lacaze Duthiers,[69] is parasitic on the
stem of the “Black Coral,” _Gerardia_ (vol. i. p. 406); it has the shape
of a broad bean, the body being entirely enclosed in a soft mantle, with
the orifice in the position corresponding to the hilum of a bean. The
body lying in the mantle is composed of eleven segments, and is curved
into an =S=-shape. Its internal anatomy is entirely on the plan of an
ordinary Cirripede.
_Petrarca bathyactidis_, G. H. Fowler,[70] was found in the mesenteric
chambers of the coral _Bathyactis_, dredged by the _Challenger_ from
4000 metres. The body is nearly spherical, and the mantle-opening forms
a long slit on the ventral surface. The mantle is soft, but is furnished
on the ventral surface with short spines.
The antennae, which form the organs of fixation, remain very much in the
state characteristic of the Cypris larvae of other Cirripedes, being
furnished with two terminal hooks by which attachment is effected. The
thoracic appendages, of which there are the normal number six, are
reduced flabellate structures, and the abdomen forms an indefinitely
segmented lobe of considerable size.
The animal appears to be in an arrested state of development, and so
retains some of the characteristics of the Cypris larvae, but it is very
doubtful how far these characters can be considered primitive.
Other forms are _Dendrogaster astericola_ on Echinoderms, and _Synagoga
mira_ on the “Black Coral,” _Parantipathes larix_, at Naples.
=Sub-Order 5. Apoda.=
[Illustration:
FIG. 65.—_Proteolepas bivincta_, × 26. _A_, Antennae; _a_, _b_, 1st
and 2nd abdominal segments; _O_, ovary; _P_, penis; _T_, telson;
1–8, thoracic segments. (After Darwin.)
]
Darwin described a small hermaphrodite parasite in the mantle chamber of
_Alepas cornuta_ from Saint Vincent, West Indies, which he named
_Proteolepas bivincta_.
The body (Fig. 65) is distinctly segmented into eleven somites, the last
three of which are supposed to belong to the abdomen; there are no
appendages except the antennae by which fixation is effected. The
mouth-parts are of normal constitution.
This animal has not been found again since Darwin’s discovery, but
Hansen[71] describes a number of peculiar Nauplius larvae taken in the
plankton of various regions, which he argues probably belong to members
of this group. A wide field of work is offered in attempting to find the
adults into which various larvae grow.
=Sub-Order 6. Rhizocephala.=[72]
These remarkable animals are Cirripedes which have taken to living
parasitically on various kinds of Crustacea; the majority infest species
of Decapoda, e.g. _Peltogaster_ on Hermit-Crabs, _Sacculina_ on a number
of Brachyura, _Sylon_ on Shrimps, _Lernaeodiscus_ on _Galathea_; but one
genus, _Duplorbis_, has been found in the marsupium of the Isopod
_Calathura brachiata_ from Greenland. Most of the species are solitary,
but a few, e.g. _Peltogaster sulcatus_, are social. In the adult state
the body consists of two portions: a soft bag-like structure, external
to the host, carrying the reproductive, nervous, and muscular organs,
and attached to some part of the host’s abdomen by means of a chitinous
ring; and a system of branching roots inside the host’s body, which
spring from the ring of attachment and supply the external body with
nourishment.
[Illustration:
FIG. 66.—Nearly median longitudinal section (diagrammatic) of
_Peltogaster_. _gn_, Brain; _m_, mantle; _mc_, mantle-cavity; _mes_,
mesentery; _op_, mantle-opening; _ov_, ovary; _ovd_, oviduct;
_ring_, ring of attachment; _t_, testis; _vd_, _vas_ deferens.
]
[Illustration:
FIG. 67.—Diagrammatic median longitudinal section through a normal
Cirripede, _gn_, Brain; _op_, mantle-opening; _ovd_, oviduct; _vd_,
vas deferens.
