VI ARTHROPODA 



231 



which was discredited for a time, that wings may have evolved out of 

 such " tracheal gills " on the thoracic segments which have become more 

 and more freely movable so as to function as organs of propulsion. 



The alimentary canal of the Arthropoda is characterized by the 

 tendency of the cuticle of the outer surface to be prolonged inwards as 

 a protective lining. This is brought about by an ingrowth of ectoderm 

 at the mouth end forming a stomodaeum and a similar ingrowth at the 

 anal end forming a proetodaeum. It is a quite usual feature in animals 

 for the tubular alimentary canal to have such an ectodermal section at 

 each end. But in the. Arthropoda these tend to be more highly developed 

 than in other groups — stomodaeum and proetodaeum encroaching more 

 and more, while the original endoderm-lined alimentary canal or mesen- 

 teron undergoes a corresponding shrinkage. This shrinkage is well 

 exemplified by the case of the Crayfish or Lobster, where careless dis- 

 section commonly results in the alimentary canal breaking through the 

 short mesenteric portion owing to this being devoid of the tough cuticular 

 lining elsewhere present. 



In the Insects there are usually " salivary " glands opening in the 

 neighbourhood of the mouth. No doubt these originally fulfilled a 

 function in connexion with the preparation of the food for digestion, but 

 in special cases they have become specialized for other functions. Thus 

 in blood-sucking insects the irritating secretion helps to promote the 

 flow of blood to the wound. In the larvae of various insects the secretion 

 is silk. 



The haemocoelic character of the body-cavity has already been alluded 

 to as a highly characteristic feature of the Arthropoda. It may be that 

 the degeneration of the blood-vessels to form such a body-cavity arose in 

 evolution in relation to a pecuUar type of respiratory system — ^that found 

 in the insects. In these arthropods the process of respiratory exchange 

 with the external medium takes place through parts of the cuticle which 

 have not undergone the thickening occurring elsewhere but have remained 

 extremely thin. Wherever in the animal kingdom there are such portions 

 of the surface specially devoted to respiratory exchange it is usual for 

 the progress of evolution to bring about increase in the area of these 

 respiratory surfaces. Most usually this increase is brought about by the 

 surface sprouting out into more or less elaborately branched projections 

 — the gills. In the insects the increase on the other hand has been 

 brought about by growth of the surface not outwards but inwards. 

 Each respiratory patch has come to dip down as a deep pocket into the 

 interior of the body, and this pocket has had its lining enormously 



