PENTASTOMIDA[383]
OCCURRENCE—ECONOMIC IMPORTANCE—STRUCTURE—DEVELOPMENT AND LIFE-HISTORY—
SYSTEMATIC
Pentastomids are unpleasant-looking, fluke-like or worm-like animals,
which pass their adult lives in the nasal cavities, frontal sinuses, and
lungs of flesh-eating animals, such as the Carnivora, Crocodiles, and
Snakes; more rarely in Lizards, Birds, or Fishes. From these retreats
their eggs or larvae are sneezed out or coughed up, or in some other way
expelled from the body of their primary host, and then if they are
eaten, as they may well be if they fall on grass, by some
vegetable-feeding or omnivorous animal, they undergo a further
development. If uneaten the eggs die. When once in the stomach of the
second host, the egg-shell is dissolved and a larva emerges (Fig. 260,
p. 494), which bores through the stomach-wall and comes to rest in a
cyst in some of the neighbouring viscera. Here, with occasional
wanderings which may prove fatal to the host, it matures, and should the
second host be eaten by one of the first, the encysted form escapes,
makes its way to the nasal chambers or lungs, and attaching itself by
means of its two pairs of hooks, comes to rest on some surface capable
of affording nutriment. Having once taken up its position the female
seldom moves, but the males, which are smaller than the females, are
more active. They move about in search of a mate. Further, should the
host die, both sexes, after the manner of parasites, attempt to leave
the body. Like most animals who live entirely in the dark they develop
no pigment, and have a whitish, blanched appearance.
The only species of Pentastomid which has any economic importance is
_Linguatula taenioides_ of Lamarck, which is found in the nose of the
dog, and much more rarely in the same position in the horse, mule, goat,
sheep, and man. It is a comparatively rare parasite, but occurred in
about 10 per cent of the 630 dogs in which it was sought at the
laboratory of Alfort, near Paris, and in 5 out of 60 dogs examined at
Toulouse. The symptoms caused by the presence of these parasites are not
usually very severe, though cases have been recorded where they have
caused asphyxia. The larval stages occur in the rabbit, sheep, ox, deer,
guinea-pig, hare, rat, horse, camel, and man, and by their wandering
through the tissues may set up peritonitis and other troubles.
As in the Cestoda, which they so closely resemble in their life-history,
the nomenclature of the Pentastomids has been complicated by their
double life. For long the larval form of _L. taenioides_ was known by
different names in different hosts, _e.g._ _Pentastoma denticulatum_,
Rud., when found in the goat, _P. serratum_, Fröhlich, when found in the
hare, _P. emarginatum_ when found in the guinea-pig, and so on. In the
systematic section of this article some of the species mentioned are
known in the adult state, some in the larval, and in only a few has the
life-history been fully worked out.
=Structure.=[384]—The body of a Pentastomid is usually white, though in
the living condition it may be tinged red by the colour of the blood
upon which it lives. The anterior end, which bears the mouth and the
hooks (Fig. 256), has no rings; this has been termed the cephalothorax.
The rest of the body, sometimes called the abdomen, is ringed, and each
annulus is divided into an anterior half dotted with the pores of
certain epidermal glands and a hinder part of the ring in which these
are absent.
On the ventral surface of the cephalothorax, in the middle line, lies
the mouth, elevated on an oral papilla, and on each side of the mouth
are a pair of hooks whose bases are sunk in pits. The hooks can be
protruded from the pits, and serve as organs of attachment. Their shape
has some systematic value.
[Illustration:
FIG. 256.—_Porocephalus annulatus_, Baird. =A=, Ventral view of head,
× 6; =B=, ventral view of animal, × 2.
]
There are a pair of peculiar papillae which bear the openings of the
“hook-glands,” lying just in front of the pairs of hooks, and other
smaller papillae are arranged in pairs on the cephalothorax and anterior
annuli. The entire body is covered by a cuticle which is tucked in at
the several orifices. This is secreted by a continuous layer of ectoderm
cells. Some of these subcuticular cells are aggregated together to form
very definite glands opening through the cuticle by pores which have
somewhat unfortunately received the name of stigmata. Spencer attributes
to these glands a general excretory function. There is, however, a very
special pair of glands, the hook-glands, which extend almost from one
end to the other of the body; anteriorly these two lateral glands unite
and form the head-gland (Fig. 257). From this on each side three ducts
pass, one of which opens to the surface on the primary papilla; the
other two ducts open at the base of the two hooks which lie on each side
of the mouth. Leuckart has suggested that these important glands secrete
some fluid like the irritating saliva of a Mosquito which induces an
increased flow of blood to the place where it is of use to the parasite.
