INTRODUCTION 9 



remark applies to the " hairs," " bristles," " spines," and 

 " scales " that clothe or fortify the bodies of many 

 Arthropoda : all these are merely hollow outgrowths of the 

 chitinous cuticle secreted by processes of epidermic cells ; 

 but so long as this fact is not forgotten there can be no 

 impropriety in calling them " hairs " and so forth. Even 

 those who use the terms seta (Lat. j^^a = bristle) and chceta 

 (xa'V); = hair) for the hairs and bristles, have no other name 

 than " scales " for the broad flat setae of the wings of butter- 

 flies and mosquitoes. 



Not only does the cuticle form an exoskeleton, but it 

 also sends processes inwards to form the tendons of the 

 muscles, the lining of certain parts of the alimentary canal 

 and genital ducts, the inner layer of the air-tubes (tracheae) 

 when these exist, and other internal supports. 



The digestive, respiratory, excretory, and reproductive 

 systems differ in the different classes of Arthropoda, but the 

 vascular and nervous systems have a general unity and 

 constancy throughout the phylum. One general fact about 

 the digestive system must, however, be noted ; namely, that 

 in all normal Arthropods certain paired appendages near 

 the mouth are differentiated, wholly or partly, to form jaws 

 or foot-jaws. 



The body-cavity contains blood, and so forms part of 

 the vascular system ; but a heart and blood-vessels are 

 usually present. 



The heart is dorsal in position and is enclosed, more or 

 less completely, in a pericardium ; it has no directly afferent 

 vessels, but is filled from the pericardial cavity, the blood 

 passing through valvular slits in the heart-wall. 



The blood-vessels vary in degree of development. In 

 many Arthropods the circulation is more or less lacunar, the 

 blood being driven from the heart, by a few short efferent 

 vessels, into the tissues, whence it passes, either by distinct 

 vessels or by mere vague afferent currents, to the pericardial 

 cavity, and so back to the heart again. In some Arthropods, 

 however, particularly, though not exclusively, in certain 

 larger forms that breathe by gills, there are well-developed 

 arteries and veins which may even be connected by capillaries. 



The blood is usually colourless, but in some Arthropods 



