CRUSTACEA (_CONTINUED_): COPEPODA
=Order II. Copepoda.=
The Copepods are small Crustacea, composed typically of about sixteen
segments, in which the biramous type of limb predominates. They are
devoid of a carapace. Development proceeds gradually by the addition
posteriorly of segments to a Nauplius larval form. Paired compound eyes
are absent, except in Branchiura, the adult retaining the simple eye of
the Nauplius.
In a typical Copepod, such as _Calanus hyperboreus_ (Fig. 25), we can
distinguish the following segments with their appendages: a
cephalothorax, carrying a pair of uniramous first antennae (_1^{st}
Ant._); a pair of biramous second antennae (_2^{nd} Ant._); mandibles
(_Md._) with biting gnathobases and a palp, and a pair of foliaceous
first maxillae (_Mx.^1_). Two pairs of appendages follow, which were
looked upon as the two branches of the second maxillae, but it is now
certain that they represent two pairs of appendages, which may be called
second maxillae (_Mx.^2_) and maxillipedes (_Mxp._) respectively. Behind
these are five pairs of biramous swimming feet, the first pair (_Th.^1_)
attached to the cephalothorax, the succeeding four pairs to four
distinct thoracic somites. Behind the thorax is a clearly delimited
abdomen composed of five segments, the first of which (_Abd.^1_) carries
the genital opening, and the last a caudal furca.
The Copepods exhibit a great variety of structure, and their
classification is attended with great difficulties. Claus[36] based his
attempt at a natural classification on the character of the mouth and
its appendages, dividing the free-living and semi-parasitic forms as
Gnathostomata from the true parasites or Siphonostomata. This division,
although convenient, breaks down in many places, and it is clear that
the parasitic mode of life has been acquired more than once in the
history of Copepod evolution, while the free-living groups do not
constitute a natural assemblage.
[Illustration:
FIG. 25.—_Calanus hyperboreus_, × 30. _Abd^1_, First abdominal
segment; _1st Ant_, _2nd Ant_, 1st and 2nd antennae; _Md_, mandible;
_Mx^1_, _Mx^2_, 1st and 2nd maxillae; _Mxp_, maxillipede; _Th^1_,
1st thoracic appendage. (After Giesbrecht.)
]
Giesbrecht has more recently[37] founded a classification of the
free-living pelagic Copepods upon the segmentation of the body and
certain secondary sexual characters, and he has hinted[38] that this
scheme of classification applies to the semi-parasitic and parasitic
forms. Although much detail remains to be worked out and the position of
some families is doubtful, Giesbrecht’s scheme is the most satisfactory
that has hitherto been suggested, and will be adopted in this chapter.
The peculiarity in structure of the Argulidae, a small group of
ectoparasites on fresh water fish, necessitates their separation from
the rest of the Copepods (Eucopepoda) as a separate Branch, Branchiura.
BRANCH I. EUCOPEPODA.
=Sub-Order 1. Gymnoplea.=
The division between the front and hind part of the body falls
immediately in front of the genital openings and behind the fifth
thoracic feet. The latter in the male are modified into an asymmetrical
copulatory organ.
TRIBE I. AMPHASCANDRIA.
The first antennae of the male are symmetrical, with highly-developed
sensory hairs.
=Fam. Calanidae.=—The Calanidae are exclusively marine Crustacea, and
form a common feature of the pelagic plankton in all parts of the world.
Some species of the genus _Calanus_ often occur in vast shoals, making
the sea appear blood-red, and they furnish a most important article of
fish food. These swarms appear to consist chiefly of females, the males
being taken rarely, and only at certain seasons of the year. Some of the
Calanidae are animals of delicate and curious form, owing to the
development of plumed iridescent hairs from various parts of their body,
which may often exhibit a marked asymmetry, as in the species figured,
_Calocalanus plumulosus_ (Fig. 26), from the Mediterranean.
[Illustration:
FIG. 26.—_Calocalanus plumulosus_, × 15. (After Giesbrecht.)
]
Sars makes a curious observation[39] with regard to the distribution of
certain Calanidae. He reports that along the whole route of the “Fram,”
species such as _Calanus hyperboreus_ and _Euchaeta norwegica_ were
taken at the surface, which, in the Norwegian fjords, only occur at
depths of over 100 fathoms. He suggests that the Norwegian individuals,
instead of migrating northwards as the warmer climate supervened, have
sought boreal conditions of temperature by sinking into the deeper
waters.
TRIBE II.
HETERARTHRANDRIA.
The first antennae of the male are asymmetrical, one, usually the right,
being used as a clasping organ.
The males of the Centropagidae, Candacidae and Pontellidae, besides
possessing the asymmetrically modified thoracic limbs of the fifth pair
also exhibit a modification of one of the first antennae, which is
generally thickened in the middle, and has a peculiar joint in it, or
geniculation, which enables it to be flexed and so used as a clasping
organ for holding the female.