Spencer, however, regards the secretion as having, like the secretion of
the so-called salivary cells of the Leech, a retarding action on the
coagulation of the blood of the host.
The muscles of Pentastomids are striated. There is a circular layer
within the subcuticular cells, and within this a longitudinal layer and
an oblique layer which runs across the body-cavity from the
dorso-lateral surface to the mid-ventral line, a primitive arrangement
which recalls the similar division of the body-cavity into three
chambers in _Peripatus_ and in many Chaetopods. Besides these there are
certain muscles which move the hooks and other structures.
The mouth opens into a pharynx which runs upwards and then backwards to
open into the oesophagus (Fig. 257). Certain muscles attached to these
parts enlarge their cavities, and thus give rise to a sucking action by
whose force the blood of the host is taken into the alimentary canal.
The oesophagus opens by a funnel-shaped valve into the capacious stomach
or mid-gut, which stretches through the body to end in a short rectum or
hind-gut. The anus is terminal.
[Illustration:
FIG. 257.—Diagrammatic representation of the alimentary, secretory,
nervous, and reproductive systems of a male _Porocephalus
teretiusculus_, seen from the side. The nerves are represented by
solid black lines. (From W. Baldwin Spencer.)
1, Head-gland; 2, testis; 3, hook-gland; 4, hind-gut; 5, mid-gut; 6,
ejaculatory
duct; 7, vesicula seminalis; 8, vas deferens; 9, dilator-rod sac; 10,
cirrus-bulb;
11, cirrus-sac; 12, fore-gut; 13, oral papillae.
]
There appears to be no trace of circulatory or respiratory organs,
whilst the function usually exercised by the nephridia or Malpighian
tubules or by coxal glands, of removing waste nitrogenous matter, seems,
according to Spencer, to be transferred to the skin-glands.
The nervous system is aggregated into a large ventral ganglion which
lies behind the oesophagus. It gives off a narrow band devoid of
ganglion-cells, which encircles that tube. It also gives off eight
nerves supplying various parts, and is continued backward as a ninth
pair of prolongations which, running along the ventral surface, reach
almost to the end of the body (Fig. 257). The only sense-organs known
are certain paired papillae on the head, which is the portion that most
closely comes in contact with the tissues of the host.
Pentastomids are bisexual. The males are as a rule much less numerous
and considerably smaller than the females, although the number of annuli
may be greater.
The ovary consists of a single tube closed behind. This is supported by
a median mesentery. Anteriorly the ovary passes into a right and left
oviduct, which, traversing the large hook-gland, encircle the alimentary
canal and the two posterior nerves (Fig. 258). They then unite, and at
their point of union they receive the ducts of the two spermathecae,
usually found packed with spermatozoa. Having received the orifices of
the spermatheca, the united oviducts are continued backward as the
uterus, a highly-coiled tube in which the fertilised eggs are stored.
These are very numerous; Leuckart estimated that a single female may
contain half a million eggs. The uterus opens to the exterior in the
mid-ventral line a short distance—in _P. teretiusculus_ on the last ring
but seven—in front of the terminal anus. In _L. taenioides_ the eggs
begin to be laid in the mucus of the nose some six months after the
parasite has taken up its position.
[Illustration:
FIG. 258.—Diagrammatic representation of the alimentary, secretory,
nervous, and reproductive systems of a female _Porocephalus
teretiusculus_, seen from the side. The nerves are represented by
solid black lines. (From W. Baldwin Spencer.)
1, Head-gland; 2, oviduct; 3, hook-gland; 4, mid-gut; 5, ovary; 6,
hind-gut; 7,
vagina; 8, uterus; 9, accessory gland; 10, spermatheca.
]
The testis is a single tube occupying in the male a position similar to
that of the ovary in the female. Anteriorly it opens into two vesiculae
seminales, which, like the oviducts, pierce the hook-glands and encircle
the alimentary canal (Fig. 257). Each vesicula passes into a vas
deferens with a cuticular lining. Each vas deferens also receives the
orifice of a muscular caecal ejaculatory duct, which, crowded with
mature spermatozoa, stretches back through the body. Anteriorly the vas
deferens passes into a cirrus-bulb, which is joined by a cirrus-sac on
one side and a dilator-rod sac on the other, structures containing
organs that assist in introducing the spermatozoa into the female. The
two tubes then unite, and having received a dorsally-placed accessory
gland, open to the exterior by a median aperture placed ventrally a
little way behind the mouth.
