256 ZOOLOGY FOR MEDICAL STUDENTS chap. 



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

 resemblance to Lice but unlike these animals they have mouth parts 

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

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

 acts as host for the cysticercoid stage of the tapeworm Dipylidium 



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

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

 existence 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 

 of Limulus leads us to conclude that the ancestors of the lung-breathing 

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

 ever their habit may have been at a still earlier period of evolutionary 

 history. 



The XiPHOSUEA are represented at the present day by the King-crab 

 (^Limulus)) which lives on sandy bottoms in shallow water off the eastern 

 coasts 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 

 creatures 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 

 defence — e.g. when a foot is incautiously thrust into a boot into which a 

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

 serious. 



The Araneae are the Spiders, recognizable by the plump soft-skinned 

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

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

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

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

 effects even to man. One of the characteristic structural features of tlic 

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

 large part of the opisthosoma. The silk is used for the construction of 

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

 The different 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 

 the form 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 



