2IO ZOOLOGY FOR MEDICAL STUDENTS chap. 



ease with which they can be preserved, make them special favourites 

 of collectors, while many members of the group are of directly practical 

 importance to mankind by providing food material, or by destroying 

 crops and manufactured articles, or by causing bodily injury by bites or 

 stings, or by acting as carriers of disease-producing microbes. In this 

 book only the barest outline of the characters of the phylum will be 

 attempted. 



The general plan of structure of the Arthropod is a further develop- 

 ment of that seen in the Annelid. Here again the body is metamerically 

 segmented and the individual segment carries a pair of appendages : but 

 these appendages are longer and more slender and in general much more 

 highly evolved than the stump-like parapodia of the Annehd. Here 

 again, in correlation with the mode of movement, the front end of the 

 body is specialized to form a head : but the head has reached a far 

 higher degree of complexity than that of the Annelid. The central 

 nervous system with its ventral chain of ganglia and its supra- 

 oesophageal ganglionic mass is clearly of the same type'as that of the 

 Annelid. 



But the Arthropoda have diverged from the annelidan type of structure 

 so as to develop peculiarities of their own. Two of these are of funda- 

 mental importance. 



(i) The cuticle, which in the Annelid is thin and membranous except 

 where it undergoes local thickening to form a chaeta, has in the Arthropod 

 become greatly exaggerated to form an armour coating, composed of the 

 nitrogen-containing substance ehitin, covering the entire surface of the 

 body and forming ah admirable protection against the attacks ot other 

 organisms — including disease-producing microbes. We may probably 

 take it that the development of this protective coat has been one of the 

 chief factors — if not the chief factor — in enabhng the Arthropods so suc- 

 cessfully to hold their own in the struggle for existence. As will become 

 apparent in the course of this chapter, the development of the rigid 

 exoskeleton has also brought in its train important secondary results 

 which find their expression in peculiarities of structure, function, or life- 

 history. 



(2) The other fundamental feature of arthropodan organization which 

 calls for mention at this point is that the coelome has shrunk up to 

 hardly recognizable vestiges, its place as body-cavity being taken by a 

 loose spongework filled with blood and continuous with the cavity of 

 the heart. This spongework represents the network of blood-vessels, 

 which have lost their definite tubular form and become widened out into 

 indefinite irregular spaces. Such a type of body-cavity — formed of 



