2I 6 /noi.oCY FOR MEDICAL STUDENTS CHAP. 



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

 ofot: -rns. 



(2) ! is a skeleton in the ordinary sense, supporting the soft 

 tissues and giving firm attachments for the muscles by which movements 



lied (Hit. 



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



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 

 thu> 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 

 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 



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

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



men pussrsscs small but comparatively well developed appendages 

 which are u-ed tor swimming : in the lobsters and crayfishes (Fig. 102, B) 

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



'iien i- carried bent forwards beneath the cephalo-thorax they have 

 redurrd ID the verge of disappearance, except the front two pairs, 

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

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

 m 1( bo owm- to their having a sexual function for the eggs 



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



'" ' < division the same tendency is seen. In Peripalus 



>5). or in a Myriapod. the series of appendages extends without 



to the hinder end. In the most primitive insects 



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



boxes, or the little Machilis- which may often be seen running about 



