210 



LOGY FOR MEDICAL STUDENTS CHAP. 



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 



manufactured articles, or by causing bodily injury by bites or 



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



. tin- barest outline of the characters of the phylum will be 



attcm] 



Tin- ueneral plan of structure of the Arthropod is a further develop- 

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

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

 appendages are longer and more slender and in general much more 

 hjulilv evolved than the stump-like parapodia of the Annelid. Here 

 i. in correlation with the mode of movement, the front end of the 

 oecialized to form a head : but the head has reached a far 

 .er 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 

 18 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 chitin, covering the entire surface of the 



body and forming an admirable protection against the attacks of other 



.nisms including disease-producing microbes. We may probably 



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



: ors if not the chief factor in enabling the Arthropods so suc- 



uilly t<> hold their own in the struggle for existence. As will become 



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



ton has also brought in its train important secondary results 



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



(2) The other fundamental feature of arthropodan organization which 



alls tor mention ;it this point is that the coelome has shrunk up to 



. its place as body-cavity being taken by a 



<>rk lilled with blood and continuous with the cavity of 



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



ir definite tubular form and become widened out into 



indefinite irregular spaces. Sudi a type of body-cavity formed of 



