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.
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.
