SECT. II THE OUTER BODY FORM 25 



Apodidae are but slightly modified Annelids, the 

 widening of the head being due simply to a fold of 

 the skin ; and the cuticle being for the most part thin 

 like that of the Annelids, showing, however, localised 

 thickenings in which we can recognise the commence- 

 ment of the Crustacean exoskeleton. 



The hinder part of the body — the trunk— has long 

 been recognised as worm-like, but we here sec that 

 the front or head part is also essentially Annelidan, 

 especially in its possession of a prostomium or upper 

 lip. The head of Apus differs from that of our 

 Annelid only in the development of the shield and 

 of the ridge-like fold which gives the head its great 

 breadth. 



The Annelid character of the Apodidae, thus visible 

 in the form of the body {i.e. of both head and trunk), 

 will be even more clearly seen when we come to 

 consider the appendages. 



