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,[[385]] 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[[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:—