]
The structure of the external bag-like portion is very simple, and
varies only in details, chiefly of symmetry, in the different genera. In
_Peltogaster_, which preserves the simplest symmetrical arrangement of
the organs, a diagrammatic section through the long axis of the body
(Fig. 66) shows that it consists of a muscular mantle (_m_) surrounding
a visceral mass, and enclosing a mantle-cavity (_mc_) or brood-pouch,
which stretches everywhere between mantle and visceral mass, except
along the surface by which the parasite is attached to its host, where a
mesentery (_mes_) is formed. The ring of attachment is situated in the
middle of this mesentery; the mantle-cavity, which is completely lined
externally and internally with chitin, opens anteriorly by means of a
circular aperture (_op_) guarded by a sphincter muscle. The visceral
mass is composed chiefly of the two ovaries (_ov_), which open on either
side of the mesentery by means of a pair of oviducts (_ovd_); the paired
testes (_t_) are small tubes lying posteriorly in the mesentery, and the
nervous ganglion (_gn_) lies in the mesentery between oviducts and
mantle-opening. A comparison with the condition of a normal Cirripede
(Fig. 67) shows us that the mesenterial surface of the parasite by which
it is fixed corresponds to the dorsal surface of an ordinary Pedunculate
Cirripede, and that the ring of attachment corresponds with the stalk or
peduncle of a _Lepas_. The root-system passes out through the ring of
attachment into the body of the host, and ramifies round the organs of
the crab; the roots are covered externally with a thin chitinous
investment, and consist of an epithelium and an internal mass of
branching cells continuous with the lacunar tissue in the visceral mass.
[Illustration:
FIG. 68.—Development of _Sacculina neglecta_. =A=, Nauplius stage, ×
about 70; =B=, Cypris stage, × about 70. _A_{1}_, _A_{2}_, 1st and
2nd antennae of Nauplius; _Ab_, abdomen; _Ant_, antenna of Cypris;
_E_, undifferentiated cells; _F_, frontal horn; _G_, glands of
Cypris; _H_, tendon of Cypris; _M_, mandible; _T_, tentacles.
]
The developmental history of the Rhizocephala is one of the most
remarkable that embryology has hitherto revealed. It has been most
accurately followed in the case of _Sacculina_. The young are hatched
out in great numbers from the maternal mantle-cavity as small Nauplii
(Fig. 68, A) of a typical Cirripede nature, but without any alimentary
canal. They swim near the surface of the sea, and become transformed
into Cypris larvae of a typical character (Fig. 68, B). The Cypris
larva, after a certain period of free existence, seeks out a crab and
fixes itself by means of the hooks on its antennae to a hair on any part
of the crab’s body. Various races of _Sacculina_ are known which infest
about fifty different species of crabs in various seas; the best known
are _S. carcini_ parasitic on _Carcinus maenas_ at Plymouth and Roscoff,
and _S. neglecta_ on _Inachus mauritanicus_ at Naples. The antenna, by
which the Cypris is fixed, penetrates the base of the hair; the
appendages are thrown away, and a small mass of undifferentiated cells
is passed down the antenna into the body-cavity of the crab. Arrived in
the body-cavity it appears that this small mass of cells is carried
about in the blood-stream until it reaches the spaces round the
intestine in the thorax. Here it becomes applied to the intestine,
usually at its upper part, immediately beneath the stomach of the crab
(Fig. 69), and from this point it proceeds to throw out roots in all
directions, and as it grows to extend its main bulk, called the central
tumour (_c.t_), towards the lower part of the intestine. As the
posterior border of the central tumour grows down towards the hind-gut,
the future organs of the adult _Sacculina_ become differentiated in its
substance; the mantle-cavity being excavated and surrounding the
rudiment of the visceral mass, while as the central tumour grows
downwards it leaves behind it an ever extending system of roots. When
the central tumour in process of differentiation has reached the
unpaired diverticulum of the crab’s intestine, at the junction between
thorax and abdomen, all the adult organs are laid down in miniature, and
the whole structure is surrounded by an additional sac formed by
invagination known as the perivisceral space (Fig. 70). The young
“_Sacculina_ interna” remains in this position for some time, and being
applied to the ventral abdominal tissues of the crab just at the point
where thorax and abdomen join, or a little below it, it causes the
crab’s epithelium to degenerate, so that when the crab moults, a little
hole is left in this region of the same size as the body of the
_Sacculina_, owing to the failure of the epithelium to form chitin here;
and thus the little parasite is pushed through this hole and comes to
the exterior as the adolescent “_Sacculina_ externa.” From this point
onwards the crab, being inhibited in its growth through the action of
the parasite, never moults again; so that the _Sacculina_ occupies a
safe position protruding from the crab’s abdomen, which laps over it and
protects it. The remarkable features of this development are, firstly,
the difficulty of understanding how the developing embryo is directed in
its complicated wanderings so as always to reach the same spot where it
is destined to come to the exterior; and, secondly, the loss after the
Cypris stage of all the organs and the resumption of an embryonic
undifferentiated state from which the adult is newly evolved. A certain
parallel to this history is found in that of the Monstrillidae,
described on pp. 64–66.
