102 ARTHROPODA. 



Order IX. PENTASTOMIDEA. 



LlNGUATULiNA. ACANTHOTIIECA. 



Worm-like entozoic animals without feet, but the embryo with 

 four rudimentary legs. Body long, annulated. Sexes distinct. 



These are colourless parasites, classed by Eudolphi with Tre- 

 roatoda, and having a parasitism very similar to the Cestoda. 

 They have no special organs for respiration. The mouth has 

 two pairs of hooks in lieu of limbs. They require about a year to 

 attain the adult condition. 



These parasites are found in a sexless condition in the lungs 

 and liver of hares and other herbivorous mammals and of rep- 

 tiles ; and in the sexual state in the nostrils of dogs and other 

 carnivora by whom the herbivorous mammals have been devoured. 

 The males are much smaller than the females; the latter in Penta- 

 stoma tcenioides is 3 or 4 inches long. The larvae of P. constrictum, 

 has been found encysted in the human liver. There is only one 

 genus, with upwards of twenty species. 



Pentastoma. 



Class IV. INSECTA. 



CONDYLOPODA. HEXAPODA. 



Head, thorax, and abdomen distinct. Two antennas. Three 

 pairs of legs. Somites never more than twenty. Eespiration by 

 tracheae. Sexes distinct. 



The wings, which are almost always present, are developed 

 from the second and third thoracic somites. They " are essen- 

 tially flattened vesicles, sustained by slender but firm hollow 

 tubes called nervures [but more analogous to veins], along which 

 branches of the tracheae and channels of circulation are conti- 

 nued." By Oken they were called " aerial gills," the hornologues 

 of the tergal branchiae of the Vermes. 



The eyes are either simple or compound ; the former (ocelli or 

 stemmata), situated on the vertex, are generally three in number, 

 and are found in all orders ; compound eyes, always two, but in 

 rare cases divided or apparently so, are made up of a number 

 of cones, separated from one another by a layer of pigment, the 

 external broad end hexagonal, capped by a facet or "corneule," 

 the narrow end communicating with the optic nerve. These 

 facets vary in number; there are about 40 in Myrmecina La- 



