MITES AND TICKS. 



247 



and oval in shape, and possesses four pairs of bud-like unjointed appendages, each 

 tipped with two claws. The last pair of legs projects from the extreme hinder end 

 of the body. The mouth is situated at the opposite extremity, but the only trace of 

 jaws that it presents is a pair of stylets. There appear to be no organs of respira- 

 tion or circulation ; and, unlike what obtains in all true Arachnida, the sexes are 

 united in each individual. The second group, Linguatulina or Pentastornida, is still 

 less like the Arachnida. It includes internal parasites, which in form and mode of 

 life present many points of resemblance to the intestinal worms. The body is long, 

 broad in front, narrowed behind, and divided into a vast number of rings. Near 

 the mouth there are two pairs of strong hooks, and although these are the only 

 traces of appendages that the adult presents, the embryo is furnished in addition 

 \vith two pairs of limbs, tipped with claws. It is mainly upon the evidence 

 furnished by these limbs that the Linguatulina are regarded as degraded mites. 

 One of the best known forms is Pentastomum tcenioides, which in the adult stage 

 lives in the nasal passages of dogs and wolves. From these hosts the embryos 

 escape to the outer world mixed up with the nasal mucus. Taken into the body 

 along with the food of the hare or rabbit, they emerge from the egg, penetrate the 

 walls of the intestine, and lodge themselves in the liver. Here they become 

 encysted, grow, and go through a series of changes of form, accompanied by 

 repeated ecdyses, until they pass into a state known as Pentastomum denti- 

 culatum. If the flesh of the rodent containing P. denticulatum be devoured by 

 a dog, the parasite passes into the skull of the dog, gradually takes on the form 

 of P. tcenioides, and acquires sexual organs. Another species has been found 

 living in the lungs of the Egyptian cobra, and a third in those of a species of boa. 



R. I. POCOCK. 



A SPIDER'S SPINNERETS (greatly magnified). 