[Illustration:
FIG. 69.—The mid-gut of _Inachus mauritanicus_ with a young
_Sacculina_ overlying it, × 2. _c.t_, “Central tumour” of the
parasite; _d.i_, _d.s_, inferior and superior diverticula of
alimentary canal of host; _n_, “nucleus,” or body-rudiment of
_Sacculina_; _r_, its roots; _x_, definitive position of the
parasite.
]
[Illustration:
FIG. 70.—Later stage in the development of the “_Sacculina_ interna,”
× 2. _b_, Body of _Sacculina_; _c.t_, “central tumour”; _d.i_,
_d.s_, inferior and superior diverticula of alimentary canal of
host; _o_, opening of perivisceral cavity of _Sacculina_; _r_, its
roots.
]
[Illustration:
FIG. 71.—Fourteen Cypris larvae fixed round the mantle-opening (_o_)
of a young _Sacculina_ externa, × 20.
]
The Rhizocephala are hermaphrodite with the possible exception of
_Sylon_, which appears to be female and perhaps parthenogenetic, no male
having been seen; but unlike most other hermaphrodite Cirripedes, they
reproduce by a continual round of self-fertilisation. This is the more
remarkable in that the vestiges of what appears to be a male sex are
still found in _Sacculina_ and _Peltogaster_; certain of the Cypris
larvae in these genera, instead of fixing on and inoculating other
crabs, become attached round the mantle-openings of young parasites of
the same species as themselves, which have recently attained to the
exterior of their hosts (Fig. 71). These larvae, which remind us of the
complemental males in _Scalpellum_, etc., never produce spermatozoa, but
rapidly degenerate where they are fixed, and appear never to play any
rôle in the reproduction of their species. The nature of this remarkable
phenomenon, together with the sexual condition of the Cirripedes in
general, will be discussed in the next section.
Much remains to be elucidated in the life-histories of these curious
animals, and it seems probable that intermediate stages may exist,
showing us how the extreme discontinuity of development has been
reached. Suggestive in this respect is the newly discovered parasite of
the Isopod, _Calathura_, which the author has named _Duplorbis
calathurae_.[73] This animal does not appear to possess a root-system,
but is attached to its host by a tube which passes right through the
mesentery and opens into the mantle-cavity of the parasite. It may be
suggested that this tube corresponds to the stalk of the normal
Cirripede, but its exact mode of formation would certainly throw much
light on the question of Rhizocephalan development.
=Phenomena of Growth and Sex in the Crustacea.=
In the foregoing account of the Cirripedia we have met with certain
peculiar sexual relations in which closely allied species exhibit marked
differences in regard to the distribution of the qualities of sex among
their individuals; we have seen that the majority of species are
hermaphrodite, unlike most Crustacea which, with the other exception of
the parasitic Isopoda, are normally dioecious; and that in some species
complemental males exist side by side with the hermaphrodites, while, in
yet others, the individuals are either females or dwarf males.
Before examining the causes of these conditions, it will be opportune to
consider a number of facts which throw light on the question of sex and
hermaphroditism in general. We may then return to the discussion of the
hermaphroditism found in particular in the Cirripedia and Isopoda.
=Parasitic Castration.=—Giard[74] was the first to observe that a number
of parasites exert a remarkable influence on the sexual characters of
their host, such that the generative glands become reduced, or may
completely degenerate, while the secondary sexual characters become
materially altered. This was proved to occur in the most widely
different hosts, affected by the most widely different parasites (_e.g._
Crustacea, Insecta, Worms). Moreover, it was apparent that the affection
does not consist in the parasite merely destroying the generative
organs, with which it often does not come into contact, but rather in
the general disturbance of the metabolism set up by its presence.
The most completely studied cases of parasitic castration are those of
the Rhizocephalous _Sacculina neglecta_, parasitic on the spider-crab,
_Inachus mauritanicus_,[75] and of _Peltogaster curvatus_ on the
Hermit-crab, _Eupagurus excavatus_, var. _meticulosa_.[76] The ordinary
males of _I. mauritanicus_ have the appearance shown in Fig. 72, A. The
abdomen is small and bears a pair of copulatory styles, while the
chelipedes are long and swollen. In the female (B) the abdomen is much
larger and trough-shaped, and carries four pairs of ovigerous
appendages; the chelae are small and narrow.
