3214 



THE JOINTED ANIMALS 



organs of respiration or circulation; and, unlike what obtains in all true Arachnida, 

 the sexes are united in each individual. The second group, L/inguatulina or Penta- 

 stomida, 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 with two pairs of limbs, tipped with claws. It is mainly upon the 

 evidence furnished by these limbs that the L,inguatulina are regarded as degraded 

 mites. One of the best-known forms is Pentastomum t<zniotdes, 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 dentic- 

 ulatum. 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 spe- 

 cies of boa. 



A SPIDER'S SPINNERETS (greatly magnified). 



