SECT. Ill THE APPENDACxES 39 



mandibles.^ It is difficult to see on what principle 

 this occurs ; the advantages gained by using the 

 ventral parapodia of the most powerful limbs as jaws, 

 would seem to be more than counterbalanced by the 

 disadvantage of combining locomotory with masti- 

 catory functions ; perhaps we might assume that in 

 this case the dorsal and ventral parapodia became 

 separately articulated with the body, so as to secure 

 all the advantages of the division of labour. From 

 what we know both of Crustacean and of Annelidan 

 morphology, there is no difficulty in the assumption 

 of the separate articulation with the body of two 

 parts of the same limb or parapodium. 



The Under Lips. — These, though not limbs, are best 

 described here, as they have hitherto always been mis- 

 taken for limbs or parts of limbs. They owe their 

 origin, as has already been described, to the change of 

 position of the mouth, the under edge of which must 

 naturally project backwards (as shown in Fig. I, p. 12). 

 This under lip, which was originally a straight ridge, 

 has been cut out in the middle, in order not to form a 

 barrier which would hinder the pushing of food into 

 the mouth by the maxillae. We thus find, instead of 

 the pronounced ridge right across the posterior edge 

 of the mouth, two limb-like projections, the inner 

 edges of which gradually slope down towards the 

 middle line, the remains of the ridge being easy to 

 follow from side to side in a series of sagittal sections. 



^ The different attempts of the primitive Crustacea to find the best 

 combination of limbs to function as jaws will be found in a table, 

 p. 250. 