[Illustration:
FIG. 72.—Illustrating the effect of parasitic _Sacculina neglecta_ on
_Inachus mauritanicus_, nat. size. =A=, Normal male; _Inachus_; =B=,
normal female; =C=, male infested by _Sacculina_ (final stage); =D=,
abdomen of infested female; =E=, infested male in an early stage of
its modification.
]
Now it is found that in about 70 per cent of males infected with
_Sacculina_ the body takes on to varying degrees the female characters,
the abdomen becoming broad as in the female, with a tendency to develop
the ovigerous appendages, while the chelae become reduced (Fig. 72, C).
This assumption of the female characteristics by the male under the
influence of the parasite may be so perfect that the abdomen and chelae
become typically female in dimensions, while the abdomen develops not
only the copulatory styles typical of the male, but also the four pairs
of ovigerous appendages typical of the female. The parasitised females,
on the other hand, though they may show a degenerate condition of the
ovigerous appendages (Fig. 72, D), never develop a single positively
male characteristic. On dissecting crabs of these various categories it
is found that the generative organs are in varying conditions of
degeneration and disintegration.
The most remarkable fact in this history is the subsequent behaviour of
males which have assumed perfect female external characters, if the
_Sacculina_ drops off and the crabs recover from the disease. It is
found that under these circumstances these males may regenerate from the
remains of their gonads a perfect hermaphrodite gland, capable of
producing mature ova and spermatozoa. The females appear quite
incapable, on the other hand, of producing the male primary elements of
sex on recovery, any more than they can produce the secondary. Exactly
analogous facts have been observed in the case of the hermit-crabs
parasitised by _Peltogaster_, but here the affected males produce small
ova in their testes before the parasite is got rid of. Here, too, the
females seem incapable of assuming male characters under the influence
of the parasite.
To summarise shortly the conclusions to be deduced from these facts—
certain animals react to the presence of parasites by altering their
sexual condition. This alteration consists in the female sex in an
arrest of reproductive activity, in the male sex in the arrest of
reproductive activity coupled with the assumption of all the external
characters proper to the female. But in these males it is not merely the
external characters that have been altered; their capacity for
subsequently developing hermaphrodite glands shows that their whole
organisation has been converted towards the female state. That this
alteration consists in a reorganisation of the metabolic activities of
the body is clearly suggested, and in the succeeding paragraph we
furnish some further evidence in support of this view.
[Illustration:
FIG. 73.—_Inachus mauritanicus_, × 1. =A=, Low male; =B=, middle male;
=C=, high male; the great chela of the right side is the only
appendage represented.
]
=Partial and Temporary Hermaphroditism. High and Low Dimorphism.=
The reproductive phases of animals are frequently rhythmic, periods of
growth alternating with periods of reproduction. This is well
exemplified in the case of the ordinary males of _Inachus mauritanicus_,
of some other Oxyrhynchous crabs, and of the Crayfish _Cambarus_.[77]
During the breeding season the males of _I. mauritanicus_ fall into
three chief categories: Small males with swollen chelae (Fig. 73, A),
middle-sized males with flattened chelae (B), and large males with
enormously swollen chelae (C). On dissecting specimens of the first and
third categories it is found that the testes occupy a large part of the
thoracic cavity and are full of spermatozoa, while in the middle-sized
males with female-like chelae the testes appear shrivelled and contain
few spermatozoa. These non-breeding crabs are, in fact, undergoing a
period of active growth and sexual suppression before attaining the
final state of development exhibited by the large breeding males. This
phenomenon is obviously parallel to the “high and low dimorphism”[78] so
common in Lamellicorn beetles, where the males of many species are
divided into two chief categories, viz. “low males” of small size in
which the secondary sexual characters are poorly developed, and “high
males” of large size in which these characters are proportionately much
more highly developed than in the low males. The only difference between
the two cases is that whereas in the beetles growth ceases on the
attainment of maturity in the low degree, in the Crustacea the low male
passes through a period of growth and sexual suppression to reach the
high degree of development.
The condition of the middle-sized males may be looked upon as one of
partial hermaphroditism, indications of the female state being found in
the flattened chelae and in the reduced state of the testes. This
interpretation is greatly strengthened by the state of affairs observed
in the life-history of the male Sand-hoppers, Amphipods of the genus
_Orchestia_.[79] In the young males of several species of this genus, at
the time of year when they are not actively breeding, small ova are
developed in the upper part of the testes of more than half of the male
individuals, these ova being broken down and reabsorbed as the breeding
season reaches its height. Nor is this phenomenon confined to this
genus; in the males of a number of widely different Crustacea these
small ova are found in the testes at certain periods of the life-history
(_e.g._ _Astacus_[80]), when the animal is not breeding.
