CHAP, XX LIFE-HISTORY AND STRUCTURE 489 
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 com- 
paratively 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 com- 
plicated by their double life. For long the larval form of Z. 
taenioides was known by different names in different hosts, e.g. 
Pentastoma denticulatum, Rud., when found in the goat, P. 
serratum, Frohlich, 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 
! This description is mainly based on the account of P. teretiusculus given by 
Spencer, Quart. J. Micr. Sci. xxxiv., 1893, p. 1. 
