456 



ARACHNIDA 



are not, however, equivalent to somites. The cephalothorax is very short 

 and bears on its ventral side 2 pairs of hook-like retractile claws. Excre- 

 tory, respiratory, circulatory, and sense organs are all absent. The diges- 

 tive tract is a straight tube. The nervous system consists of a sub- 

 O3sophageal ganglion and a circumoesophageal ring. The sexes are 

 separate, the male genital pore being near the mouth, the female near 

 the anus at the hinder end. The linguatulids live as parasites in the air 

 passages and also in the body cavity of certain mammals and reptiles. 

 f . 9 The eggs reach the outside world together with the 



discharges of the host and must be swallowed by a 

 mouse, rabbit, or other small animal in order to de- 

 velop. The larva (Fig. 717, B), hatching in the stomach 

 of this intermediate host, is quite mite-like in appear- 

 ance and has 2 pairs of legs and boring mouth parts. 

 It makes its way into the liver, peritoneum, or other 

 organ, where it encapsules itself and here undergoes 

 the greater part of its metamorphosis and attains the 

 form of the adult, except that it 

 has a circle of spines around each 

 ring and is not sexually mature. 

 It may then leave its cyst and 

 migrate through the tissues of the 

 host, and may reenycst in some 

 other locality. If the intermedi- 

 ate host is then eaten by the final 

 host, the parasite passes into its 

 final resting place in the latter. 

 It may, for instance, pass directly 

 from the mouth into the nasal 

 passages, or it may migrate from 

 the stomach or intestine through 

 the intestinal wall into the lungs, or air passages, where it becomes 

 mature. 



The linguatulids were formally classed among the worms, but the 

 resemblance of the body of the adult to that of eriophyid mites and of 

 the larva to the short-bodied mites, as well as their voluminous body 

 cavity and striated muscle fibres, have caused them to be placed among 

 the Arachnida. Both the adult and larva are occasionally found in man. 

 The order contains 3 genera and about 26 species. 

 Key to the genera of Linguatulida here described : 



d! Body flat 1. LINGUATULA 



a, Body cylindrical 2. POROCEPHALUS 



Fig. 717 Porocephalus crotali. A, dia- 

 gram showing anatomy ; B, a larva (Stiles). 

 1, mouth ; 2, hooks ; 3, genital pore ; 4, tes- 

 tis ; 5, intestine ; 6, sperm duct ; 7, anus. 



