332 Proceedings of the Royal Irish Academy. 



fact that in Scolopendrella, despite its marked Insectan affinities, the 

 genital opening is far forward, as in Millipedes. Therefore the differ- 

 ence i^. the position of the genital opening cannot by itself indicate 

 a very radical divergence. Although Centipedes are more nearly 

 related to Insects than to Millipedes, it is likely that the kinship of 

 Insects to the two classes of "myriapods" is equally close. The 

 Insects represent the main stem, the Centipedes and Millipedes two 

 divergent branches. 



Relationship letween the Orders of Crustacea. 



Turning next to consider the probable nature of the most primitive 

 Crustaceans, we find the prevailing opinion among modern zoologists 

 to be that the Phyllopoda, as exemplified by the many-segmented 

 Apus and Branchipus, represent, more nearly than any other living 

 order, the ancestral stock of the class. According to this view, the 

 evolution of the Crustacea has been effected by a reduction in the 

 number of body-segments until the definite and limited number 

 characteristic of the higher orders (Malacostraca) has been reached. 

 But Packard ('82), Sars ('87), and Hartog ('88) have argued that the 

 Copepoda are more primitive than the Phyllopoda. 



Now if we consider the lower orders (Entomostraca) as a whole, 

 we are struck by the quite exceptional presence of a rich segmenta- 

 tion. In the Phyllopoda the number of pairs of legs may vary from 

 four to over sixty ; whilst, in the other recent orders of the Entomo- 

 straca, the limb-bearing segments are always few. "We now know that 

 Phyllopods, closely related to Apus (Protocaris), and Ostracods, had 

 already been differentiated in the Cambrian period. Therefore, what- 

 ever may have been the segmentation of the primitive Crustaceans, 

 there had been gi'eat modification before the dawn of the earliest life- 

 epoch known to us by fossil evidence. No doubt can be entertained 

 that such poorly-developed segmentation as is shown by the Ostracods 

 must be due to reduction. But has such reduction been the constant 

 mle in Crustacean development ? It may be of interest to consider in 

 this connexion that most ancient of Crustacean orders known to us — 

 the Trilobita. 



Nearly all Trilobites are composed of a number of segments greater 

 than that characterising the Malacostraca. After the recent researches 

 of Beecher ('00) and others, there can be no reasonable doubt that 

 these animals were true Crustacea, and that they combined to some 

 extent the characters of the Branchiopoda and the lower Malacostraca. 