The foregoing facts indicate unmistakably that the males of a number of
Crustacea under certain metabolic conditions, _i.e._ when a stage of
active growth as opposed to a stage of reproductive activity is
initiated, alter their sexual constitution in such a way that the latent
female characteristics are developed, and the organism appears as a
partial hermaphrodite. In the preceding paragraph we saw that the males
of a number of animals, especially Crustacea, react to the metabolic
disturbance set up by the presence of a parasite in exactly the same
way, _i.e._ by developing into partial or total hermaphrodites. From
these two converging bodies of facts we may conclude, firstly, that sex
and metabolism are two closely connected phenomena; and, secondly, that
the male sex is especially liable to assume hermaphrodite characters
whenever its metabolic requirements are conservative, assimilatory, or
in a preponderating degree anabolic, as when a phase of active growth is
initiated, or the drain on the system, due to the presence of a
parasite, is to be made good.
=Normal Hermaphroditism in Cirripedia and Isopoda Epicarida.=
The above-mentioned groups contain the only normally hermaphrodite
Crustacea, and since they are in most respects highly specialised, we
may be certain that they have been secondarily derived from dioecious
ancestors. They both lead a sessile or parasitic life, and it is
noteworthy that this habit is often associated with hermaphroditism,
_e.g._ in Tunicates. A sessile or parasitic mode of life is one in which
the metabolic functions are vegetative and assimilatory rather than
actively kinetic or metabolic. It is in this state that we have seen the
males of a number of Crustacea taking on a temporary or partial
hermaphroditism. We may, therefore, inquire, whether in these cases of
normal hermaphroditism there is any evidence to show that here too the
hermaphroditism has been acquired by the male sex as a response to the
change in the metabolic conditions. In the parasitic Isopoda Epicarida
(see pp. 129–136) the hermaphroditism is of a very simple kind; all the
individuals are at first males, whose function it is to fix on and
fertilise the adult parasites. These subsequently develop into females
which are in their turn cross-fertilised by the young larvae derived
from a previous generation. All the individuals being alike, it seems
probable that they have been derived from one sex, and the general
nature of hermaphroditism deduced above may lead us to suppose that that
sex was originally male, the female having been suppressed. In certain
Cirripedia, _e.g._ most species of _Scalpellum_, there exist, besides
the hermaphrodite individuals, complemental males, so that here a
superficial conclusion might be drawn that the hermaphrodites represent
the female sex. But if we can suggest that the complemental males are in
reality similar in derivation to the hermaphrodite individuals, we shall
be in a position to claim that the hermaphrodite Cirripedes are similar
to the Isopoda Epicarida, and have probably also been derived from the
male sex. There is decided evidence pointing to this conclusion. In the
first place, the complemental males of at least one species of
_Scalpellum_, _S. peronii_, do show an incipient hermaphroditism[81] in
the presence of small ova in their generative glands, which, however,
never come to maturity.
The condition of the degenerate males in the Rhizocephala may also be
interpreted in the same manner. These never pass beyond the Cypris stage
of development, in which they resemble in detail the Cypris larvae of
the ordinary hermaphrodite individuals, and they are quite useless in
the propagation of their species.
It is more reasonable to suppose that these Cypris larvae, which fix on
the mantle-openings of adult parasites, are in reality identical with
the ordinary Cypris which infest crabs and develop into the
hermaphrodites, than that they represent a whole male sex doomed
beforehand to uselessness and degeneration. If we suppose that the
Cirripedes have passed through a state of protandric hermaphroditism
similar to that of the Isopoda Epicarida, it is plain that all the
larvae must have originally possessed the instinct of first fixing on
the adult parasites, and we may suppose that this instinct has been
retained in the Rhizocephala, but is now only actually fulfilled by a
certain proportion of the larvae, which, under existing circumstances,
are useless and fail to develop further; while the rest of the larvae,
not finding an adult parasite to fix upon, go straight on to infect
their hosts and develop into the adult hermaphrodites.
The same explanation would apply to the complemental males in
_Scalpellum_, etc., these individuals being also potential
hermaphrodites, which are arrested in development, though not so
completely as in the Rhizocephala, owing to the position they have taken
up.