=Fam. 1.—Centropagidae.=—These Copepods are very common in the pelagic
plankton, and some of the species vie with the Calanidae in plumed
ornaments, e.g. _Augaptilus filigerus_, figured by Giesbrecht in his
monograph. The use of these ornaments, which are possessed by so many
pelagic Copepods, is entirely obscure.[40] Certain of the Centropagidae
live in fresh water. Thus _Diaptomus_ is an exclusively fresh-water
genus, and forms a most important constituent of lake-plankton; various
species of _Heterocope_ occur in the great continental lakes, and
certain _Eurytemora_ go up the estuaries of rivers into brackish water.
An excellent work on the fresh-water Copepods of Germany has been
written by Schmeil,[41] who gives analytical tables for distinguishing
various genera and species. The three fresh-water families are the
Centropagidae, Cyclopidae, and Harpacticidae (see p. 62). The
Centropagidae may be sharply distinguished from the other fresh-water
families by the following characters:—The cephalothorax is distinctly
separated from the abdomen; the first antennae are long and composed of
24–25 segments, in the male only a single antenna (generally the right)
being geniculated and used as a clasping organ. The fifth pair of limbs
are not rudimentary; a heart is present, and only one egg-sac is found
in the female. The second antennae are distinctly biramous.
_Diaptomus._—The furcal processes are short, at most three times as
long as broad; endopodite of the first swimming appendage 2–jointed,
endopodites of succeeding legs 3–jointed.
_Heterocope._—The furcal processes are short, at most twice as long as
broad; endopodites of all swimming legs 1–jointed.
_Eurytemora._—The furcal processes are long, at least three and a half
times as long as broad; the endopodite of the first pair of legs
1–jointed, those of the other pairs 2–jointed.
[Illustration:
FIG. 27.—Dorsal view of _Anomalocera pattersoni_, ♂, × 20. (After
Sars.)
]
It has been known for a long time that some of the marine Copepods are
phosphorescent, and, indeed, owing to their numbers in the plankton,
contribute very largely to bring about that liquid illumination which
will always excite the admiration of seafarers. In northern seas the
chief phosphorescent Copepods belong to _Metridia_, a genus of the
Centropagidae; but in the Bay of Naples Giesbrecht[42] states that the
phosphorescent species are the following Centropagids: _Pleuromma
abdominale_ and _P. gracile_, _Leuckartia flavicornis_ and _Heterochaeta
papilligera_; _Oncaea conifera_ is also phosphorescent. It is often
stated that _Sapphirina_ (p. 69) is phosphorescent, but its wonderful
iridescent blue colour is purely due to interference colours, and has
nothing to do with phosphorescence. Giesbrecht has observed that the
phosphorescence is due to a substance secreted in special skin-glands,
which is jerked into the water, and on coming into contact with it emits
a phosphorescent glow. This substance can be dried up completely in a
desiccated specimen and yet preserve its phosphorescent properties, the
essential condition for the actual emission of light being contact with
water. Similarly, specimens preserved in glycerine for a long period
will phosphoresce when compressed in distilled water. From this last
experiment Giesbrecht concludes that the phosphorescence can hardly be
due to an oxidation process, but the nature of the chemical reaction
remains obscure.
=Fam. 2. Candacidae.=—This family comprises the single genus _Candace_,
with numerous species distributed in the plankton of all seas. Some
species, e.g. _C. pectinata_, Brady, have a practically world-wide
distribution, this species being recorded from the Shetlands and from
the Philippines.
=Fam. 3. Pontellidae.=—This is a larger family also comprising widely
distributed species found in the marine plankton. _Anomalocera
pattersoni_ (Fig. 27) is one of the commonest elements in the plankton
of the North Sea.
=Sub-Order 2. Podoplea.=
The boundary between the fore and hind part of the body falls in front
of the fifth thoracic segment. The appendages of the fifth thoracic pair
in the male are never modified as copulatory organs.
TRIBE I. AMPHARTHRANDRIA.
The first antennae in the male differ greatly from those in the female,
being often geniculated and acting as prehensile organs.
[Illustration:
FIG. 28.—_Euterpe acutifrons_, ♀, × 70. _Abd.1_, 1st abdominal
segment; _Th.5_, 5th thoracic segment. (After Giesbrecht.)
]
[Illustration:
FIG. 29.—First antenna of _Euterpe acutifrons_, ♂. (After Giesbrecht.)
