234 THE APODID^: PART n 



highly developed. The use of such a specialised 

 limb in the Trilobites, however, is difficult at first 

 sight to see. In Apus we find it developed as a 

 sensory organ on the principle of the division of 

 labour. In the Trilobites it is clearly locomotory, and 

 as such seems rather out of place among the smaller 

 and less powerful crawling legs of the other trunk 

 segments. In discussing the manner of life of Eury- 

 pterus and Pterygotus, we shall find that they throw 

 some light on the probable use of this limb in the 

 Trilobites. 



It is especially interesting to find the gradual 

 simplification of the limbs from front to back, 

 which is evident towards the posterior end of the 

 body (Fig. 49, cf. with the Frontispiece). There can 

 hardly be any doubt that the gradual dwindling of 

 the limbs in the Trilobites admits of the same 

 explanation as a similar dwindling of the limbs in 

 Apus. Such a singular morphological occurrence, 

 in two animals so like in other respects also, can 

 hardly be a case of analogy. 



TJie Eyes. Packard has shown that the hard part 

 of the eyes of Trilobites, which alone have been pre- 

 served in the fossils, are identical with those of Limu- 

 lus. As we have already seen in discussing the eyes 

 of Apus, we consider the eye of Limulus as a more 

 primitive stage in the development of the Crustacean 

 eye out of the Annelidan eye-spots. In this respect 

 Apus is more highly developed than both Limulus 

 and the Trilobites, as indeed we should expect from 

 its free-swimming life. 



