2i6 ZOOLOGY FOR MEDICAL STUDENTS chap. 



desiccation, from the effects of harmful substances, and from the attacks 

 of other organisms. 



(2) It serves as a skeleton in the ordinary sense, supporting the soft 

 tissues and giving firm attachments for the muscles by which movements 

 are carried out. 



(3) Formed as it is of chitin — a compound of carbon, hydrogen, 

 oxygen, and nitrogen — it constitutes so much waste, or excretory, 

 nitrogenous material which, instead of being got rid of as soon as formed, 

 is deposited in the cuticle and cast off periodically at the ecdyses. It 

 thus plays an important part in the function of nitrogenous excretion. 



While the hard jointed exoskeleton is highly characteristic of the 

 typical arthropod, yet under certain conditions it is liable to revert to 

 a thin flexible condition. Thus in Hermit Crabs this has happened on 

 the hinder, abdominal portion of the body, which is kept tucked snugly 

 away in the shelter of a Gasteropod (p. 267) shell. Again, in arthropods 

 which have assumed parasitic habits the same may happen over the 

 whole surface of the body. 



Appendages 



The limbs of the Arthropoda are primitively numerous, arranged in 

 pairs, one to each segment of the body, and alike. But it has been a 

 characteristic feature in the evolution of the phylum that the appendages 

 towards the hinder end of the body have tended to become reduced and 

 indeed to disappear entirely. Thus in the prawns and shrimps the 

 abdomen possesses small but comparatively well developed appendages 

 which are used for swimming : in the lobsters and crayfishes (Fig. 102, B) 

 these have become further reduced in size : in male crabs — in which the 

 abdomen is carried bent forwards beneath the cephalo-thorax — they have 

 been reduced to the verge of disappearance, except the front two pairs, 

 which have been preserved owing to their performing a sexual function. 

 In the female crab the whole series has been preserved from reduction, 

 in this case also owing to their having a sexual function — for the eggs 

 are carried about cemented on to the bristles which project from these 

 appendages. 



In the tracheate division the same tendency is seen. In Peripaius 

 (Fig. 95), or in a Myriapod, the series of appendages extends without 

 any reduction in size to the hinder end. In the most primitive insects — 

 such as the " Silver fish " (Lepisma), sometimes imported with sugar 

 boxes, of the little Machilis — which may often be seen running about 