=Life-history.=—The egg undergoes a large portion of its development
within the body of the mother. In _Linguatula taenioides_, which lives
in the nasal cavities of the dog, the eggs pass away with the nasal
excretions. If these, scattered about in the grass, etc., be eaten by a
rabbit, the egg-shell is dissolved in the stomach of the second host and
a small larva is set free. In _Porocephalus proboscideus_ and others,
which inhabit the lungs of snakes, the eggs pass along the alimentary
canal and leave the body with the faeces. They also must be eaten by a
second host if development is to proceed.
[Illustration:
FIG. 259.—A late larval stage of _Porocephalus proboscideus_, seen
from the side. Highly magnified. (From Stiles.) 1, primordium of
first pair of chitinous processes; 2, primordium of second pair of
chitinous processes; 3, mouth; 4, ventral ganglion; 5, receptaculum
seminis; 6, oviduct; 7, ovary; 8, anus; 9, vagina.
]
The larva which emerges when the egg-shell is dissolved has a rounded
body provided with two pairs of hooked appendages, and a tail which is
more or less prominent in different species (Figs. 259, 260). Each
appendage bears a claw, and is strengthened by a supporting rod or
skeleton. Anteriorly the head bears a boring apparatus of several
chitinous stylets. The various internal organs are in this stage already
formed, though in a somewhat rudimentary state, and it is doubtful if
the anus has yet appeared.
[Illustration:
FIG. 260.—Larva of _Porocephalus proboscideus_, seen from below.
Highly magnified. (From Stiles.) 1, Boring, anterior end; 2, first
pair of chitinous processes seen between the forks of the second
pair; 3, ventral nerve-ganglion; 4, alimentary canal; 5, mouth; 6
and 7, gland-cells.
]
By means of its boring apparatus, and aided by its hooked limbs, the
larva now works its way through the stomach-walls of its second host,
and comes to rest in the liver or in some other viscus. Its presence in
the tissues of its second host causes the formation of a cyst, and
within this the larva rests and develops. In man, at least, the cysts
often undergo a calcareous degeneration, and Virchow states “dass beim
Menschen das _Pentastomum_ am häufigsten von allen Entozoen zu
Verwechselungen mit echten Tuberkeln Veranlassungen giebt.” The larva
moults several times, and loses its limbs, which seem to have no
connexion with the paired hooks in the adult (Fig. 256). The internal
organs slowly assume the form they possess in the adult. The larva is at
first quite smooth, but as it grows the annulations make their
appearance, arising in the middle and spreading forward and backward
(Fig. 259). In this encysted condition the larva remains coiled up for
some months, according to Leuckart; six in the case of _L. taenioides_,
and a somewhat shorter period, according to Stiles,[385] in the case of
_P. proboscideus_.
[Illustration:
FIG. 261.—Encysted form of _Porocephalus protelis_, × 1, lying in the
mesentery of its host. (From Hoyle.)
]
The frequency of what used to be called _Pentastoma denticulatum_ (= the
larval form of _L. taenioides_) in the body of man depends on the
familiarity of man with dogs. Klebs and Zaeslin found one larva in 900
and two in 1914 autopsies. Laenger[386] found the larva fifteen times in
about 400 dissections, once in the mesentery, seven times in the liver,
and seven times in the wall of the intestine. After remaining encysted
for some time it may escape, and begins wandering through the tissues,
aided by its hooks and annulations, a proceeding not unaccompanied by
danger to its host. Should the latter be eaten by some carnivorous
animal, the larva makes its way into the nasal cavities or sinuses, or
into the lungs of the flesh-eating creature, and there after another
ecdysis it becomes adult. If, however, the second host escapes this
fate, the larvae re-encyst themselves, and then if swallowed they are
said to bore through the intestine of the flesh-eater, and so make their
way to their adult abode.
=Systematic.=[387]—The Pentastomida are a group much modified by
parasitism, which has so deeply moulded their structure as to obscure to
a great extent their origin and affinities. The larva, with its clawed
limbs, recalls the Tardigrades and certain Mites, e.g. _Phytoptus_,
where only two pairs of limbs persist, and where the abdomen is
elongated and forms a large proportion of the body. The resemblances to
a single and somewhat aberrant genus must not, however, be pressed too
far. The striated muscles, the ring-like nature of the reproductive
organs and their ducts, perhaps even the disproportion both in size and
number of the females to the males, are also characters common to many
Arachnids.
The Pentastomida include three genera, _Linguatula_, Fröhlich,
_Porocephalus_, Humboldt, and _Reighardia_, Ward.[388] The first two
were regarded by Leuckart as but sub-genera, but Railliet[389] and
Hoyle[390] have raised them to the rank of genera. They are
characterised as follows:—
_Linguatula_, body flattened, but dorsal surface arched; the edges of
the fluke-like body crenelated; the body-cavity extends as diverticula
into the edges of the body.