]
=Fams. 1–2.= =Cyclopidae= and =Harpacticidae=, and other allied
families, are purely free-living forms; they are not usually pelagic in
habit, but prefer creeping among algae in the littoral zone or on the
sea-bottom, or especially in tidal pools. Some genera are, nevertheless,
pelagic; e.g. _Oithona_ among Cyclopidae; _Setella_, _Clytemnestra_, and
_Aegisthus_ among Harpacticidae.
The sketch (Fig. 28) of _Euterpe acutifrons_ ♀, a species widely
distributed in the Mediterranean and northern seas, exhibits the
structure of a typical Harpacticid, while Fig. 29 shows the form of the
first antenna in the male.
Several fresh-water representatives of these free-living families occur.
The genus _Cyclops_ (Cyclopidae) is exclusively fresh-water, while many
Harpacticidae go up into brackish waters: for example on the Norfolk
Broads, Mr. Robert Gurney has taken _Tachidius brevicornis_, Müller, and
_T. littoralis_, Poppe; _Ophiocamptus brevipes_, Sars; _Mesochra
lilljeborgi_, Boeck; _Laophonte littorale_, T. and A. Scott; _L.
mohammed_, Blanchard and Richard; and _Dactylopus tisboides_, Claus.
Schmeil[43] gives the following scheme for identifying the fresh-water
Cyclopidae and Harpacticidae (see diagnosis of Centropagidae on p. 59):—
=Fam. 1. Cyclopidae.=—The cephalothorax is clearly separated from the
abdomen. The first antennae of the female when bent back do not stretch
beyond the cephalothorax; in the male both of them are clasping organs.
The second antennae are without an exopodite. The fifth pair of limbs
are rudimentary, there is no heart, and the female carries two egg-sacs.
_Cyclops._—Numerous species, split up according to segmentation of
rudimentary fifth pair of legs, number of joints in antennae, etc.
=Fam. 2. Harpacticidae.=—The cephalothorax is not clearly separated from
the abdomen. The first antennae are short in both sexes, both being
clasping organs in the male. The second antennae have a rudimentary
exopodite. The fifth pair of limbs are rudimentary and plate-shaped; a
heart is absent, and the egg-sacs of the female may be one or two in
number.
1. _Ophiocamptus_ (_Moraria_).—Body worm-shaped; first antennae of
female 7–jointed, rostrum forming a broad plate.
2. Body not worm-shaped; first antennae of female 8–jointed, rostrum
short and sharp.
(_a_) Endopodites of all thoracic limbs 3–jointed. The first
antennae in female distinctly bent after the second joint.
_Nitocra._
(_b_) Endopodite of at least the fourth limb 2–jointed; first
antennae in female not bent. _Canthocamptus._
3. _Ectinosoma._—Body as in 2, but first antennae are very short,
and the maxillipede does not carry a terminal hooked seta as in 1
and 2.
=Fam. 3. Peltiidae.=[44]—This is an interesting family, allied to the
Harpacticidae, and includes species with flattened bodies somewhat
resembling Isopods, and a similar habit of rolling themselves up into
balls. No parasitic forms are known, though _Sunaristes paguri_ on the
French and Scottish coasts is said to live commensally with
hermit-crabs.
We have now enumerated the chief families of free-living Copepods; the
rest are either true parasites or else spend a part of their lives as
such. A number of the semi-parasitic and parasitic Copepods can be
placed in the tribe Ampharthrandria owing to the characters of their
antennae; but it must be remembered that many parasitic forms have given
up using the antennae as clasping organs; however, the sexual
differences in the antennae, and the fact that many of the species which
have lost the prehensile antennae in the male have near relations which
preserve it, enable us to proceed with some certainty. The adoption of
this classification necessitates our separating many families which
superficially may seem to resemble one another, _e.g._ the
semi-parasitic families Lichomolgidae and Ascidicolidae, and the
Dichelestiidae from the other fish-parasites; it also necessitates our
treating the presence of a sucking mouth as of secondary importance.
This characteristic must certainly, however, have been acquired more
than once in the history of the Copepods, for instance in the
Asterocheridae and in the fish-parasites, while it sometimes happens
that genera belonging to a typically Siphonostomatous group possess a
gnathostome, or biting mouth, _e.g._ _Ratania_ among the Asterocheridae.
Again, it is impossible even if we use the character of the mouth as a
criterion to place together all the true parasites on fishes in one
natural group, because the Bomolochidae and Chondracanthidae, which are
otherwise closely similar to the rest of the fish-parasites, possess no
siphon. It seems plain, therefore, that the parasitic habit has been
acquired several times separately by diverging stocks of free-swimming
Copepods, and that it has resulted in the formation of convergent
structures.
[Illustration:
FIG. 30.—_Haemocera danae_, × 40. =A=, Side view ♀; =B=, ventral view
♂. _Ant.1_, 1st antenna; _e_, eye; _ov_, ovary; _ovd_, oviduct;
_St_, stomach; _Th.1_, 1st thoracic appendage; _Th.5_, 5th thoracic
segment; _vd_, vas deferens. (After Malaquin.)
