PENTASTOMIDA, or LINGUATULINA  , vermiform entoparasitic animals, of which the exact zoological position is unknown, although they are usually regarded as highly modified degenerate Arachnida of the order Acari . The body is sub-cylindrical or somewhat convex above, flatter below, broad and oval in front and narrowed and elongate behind . Its integument is marked by a large number of transverse grooves simulating the segmentation of Annelids, and near the anterior extremity close to the mouth are two pairs of recurved chitinous hooks . The alimentary canal is a simple tube traversing the body from end to end, the anus opening at the extremity of its narrowed tail-like termination . The nervous system is represented by an oesophageal collar and a suboesophageal ganglion, whence paired nerves pass outwards to innervate the anterior extremity and backwards towards its posterior end . No respiratory or circulatory organs are known . The sexes are distinct but dissimilar in size, the female being usually much larger than the male . The generative organs occupy a large part of the body cavity . In the female the ovary is a large unpaired organ from the anterior end of. which arise two oviducts, and connected with the latter are a pair of large so-called copulatory pouches, which perhaps act as receptacula seminis . These and the oviducts lie on the anterior half of the body; but the oviducts themselves soon unite to form a single tube of great length, which runs backwards to its posterior extremity, terminating in the genital orifice close to the anus . In the male, on the contrary, this orifice is situated in the anterior half of the body, not far behind the mouth . The orifice leads into a large pouch lodging a pair of very long penes, which are coiled up when not in use .

The two testicles, which extend far back into the posterior part of the body, are long and tubular . Anteriorly their

vasa deferentia soon unite into a common duct, which opens into the pouch containing the penes . Also communicating with this pouch is a pair of long slender flagelliform tubes, of which the function is unknown . The structure of the adult Linguatula or Pentastomum, above described, does not supply convincing evidence of relationship with the Acari . At the same time some Acari, like Eriophyes (Phytoptus) and Demodex, have the body elongated and annulated, but In these groups the elongation of the body is caudal or post-anal, as is attested by the position of the anus far forwards on its ventral surface . Again, the adult Pentastomum shows no trace of appendages, unless the two pairs of chitinous hooks are to be regarded as the vestiges of jaws or ambulatory limbs . In the embryo, however, what have been regarded as remnants of limbs may be seen . In the mature stage Pentastomida live in the respiratory passages of mammalia, principally in the nasal cavities . The remarkable life-history of one species, Linguatula taenioides, has been worked out in detail and presents a close analogy to that of some Cestodes . The adults live in the nose of dogs, where they have been known to survive over fifteen months . Each female lays a vast number of eggs, about 500,00o being the estimated amount . These are expelled along with mucus by the sneezing of the host .

If they fall on pasture

land or fodder of any kind and are eaten by any herbivorous animal, such as a hare, rabbit, horse, sheep or ox, the active embryos or larvae are set free in the alimentary canal of the new host . These larvae are minute oval creatures with a comparatively short apically fringed caudal prolongation and furnished with two pairs of short two-clawed processes, which may represent the limbs of anthropods and possibly the two pairs of legs found in Acari of the family Eriophyidae . The larva is also armed anteriorly with a median piercing probe and a pair of sharp hooks by means of which it perforates the walls of the alimentary tract and makes its way into the body cavity, lungs or liver . Here it becomes encysted, and losing its boring apparatus and claw-bearing processes remains for a time quiescent . After a series of moults it passes into the second larval stage, somewhat like the parent but differing in having each integumental ring armed with a fringe of backwardly directed short bristles . This sexually immature stage, regarded at one time as representing a distinct species and named Linguatula denticulata, is reached in about six or seven months and measures from 6 to 8 mm. in length . In the event of the host escaping being killed and eaten it is believed that some of these larvae wander about or ultimately make their way to the exterior, possibly through the bronchi; nevertheless it seems to be certain that they can only reach sexual maturity in the nasal passages of some carnivorous animal, and the chance of attaining this environment is afforded when the viscera of the host are devoured by some flesh-eating mammal . The adult female of L. taenioides measures about 4 in. long and the male barely one-fourth of that . The adult and immature stages are, however, by no means confined respectively to carnivorous and herbivorous species of mammals . The adult stage, for example, has been found in the nasal passages of sheep, goats,horses and even of man, and the larval stage in the pleural and peritoneal cavities of dogs and cats . (R . I .