12 



THE 



PART 1 



Owing to the bending on itself of the cylindrical 

 Annelidan body, the original head must have been 

 anteriorly almost completely hemispherical (Figs, i and 

 46). This form of the original Crustacean-Annelid head 



< 6 



3 -J 



9-' 



FIG. i. Diagram showing the first six segments of a carnivorous Annelid ; the first 

 five being bent round ; /, the prostomium with two pairs of eyes and a median 

 cirrus, i, the ist segment carrying a pair of antennae, its under edge projecting 

 backwards as the lower lip(/). 2, the 2nd segment with a pair of antennal para- 

 podia. 3, the 3rd segment with rudimentary dorsal parapodia, the ventral 

 parapodia developing into mandibles. 4, the 4th segment with a pair of maxilla;, 

 the dorsal parapodium slightly less degenerated. 5, the 5th and last head 

 segment, the dorsal parapodium with large aciculum and gland. 6, the 6th 

 segment (ist free segment) with large dorsal parapodium carrying gill (g) and 

 sensory cirrus (c). 



was, however r very clearly modified. In most Trilobites 

 traces of it are still visible in the glabella (Fig. 47, p. 2 1 3). 

 In Apus, the disguise is very complete, the whole head 



