352 Proceedings of the Royal Irish Academy. 



and just as the origin of a Crustacean from a Nauplius is reasonable 

 and natural, so the origin of the Nauplius itself from some soft-skinned, 

 poorly-segmented trochopliore-like form, is quite possible. Perhaps 

 belief in the Eotifera as the connecting links between the Annelids 

 and Arthropods, will yet be justified, for tbe divergence between the 

 great main types of animal structure must have begun when all the 

 life-forms were aquatic and all microscopic. 



This view of the relationship between Arthropods and Annelids 

 throws light on the origin of the Peripatids. Before they became 

 adapted to life on land they must have been marine forms with free- 

 swimming larvse. The three pairs of ISTaupliar appendages correspond 

 with the three pairs of appendages in the head of Peripatus. There- 

 fore it is probable that the microscopic ancestors of the Malacopoda 

 had acquired three pairs of appendages, but that these had not yet 

 become jointed or chitinised. Thus the far-off ancestors of the Arthro- 

 poda gave origin to a lower soft-skinned branch, whence sprang the 

 Malacapoda, subsequently adapted to a terrestrial life ; and to a higher 

 firm-skinned, and truly " arthropodous " branch, with a definite number 

 of segments, from which have descended the Crustacea, the Insecta 

 and their allies, and the Arachnida. 



Conclusion. 



It may be convenient briefly to sum up the principal conclusions 

 set forth in this essay : — 



1. The Arthropoda are a natural, monophyletic assemblage of 



animals. 



2. There is exact numerical correspondence between the segmenta- 



tion of typical Insects, Crustaceans, and Arachnids. 



3. Such correspondence in three distinct classes cannot reasonably 



be explained as the' result of convergent evolution from 

 ancestors with very numerous segments, which indepen- 

 dently became diminished to exactly the same extent. 



4. The ancestral Arthropods must therefore have possessed a fixed 



and definite segmentation; and the various foims with 

 veiy numerous segments (Phyllopods, Millipedes, &c.) 

 have undergone abnormal elongation. 