]
[Illustration:
FIG. 31.—Free-swimming Nauplius larva of _Haemocera danae_; _Ant.1_,
_Ant.2_, 1st and 2nd antennae; _e_, remains of eye; _Md_, mandible.
(After Malaquin.)
]
=Fam. 4. Monstrillidae.=[45]—These are closely related to the
Harpacticidae. The members of this curious family are parasitic during
larval life and actively free-swimming when adult. There are three
genera, _Monstrilla_, _Haemocera_, and _Thaumaleus_. The best known type
is _Haemocera danae_ (often described as _Monstrilla danae_). In the
adult state (Fig. 30) there are no mouth-parts; the mouth is exceedingly
small and leads into a very small stomach, which ends blindly, while the
whole body contains reserve food-material in the form of brown
oil-drops. The sole appendages on the head are the first antennae; but
on the thorax biramous feet are present by means of which the animal can
swim with great rapidity. This anomalous organisation receives an
explanation from the remarkable development through which the larva
passes. The larva is liberated from the parent as a Nauplius with the
structure shown in Fig. 31; it does not possess an alimentary canal. It
makes its way to a specimen of the Serpulid worm, _Salmacina dysteri_,
into the epidermis of which it penetrates by movements of the antennae,
hanging on all the time by means of the hooks on the mandibles. From the
epidermis it passes through the muscles into the coelom of the worm, and
thence into the blood-vessels, usually coming to rest in the ventral
blood-vessel. As the Nauplius migrates, apparently by amoeboid movements
of the whole body, it loses all its appendages, the eye degenerates, and
the body is reduced to a minute ovoid mass of cells, representing
ectoderm and endo-mesoderm, surrounded by a chitinous membrane (Fig. 32,
A). Arrived in the ventral blood-vessel it begins to grow, and the first
organ formed is a pair of fleshy outgrowths representing the second
antennae (Fig. 32, B), which act as a nutrient organ intermediary
between host and parasite. The adult organs now begin to be
differentiated, as shown in Fig. 32, C, from the undifferentiated
cellular elements of the Nauplius, the future adult organism being
enclosed in a spiny coat from which it escapes. At this stage it
occupies a large part of its host’s body, lying in the distended ventral
blood-vessel, and it escapes to the outside world by rupturing the
body-wall of the worm, leaving behind it the second antennae, which have
performed their function as a kind of placenta. Malaquin, to whom we owe
this account, makes the remarkable statement that if two or three
Monstrillid Nauplii develop together in the same host they are always
males, if only one it may be either male or female. The only parallel to
this extraordinary life-history is found in the Rhizocephala (see pp.
96–99).
[Illustration:
FIG. 32.—Later stages in the development of _Haemocera danae_. _Abd_,
Abdomen; _Ant.1_, _Ant.2_, 1st and 2nd antennae; _ch_, chitinous
investment; _e_, eye; _Ect_, ectoderm; _En_, endoderm; _Mes_,
mesoderm; _Mes & en_, mesoderm and endoderm; _R_, rostrum; _St_,
mouth and stomach; _Th_, thoracic appendages. (After Malaquin.)
]
[Illustration:
FIG. 33.—Side view of _Doropygus pulex_, ♀, × 106. _Abd.1_, 1st
abdominal segment; _Ant.1_, 1st antenna; _b.p_, brood-pouch; _Th.1_,
1st thoracic appendage; _Th.4_, 4th thoracic segment. (After Canu.)
]
=Fam. 5. Ascidicolidae.=[46]—Although the members of this family, which
live semiparasitically in the branchial sac or the gut of Ascidians,
betray their Ampharthrandrian nature by the sexual differences of their
first antennae, only two genera, _Notodelphys_ and _Agnathaner_, possess
true prehensile antennae. According as the parasitism is more or less
complete, the buccal appendages either retain their masticatory
structure or else become reduced to mere organs of fixation. In
_Notodelphys_ both sexes can swim actively and retain normal
mouth-parts; they live parasitically, or perhaps commensally, in the
branchial cavities of Simple or Compound Ascidians, feeding on the
particles swept into the respiratory chamber of the host. They leave
their host at will in search of a new home, and are frequently taken in
the plankton.