This theory throws light on another dark feature of Cirripede
life-history, namely, the gregarious instinct. The associations of
Cirripedes are not formed by a number of Cypris larvae fixing together
on the same spot, but rather by the Cypris larvae seeking out adolescent
individuals of their own species and fixing on or near them. Now, if we
suppose that the Cirripedes have passed through a condition of
protandric hermaphroditism similar to that of the Isopoda Epicarida, it
is clear that a slight modification of the sexual instinct of the larvae
would lead to the gregarious habit, while its retention in some
individuals in its original form accounts for their finding their way to
the mantles of adult individuals and developing into the so-called
complemental males.
Certain Cirripedes, viz. certain species of _Scalpellum_ and _Ibla_ and
all the Acrothoracica, are dioecious. It is impossible to decide at
present whether these species retain the primitive dioecious condition
of the ancestral Cirripedes, or whether they too have been derived from
an hermaphrodite state, but in the present state of knowledge they
hardly affect the validity of the theory that has been proposed to
account for the nature of the complemental males and the hermaphrodite
individuals.
=Order IV. Ostracoda.=
The Ostracoda are small Crustacea, the body consisting of very few—about
eight—segments, and being completely enclosed in a carapace, which has
the form of a bivalve shell. Development is direct, without a Nauplius
stage.
The Ostracoda[82] are marine and fresh-water animals that can be divided
into several families, differing slightly in habits and in structures
correlated with those habits.
[Illustration:
FIG. 74.—_Candona reptans._ =A=, Natural size; =B=, X 15. _a_, 1st
antennae; _b_, 2nd antennae; _c_, walking legs. (After Baird.)
]
The =Cypridae= and =Cytheridae= include all the fresh-water and a vast
majority of marine genera, adapted for a sluggish life among
water-plants, though some can swim with considerable activity. The
common _Cypris_ and _Candona_ of our ponds and streams are familiar
instances. The movements of these animals are effected by means of the
two pairs of uniramous pediform antennae which move together and in a
vertical straight line. In the Cypridae (Fig. 74) there are, besides the
mandibles, two pairs of maxillae, a pair of walking legs, and, lastly, a
pair of appendages, which are doubled up into the carapace, and are used
for cleaning purposes. In the marine Cytheridae there is only one
maxilla, the last three appendages being pediform and used in walking.
The telson in the Cytheridae is rudimentary, but is well developed in
the Cypridae. The heart is altogether absent.
In many of the fresh-water forms, _e.g._ common species of _Candona_ and
_Cypris_, males are never found, and parthenogenetic reproduction by the
females appears to proceed uninterruptedly. Weismann[83] kept females of
_Cypris reptans_ breeding parthenogenetically for eight years. He also
remarks on the fact that these, and indeed all parthenogenetic female
Ostracoda, retain the receptaculum seminis, used normally for storing
the spermatozoa derived from the male, unimpaired.
Some of the Cytheridae occur in deep water. Thus _Cythere dictyon_ was
frequently taken by the _Challenger_ in depths of over 1000 fathoms, but
the majority prefer shallow water.
[Illustration:
FIG. 75.—_Asterope oblonga_, ♀, removed from its carapace, × 25. _A_,
Alimentary canal; _A_{1}_, _A_{2}_, 1st and 2nd antennae; _E_, eye;
_G_, gills; _G.O_, generative opening; _H_, heart; _M_, mandible;
_T_, 6th appendage; _T′_, last appendage (cleaning foot). (After
Claus.)
]
The =Halocypridae= and =Cypridinidae= comprise marine genera of a
pelagic habit. The first antennae are chiefly sensory, but the second
antennae are biramous, and they do not merely move up and down, as in
the preceding families, but sideways like oars, the valves of the shells
being excavated to admit of free movements. There are two pairs of
maxillae; the succeeding limbs differ in the two families. In the
Cypridinidae, _e.g._ _Asterope_ (Fig. 75), the first leg (T) is
lamelliform and is used as an accessory maxilla, while the second leg
(T’) is turned upwards into the shell as a cleaning organ. In the
Halocypridae the first leg is pediform, and differs in the two sexes,
while the second leg is rudimentary and points backwards. In _Asterope_
peculiar branchial organs (_G_) are present on the back. Both families
possess a heart; the Halocypridae are blind, while the Cypridinidae
possess eyes.
The =Polycopidae= and =Cytherellidae= are curious marine families of a
pelagic habit, with biramous second antennae well adapted for swimming,
and very broad. The first maxilla in the Polycopidae is also employed in
swimming, while the second is modified into a branchial organ; the
maxillae of the Cytherellidae are more normal in structure, but both
carry branchial lamellae. The posterior limbs are altogether absent in
Polycopidae, and in the Cytherellidae are only represented by the
copulatory organs of the male.