ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



of Birds and Mammals. As the name indicates they have a general 



ahlance to Lice but unlike these animals they have mouth parts 



[or biting, not for sucking blood. A species of Trichodectes is 



i the common parasites of the Dog and, as will be remembered, it 



as host for the cysticercoid stage of the tapeworm Dipylidium 



:ini. 



IV. The ARACHNIDA include a number of very different -looking 

 types of animal, all of them adapted to an air-breathing terrestrial 

 , nee with the exception of the King-crab (Limulus). The fact that 

 the lungs of the air-breathing arachnids in early stages of their develop- 

 ment resemble, as mentioned on p. 223, the appendage-borne gill-books 

 n nl us leads us to conclude that the ancestors of the lung-breathing 

 Imids of to-day passed through an aquatic phase of evolution, what- 

 iheir habit may have been at a still earlier period of evolutionary 

 history. 



The Xii-iiosuRA are represented at the present day by the King-crab 

 (Limulns), which lives on sandy bottoms in shallow water off the eastern 

 is of North America and Asia an interesting example of the dis- 

 continuous geographical distribution often met with in ancient types of 

 animal life. 



The SCORPIONIDEA include the many species of scorpion, insect-eating 



itures of nocturnal habit, found in all warm countries. They are of 



practical interest from the conversion of the telson into a sting by which 



poison is injected into the prey. The sting is also made use of for 



when a foot is incautiously thrust into a boot into which a 



ion has retired for the day, and the effects of the poison may be 



ous. 



Thr ARANEAE are the Spiders, recognizable by the plump soft-skinned 



opisthosoma attached to the prosoma by a narrow " waist." They also 



provided with poison-glands, but these open at the tip of the first 



pair ot appendages (chelicerae). The poison is powerful and the bite of 



nil species in different parts of the world is credited with serious 



-en to man. One of the characteristic structural features of the 



is the complicated arrangement of silk-glands which fill up a 



rt ot the opisthosoma. The silk is used for the construction of 



egg-bags, nests, or more or less elaborate webs for the capture of flies. 



rent silk-glands differ in the quality of their product and in the 



case of the more elaborate webs the last portion to be constructed is in 



of a fine thread carried round and round the web in a polygonal 



pattern and composed of a highly elastic core with a coating of very 