_Doropygus_ (Fig. 33), a genus widely distributed in the North Sea and
Mediterranean, also inhabiting the branchial sac of Ascidians, is more
completely parasitic, and the female cannot swim actively. Forms still
more degraded by a parasitic habit are _Ascidicola rosea_ (especially
abundant in the stomach of _Ascidiella scabra_ at Concarneau), in which
the female has lost its segmentation, the mouth-parts and thoracic legs
being purely prehensile, and various species of _Enterocola_, parasitic
in the stomach of Compound Ascidians, in which the female is a mere sac
incapable of free motion, while the male preserves its swimming powers
and a general _Cyclops_-form (Fig. 34). We have here the first instance
of the remarkable parallelism between the degree of parasitism and the
degree of sexual dimorphism, a parallelism which holds with great
regularity among the Copepoda, and can be also extended to other classes
of parasitic animals.
[Illustration:
FIG. 34.—_Enterocola fulgens._ =A=, Ventral view of ♀, × 35; =B=, side
view of ♂, × 106. _Abd.1_, 1st abdominal segment; _Ant.1_, _Ant.2_,
1st and 2nd antennae; _c.m_, gland-cells; _n_, ventral nerve-cord;
_og_, oviducal gland; _ov_, ovary; _po_, vagina; _Th.1_, 1st
thoracic appendage; _Th.4_, _Th.5_, 4th and 5th thoracic segments.
(After Canu.)
]
[Illustration:
FIG. 35.—_Asterocheres violaceus_, ♀, with egg-sacs, × 57. (After
Giesbrecht.)
]
[Illustration:
FIG. 36.—Diagrammatic transverse section through the distal part of
the siphon of _Rhynchomyzon purpurocinctum_ (Asterocheridae). _Md_,
mandible. (After Giesbrecht.)
]
=Fam. 6. Asterocheridae.=[47]—These forms retain the power of swimming
actively, and are very little modified in outward appearance by their
parasitic mode of life (Fig. 35), though they possess a true siphon in
which the styliform mandibles work. The siphon is formed by the upper
and lower lips, which are produced into a tube with three longitudinal
ridges; in the outer grooves are the mandibles, while the inner groove
forms the sucking siphon (see transverse section, Fig. 36). In
_Ratania_, however, there is no siphon. The first antennae possess a
great number of joints, and may be geniculated in the male
(_Cancerilla_). The members of this family live as ectoparasites on
various species of Echinoderms, Sponges, and Ascidians, but they
frequently change their hosts, and it appears that one and the same
species may indifferently suck the juices of very various animals, and
even of Algae. _Cancerilla tubulata_, however, appears to live only on
the Brittle Starfish, _Amphiura squamata_.
=Fam. 7. Dichelestiidae.=—The males and females are similarly parasitic,
and the body in both is highly deformed, the segmentation being
suppressed and the thoracic limbs being produced into formless fleshy
lobes; they are placed among the Ampharthrandria owing to sexual
differences in the form of the first antennae. There is a well-developed
siphon in which the mandibular stylets work, except in _Lamproglena_,
parasitic on the gills of Cyprinoid fishes; the succeeding mouth-parts
are prehensile.
The majority of the species are parasitic on the gills of various fish
(_Dichelestium_ on the Sturgeon, _Lernanthropus_[48] on _Labrax lupus_,
_Serranus scriba_, etc.), but Steuer[49] has recently described a
Dichelestiid (_Mytilicola_) from the gut of _Mytilus galloprovincialis_
off Trieste. This animal and _Lernanthropus_ are unique among Crustacea
through the possession of a completely closed blood-vascular system
which contains a red fluid; the older observers believed this fluid to
contain haemoglobin, but Steuer, as the result of careful analysis,
denies this. The parasite on the gills of the Lobster, _Nicothoe
astaci_, possibly belongs here.
The inclusion of _Nicothoe_ and the Dichelestiidae among the
Ampharthrandria rests on a somewhat slender basis; this basis is
afforded by the fact that none of the parasitic Isokerandria have more
than seven joints in the first antennae, whereas _Nicothoe_ and some of
the Dichelestiidae[50] have more numerous joints. In most of the
Dichelestiidae, however, the number of joints is less than seven and
practically equal in the two sexes.
TRIBE II. ISOKERANDRIA.
The first antennae are short, similar in the two sexes, and are never
used by the male as clasping organs. This function may be subserved by
the second maxillae.
FAMS. ONCAEIDAE, CORYCAEIDAE, LICHOMOLGIDAE, ERGASILIDAE, BOMOLOCHIDAE,
CHONDRACANTHIDAE, PHILICHTHYIDAE, NEREICOLIDAE, HERSILIIDAE, CALIGIDAE,
LERNAEIDAE, LERNAEOPODIDAE, CHONIOSTOMATIDAE.
The families Oncaeidae and Corycaeidae contain pelagic forms of
flattened shape and great swimming powers, but the structure of the
mouth-parts in the Corycaeidae points to a semi-parasitic habit.
=Fam. 1. Oncaeidae.=—This family, including the genera _Oncaea_,
_Pachysoma_, etc., does not possess the elaborate eyes of the next
family, nor is the sexual dimorphism so marked.