_Porocephalus_, body cylindrical, with no diverticula of the
body-cavity.
_Reighardia_, devoid of annulations, transparent, with poorly developed
hooks and a mouth-armature.
The following is a list of the species with their primary and secondary
or larval hosts:—
i. _Linguatula pusilla_, Diesing, found in the intestine of the
fresh-water fish _Acara_, a South American genus of the Cichlidae.
This is possibly the immature form of _L. subtriquetra_.
ii. _L. recurvata_, Diesing, found in the frontal sinuses and the
trachea of _Felis onca_.
iii. _L. subtriquetra_, Diesing, found in the throat of _Caiman
latirostris_ and _C. sclerops_, perhaps the mature form of _L.
pusilla_.
iv. _L. taenioides_, Lamarck, found in the frontal sinuses and nasal
chambers of the dog and ounce, and in the nasal cavities of the
wolf, fox, goat, horse, mule, sheep, and man, and in the trachea of
the ounce. The immature form has been found in or on the liver of
the cat, guinea-pig, and horse; in the lungs of the ox, cat,
guinea-pig, porcupine, hare, and rabbit; in the liver and connective
tissue of the small intestine of man; and in the mesenteric glands
of the ox, camel, goat, sheep, antelope, fallow-deer, and mouse.
v. _Porocephalus annulatus_, Baird, found in the lungs of the Egyptian
cobra, _Naja haje_; the immature form is thought to live encapsuled
in a species of _Porphyrio_[391] and in the Numidian Crane.
vi. _P. aonycis_, Macalister, from the lungs of an Indian otter taken
in the Indus.
vii. _P. armillatus_, Wyman, found in the adult state in the lungs of
certain African pythons, and in the lion; in the larval form it
occurs encysted in the abdomen of the Aard-wolf, the mandril, and
man—usually in negroes. Its migrations in the body of its second
host sometimes cause fatal results.
viii. _P. bifurcatus_, Diesing, found in the body-cavity of certain
snakes, and in the lungs of boa-constrictors and the legless lizard,
_Amphisbaena alba_. Possibly an immature form.
ix. _P. clavatus_, Lohrmann, found in the lungs of the Monitor lizard.
x. _P. crocidura_, Parona, found in the peritoneum of the “musk-rat”
_Crocidura_ in Burmah. Probably a larval form.
xi. _P. crotali_, Humboldt, found in the lungs, body-cavity, kidneys,
spleen, and mesentery of many snakes and lizards, and of the lion
and leopard. The immature forms occur in the liver and abdominal
cavity of species of opossum, armadillo, mouse, raccoon, bat, and
marmoset.
xii. _P. geckonis_, Dujardin, found in the lungs of a Siamese gecko.
xiii. _P. gracilis_, Diesing, found free in the body-cavity or
encapsuled on the viscera and mesenteries of South American fishes,
snakes, and lizards,
xiv. _P. heterodontis_; Leuckart, found encapsuled in the abdominal
muscles and mesentery of a species of _Heterodon_.
xv. _P. indicus_,[392] v. Linst., found in the trachea and lungs of
_Gavialis gangeticus_.
xvi. _P. lari_, Mégnin, found in the air-sacs of the Burgomaster or
Glaucous gull, _Larus glaucus_ of the Polar seas.
xvii. _P. megacephalus_, Baird, found embedded in the flesh of the
head of an Indian crocodile, _C. palustris_, the “Mugger.” Probably
a larval form.
xviii. _P. megastomus_, Diesing, found in the lungs of a fresh-water
tortoise, _Hydraspis geoffroyana_.
xix. _P. moniliformis_, Diesing, found in the lungs of pythons.
xx. _P. najae sputatricis_, Leuckart, found encapsuled in the
abdominal muscles and peritoneum of the cobra, _Naja tripudians_.
Probably a larval form.
xxi. _P. oxycephalus_, Diesing, found in the lungs of crocodiles and
alligators.
xxii. _P. platycephalus_, Lohrmann, habitat unknown.
xxiii. _P. subuliferus_, Leuckart, in the lungs of the cobra _Naja
haje_.
xxiv. _P. teretiusculus_, Baird, found in the lungs and mouth of
certain Australian snakes.
xxv. _P. tortus_, Shipley, found in the body-cavity of a snake,
_Dipsadomorphus irregularis_, taken in New Britain.
xxvi. _Reighardia_, sp., Ward, found in the air-sacs of Bonaparte’s
gull and the common North American tern.
PYCNOGONIDA
BY
D’ARCY W. THOMPSON, C.B., M.A. TRINITY COLLEGE
Professor of Natural History in University College, Dundee