ARTHROPODA 



I8 5 



the mouth parts are modified according to the life habits of the 

 animal into organs for biting or piercing and sucking. The 

 respiratory organs are tracheae opening by a pair of stigmata, 

 but in the water mites and some parasites there are 110 special 

 organs of respiration, — in such 

 cases respiration probably takes 

 place through the skin. There 

 is one mite that lives in the hair 

 follicles of domestic animals, 

 such as the cat, dog, cow, horse, 

 and sheep, and many also occur 

 in man ; this genus has a greatly 

 elongated, vermiform body (Fig. 

 179). The itch mites (Fig. 180) 

 live in or upon the skin of the 

 higher animals and produce the 

 diseases known as the itch and 

 the mange. Cheese mites occur 

 on animal and vegetable matter. Others live as parasites 

 on beetles, water insects, and birds. The ticks (Fig. 181 ) may 

 be regarded as blood-sucking mites ; they live in forests, and the 

 females crawl on to man and other Mammalia, sucking the blood 

 until they become greatly swollen ; they are excessively annoy- 

 ing in the tropics, where they attain a considerable size. 



We come now to three orders whose systematic position is 

 doubtful ; as they may possibly have some relationship to the 

 Arachnida, it is convenient to take them up at this point without 

 assigning them a definite position in the scheme of classification. 



Fig. 181. Dermacei/fi'r americanus, 5 .the 

 common dog tick or wood tick of the 

 United States. Natural size, about five 

 millimeters long. (Drawn from a pre- 

 served specimen.) 



Supplementary Order Linguatulida 



The Linguatulida (Lat. linguatulus, tongued), sometimes called 

 the Pentastomida (Gr. 7revTdaro/Mo<;, five-mouthed), are elongated, 

 vermiform animals, which are marked externally by a large num- 

 ber of rings ; these, however, do not indicate segmentation. The 

 adult individuals live as parasites in the nasal and some adjacent 

 cavities of some of the higher animals, such as the fox, wolf, 

 dog, tiger, lion, and exceptionally in man. The animal is very 

 degenerate in structure. The mouth is near the anterior end of 