=Fam. 2. Corycaeidae.=—These are distinguished from the Oncaeidae, not
only by their greater beauty, but also by the possession of very
elaborate eyes, which are furnished with two lenses, one at each end of
a fairly long tube. The females of _Sapphirina_ are occasionally found
in the branchial cavity of Salps, and their alimentary canal never
contains solid particles, but is filled with a fluid substance perhaps
derived by suction from their prey. _S. opalina_ may occur in large
shoals, when the wonderful iridescent blue colour of the males makes the
water sparkle as it were with a sort of diurnal phosphorescence. The
animal, however, despite the opinion of the older observers, is not
truly phosphorescent. It may be that the ornamental nature of some of
the males is correlated with the presence of the curious visual organs,
which are on the whole better developed in the females than in the
males. As in so many pelagic Copepods, the body and limbs may bear
plumed setae of great elaboration and beautiful colour, _e.g._ _Copilia
vitrea_ (Fig. 37).
We now pass on to the rest of the parasitic Copepods,[51] which probably
belong to the tribe Isokerandria, and we meet with the same variety of
degrees of parasitism as in the Ampharthrandria, often leading to very
similar results.
[Illustration:
FIG. 37.—_Copilia vitrea_ (Corycaeidae), ♀, × 20. (After Giesbrecht.)
]
In the first seven families mentioned below there is no siphon. The
Lichomolgidae and Ergasilidae have not much departed from the
free-living forms just considered, retaining their segmentation, though
in the Ergasilidae the body may be somewhat distorted (Fig. 39). In both
families the thoracic swimming feet are of normal constitution.
[Illustration:
FIG. 38.—_Lichomolgus agilis_, × 10. _Abd. 1_, 1st abdominal segment;
_cpth_, cephalothorax; _Th.1_, 1st thoracic segment; _Th.5_, 5th
thoracic appendage. (After Canu.)
]
=Fam. 3. Lichomolgidae.=[52]—These are semi-parasitic in a number of
animals living on the sea-bottom, such as Actinians, Echinoderms,
Annelids, Molluscs, and Tunicates. _Lichomolgus agilis_ (Fig. 38) occurs
in the North Sea, Atlantic, and Mediterranean, on the gills of large
species of the Nudibranch, _Doris_, while _L. albeus_ is found in the
peribranchial cavity and cloaca of various Ascidians. _Sabelliphilus_
may infect the gills of Annelids such as _Sabella_, and is common at
Liverpool.
=Fam. 4. Ergasilidae.=—_Thersites_ (Fig. 39) is parasitic on the gills
of various fishes, _e.g._ _T. gasterostei_, which is common on
_Gasterosteus aculeatus_ on the French and North Sea coasts, and may
even be found on specimens of the fish that have run up the River Forth
into fresh water. The animal possesses claw-like second antennae by
which it clings to its host.
[Illustration:
FIG. 39.—_Thersites gasterostei._ =A=, ♀, × 10; =B=, ♂, × 20. _Abd. 1
& 2_, Fused 1st and 2nd abdominal segments; _Ant.1_, _Ant.2_, 1st
and 2nd antennae; _e.s_, egg-sac; _Th_, thoracic appendages. (After
Gerstaecker.)
]
Similarly characterised by the absence of a siphon are three other
families of fish-parasites, the Bomolochidae, Chondracanthidae, and
Philichthyidae.
=Fam. 5. Bomolochidae.=—_Bomolochus_ (Fig. 40), parasitic on the skin of
the Sole (_Solea_) and in the nostrils of Cod (_Gadus_), is held to be
related to the Ergasilidae. The first thoracic limb is remarkably
modified. Were it not for the absence of a siphon, it would be hard to
separate this family from the Caligidae.
[Illustration:
FIG. 40.—_Bomolochus_, sp. (Bomolochidae), × 8. _Abd. 1_, 1st
abdominal segment; _Ant.1_, _Ant.2_, 1st and 2nd antennae; _Mx.1_,
_Mx.2_, 1st and 2nd maxillae; _Mxp_, maxillipede; _Th.1_, 1st
thoracic appendage. (After Gerstaecker.)
]
[Illustration:
FIG. 41.—_Chondracanthus zei_, ♀, × 4.
]
[Illustration:
FIG. 42.—Dwarf male of _Lernentoma cornuta_ (Chondracanthidae), × 10.
_Ant.1_, _Ant.2_, 1st and 2nd antennae; _Th.1_, 1st thoracic
segment. (After Gerstaecker.)
