CHAPTER XX. Pentastomida[383]

PENTASTOMIDA

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.=—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.

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.

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.

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.

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.

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, in the case of P. proboscideus.

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 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.=—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. The first two were regarded by Leuckart as but sub-genera, but Railliet and Hoyle 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 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, 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

D’ARCY W. THOMPSON, C.B., M.A. TRINITY COLLEGE

Professor of Natural History in University College, Dundee