SECT, xil RELATION OF APUS TO CRUSTACEA 195 



been forced outwards and forwards, and the under lip 

 backwards, so as to admit of the working of the five 

 pairs of jaws between them. If it at first sight seems 

 unlikely that the paragnatha should move so far back 

 as to come behind the first pair of trunk feet, it must be 

 remembered that, when the mouth of the Crustacean- 

 Annelid first stretched out so as to admit of the working 

 of the five ventral parapodia as jaws, the parts were 

 more flexible. It is also some confirmation of this 

 homology to find that the sternal plate, the sinewy 

 mass of the musculature originally in the angle of the 

 bent intestine and thus close above the under lip, is 

 also drawn back as far as this under lip, showing that 

 the whole region has been drawn out of its original 

 shape. The origin of the division of the under lip 

 has been already explained (p. 40). 



In Limulus, as already said, we do not find, as in 

 Apus, a gradual change in the limbs from the more 

 Crustacean form anteriorly to the more Annelidan, 

 i.e. parapodial, posteriorly. The transition is sudden. 

 The limb corresponding with the second trunk limb 

 of Apus forms the flat operculum to cover the follow- 

 ing five pairs of Phyllopodan (i.e. parapodia-like) 

 limbs. 



The typical parts of these abdominal limbs can still 

 be more or less clearly recognised. The ventral para- 

 podia of each pair of limbs have fused in the middle 

 line, forming the basal plate ; the dorsal parapodium 

 is represented by a row of four joints approaching 

 the middle line (see Fig. 45). On the outside of 

 these come the large gill and somewhat smaller flabel- 



O 2 