]
=Fam. 6. Chondracanthidae.=—These Copepods infest the gills and even the
mouth of various marine fish, such as the Gurnard, Plaice, Skate, Sole,
and many others. The sexual dimorphism is very marked, the female being
large, indistinctly segmented, and with irregular paired processes
protruding from the sides of the body, giving the animal a monstrous
form (Fig. 41); while the male (Fig. 42) is very small, has a completely
segmented thorax, and lives clinging on to the female by its prehensile
second antennae—_Chondracanthus_, _Lernentoma_.
=Fam. 7. Philichthyidae.=—These parasites, which are hardly known to
occur in British waters,[53] are mucus-feeders and infest the skin of
Teleosts, _e.g._ the Sole; often taking up a position in the lateral
line or in a slime canal. They show a similar sexual dimorphism to the
foregoing family, the adult female being extraordinarily drawn out into
finger-like processes (_e.g._ _Philichthys_)[54] or else long, slender,
and Nematode-like, with much reduced appendages (_Lernaeascus_), while
the male retains a more normal structure. As in all the foregoing forms
there is no siphon.
We now return to two semi-parasitic families, =Fam. 8, Nereicolidae=,
and =Fam. 9, Hersiliidae=, in which there is certainly no well-developed
siphon, but the upper and under lips protrude, forming a hollow between
them in which the mouth-parts work. Both families are ectoparasites
which frequently leave their hosts, and they retain their segmentation
and powers of swimming. Perhaps the best-known form is the Hersiliid,
_Giardella callianassae_, which lives in the adult state in the
galleries excavated in the sand by _Callianassa subterranea_, gaining
its nourishment as an ectoparasite on the Decapod. The larvae are
pelagic, and are said by Thomson[55] to occur in Liverpool Bay.
List[56] describes _Gastrodelphys_, a parasite of doubtful position,
from the gills of tubicolous worms, such as _Myxicola_ and _Sabella_,
which possesses a perfectly siphonostomatous mouth.
The remaining families to be dealt with are those containing all the
fish-parasites which possess a true siphonostome, as well as the
siphonostomatous family Choniostomatidae, which is parasitic on other
Crustacea. In all these forms the mouth is prolonged into a tube in
which the styliform mandibles work.
=Fam. 10. Caligidae.=—Ectoparasites on fish, lodging most frequently in
the gill-chamber. In most of the genera the segmentation and power of
swimming are retained in both sexes, the sexual dimorphism not being
very well marked, though the males are smaller than the females, and
were in some cases originally described as belonging to a special genus
_Nogagus_. The females carry two long egg-sacs; the general structure
may be made out from the ventral view of _Caligus nanus_ (Fig. 43).
Some of the Caligidae are distinguished by the terga of the thoracic
segments being expanded to form large chitinous elytra, e.g. _Cecrops_,
found parasitic on the gills of the Tunny and on the Sun-fish
(_Orthagoriscus mola_). _Caligus rapax_ is parasitic on the skin and in
the gills of Sea-Trout, Pollan, etc.; and _C. lacustris_ is common in
fresh-water lakes and streams on Pike and Carp.
[Illustration:
FIG. 43.—_Caligus nanus_, × 10. _Abd.1_, 1st abdominal segment;
_Ant.1_, _Ant.2_, 1st and 2nd antennae; _Mx.1_, _Mx.2_, 1st and 2nd
maxillae; _Mxp_, maxillipede; _s_, siphon; _Th.1_, _Th.5_ 1st and
5th thoracic appendages. (After Gerstaecker.)
]
[Illustration:
FIG. 44.—_Lernaea branchialis_ from the Haddock, ♀, × 1. _Ceph_,
cephalothorax; _e.s_, egg-sacs. (After Scott.)
]
=Fam. 11. Lernaeidae.=—These parasites burrow with their heads deep into
the skin, or even into the blood-vessels or body-cavity, of various
marine fish. The body of the adult female _Lernaea_ is extraordinarily
deformed, consisting of a mere shapeless sac with irregular branched
processes on the head, and two egg-sacs attached behind (Fig. 44).
_Pennella sagitta_[57] bores so deeply into the flesh of its host,
_Chironectes marmoratus_, that only the egg-sacs and some remarkable
branchial processes attached to its abdomen protrude outside the host to
the exterior. _Peroderma cylindricum_ bores similarly into the flesh of
the Sardine, and where it is common, inflicts considerable damage. The
males of these curious animals are of more normal structure (Fig. 45).
Claus[58] states that fertilisation takes place when both sexes are
free-swimming, and of a more or less similar structure, and that
subsequently the female becomes fixed to her host and degenerates into
the shapeless mass shown in Fig. 44.
[Illustration:
FIG. 45.—_Lernaea branchialis_, ♂, × 10. _Ant.1_, _Ant.2_, 1st and 2nd
antennae; _Br_, brain; _e_, eye; _g_, stomach; _t_, testis; _vd_,
vas deferens; _ves. sem_, vesicula seminalis. (After Claus.)
]
[Illustration:
FIG. 46.—_Achtheres percarum_. =A=, ♀, × 4; =B=, ♂, × 4. _Ant.2_, 2nd
antenna; _g_, stomach; _Mx.2_, 2nd maxilla; _Mxp_, maxillipede;
_ov_, ovary; _ovd_, oviduct. (After Gerstaecker.)
]
=Fam. 12. Lernaeopodidae.=—This family may be illustrated by the common
gill-parasite of Perch and Trout, known as _Achtheres percarum_. The
female (Fig. 46), which is much larger than the male, and is not clearly
segmented, is attached to the host by means of the maxillipedes, which
are fused distally into a pad armed with chitinous hooks. In the male
the maxillipedes are prehensile, but are not so fused. Besides
_Achtheres_ there are other fresh-water forms, _e.g._ _Lernaeopoda
salmonea_ on Salmon, and a number of marine genera. It appears that the
larvae fix themselves to their hosts by means of a long glandular
thread, which proceeds from the middle of the forehead.[59]
[Illustration:
FIG. 47.—Ventral view of _Stenochotheres egregius_ (Choniostomatidae),
♂. _A_, _A′_, 1st and 2nd antennae; _M_, mouth; _Mx_, 2nd maxilla;
_T_, 1st thoracic leg. (After Hansen.)
]
=Fam. 13. Choniostomatidae.=[60]—The members of this family are all
parasitic on other Crustacea. The majority live parasitically in the
marsupial pouches of female Amphipods, Isopods, Mysidae, and Cumacea,
_e.g._ _Sphaeronella_ and _Stenochotheres_ in the marsupia of Gammarids;
but _Choniostoma_ occurs in the branchial cavity of _Hippolyte_,
_Homoeoscelis_ is common in the branchial cavity of _Diastylis_ and
_Iphinoe_, and _Aspidoecia_ on the outside of the body of the Mysid
_Erythrops_. The males and females live together in the same marsupium,
but the adult males retain the power of roving about, and do not feed so
much as the females, though their mouth-parts are similarly constructed
(Fig. 47). Representatives occur all over the world, but the majority of
species known at present are from the North Sea, the most abundant being
_Stenochotheres egregius_, parasitic on the Gammarid _Metopa bruzelii_,
Goës.
The male bears a median glandular thread on the forehead by which it
attaches itself to the females or to the host. Hansen considers that the
family is most closely allied to the Lernaeopodidae.
[Illustration:
FIG. 48.—_Argulus foliaceus_, young ♂, × 15. _a^1_, _a^2_, First and
second antennae; _ab_, abdomen; _E_, compound eye; _l_, liver; _m_,
mandibles and first maxillae; _mx_, second maxilla (the median eye
is seen between the two second maxillae); _mxp_, maxillipede; _s.g_,
shell-gland; _sp_, spine; _t_, testis; 1, 4, first and fourth
swimming appendages. (After Claus.)
]
BRANCH II. BRANCHIURA.
=Fam. Argulidae.=[61]—We have yet to mention this group of
fish-parasites, related to the Copepoda, but occupying an isolated
position. They are ectoparasites upon various species of fish, _Argulus
foliaceus_ being common in the fresh waters of Europe, infesting the
branchial chamber or the skin of fresh-water fish, but being frequently
taken swimming freely in the water. Both males and females can swim with
great agility, and they leave their hosts regularly at the breeding
season in spring and autumn; fertilisation is internal, and the female
deposits the eggs on stones and other objects. After leaving its host,
an _Argulus_, if it cannot find a fish of the same species, can live on
almost any other species, and may even attack Frog tadpoles; while the
kinds that infest migratory fish can change with their hosts from salt
to fresh water, or the reverse. America appears to be the home of the
Argulidae.[62]
The structure of an Argulid is exhibited in Fig. 48. In front of the
siphon, within which the styliform mandibles and first maxillae work,
there is a poison-spine (_sp_); the appendages which correspond to the
second maxillae (_mx_) are modified into sucking discs, but in the genus
_Dolops_ they terminate in normal claws. The next pair of appendages,
usually spoken of as maxillipedes (_mxp_), are clasping organs, and
behind follow four pairs of thoracic swimming feet (1–4). The body is
foliaceous, and they always apply themselves to their hosts with the
long axis pointing forwards and parallel to that of the host, while on
various parts of the under surface of the body are spines pointing
backward which prevent the parasite being brushed off by the passage of
the host through the water. These animals, alone among the Copepoda,
possess compound eyes.
A short sketch has now been given of the variations in Copepod
organisation, but we cannot leave the subject without pointing out the
rich field which still remains for the morphologist, especially in
determining the true relationships of the parasitic families.