THE MALACOSTRACA 147 



endopodite known as the appendix interna. The sixth pair of 

 abdominal appendages, known as the uropods, are larger than the 

 others, with a short, unsegmented protopodite and broad lamellar 

 rami which lie at the sides of the telson and form with it the 

 " tail-fan " which is used in swimming, or rather springing, back- 

 wards by sudden flexion of the whole abdomen. 



The Leptostraca alone among the more primitive orders of 

 existing Malacostraca stand apart from the scheme outlined above, 

 and seem to have diverged from the main line of Malacostracan 

 descent before the assumption of tho caridoid form. Their 

 systematic relations will be discussed more fully later. 



CLASSIFICATION OF THE MALACOSTRACA. 



The group Malacostraca, established by Latreille in 1806, has 

 been accepted as a natural division by nearly all subsequent 

 writers. Almost the only divergences of opinion as to its limits 

 have had reference to the Leptostraca, which many zoologists 

 following Milne -Ed wards have referred to the Branchiopoda or 

 have regarded as occupying an intermediate place between 

 Malacostraca and "Entomostraca." Claus's investigations on the 

 structure of Nebalia, however, have been generally accepted as 

 demonstrating its Malacostracan affinities. In the arrangement 

 here adopted (following Grobben) the order Nebaliacea is included 

 within the sub-class Malacostraca, but its distinctness from the 

 other orders is marked by placing it in a separate division 

 (Leptostraca) opposed to the other orders grouped together as 

 Eumalacostraca. 



In the arrangement of the Eumalacostraca most carcinologists 

 hitherto have followed the lines laid down by Leach, who, in 1815, 

 divided the group into two legions the Podophthalma and the 

 Edriophthalma according to the condition of the eyes, movably 

 pedunculate in the one and sessile in the other. As originally 

 defined, the two groups were also distinguished from each other 

 by the presence in the Podophthalma of a carapace which was 

 absent in the Edriophthalma, this character giving occasion for the 

 names Thoracostraca and Arthrostraca applied to the same groups 

 by Burmeister in 1834. The progress of research, however, 

 rendered it increasingly difficult to frame satisfactory definitions 

 of the two divisions. Thus, the sessile-eyed Tanaidacea were 

 found to possess a true, though reduced, carapace, while the 

 Cumacea were still more plainly intermediate between the two 

 groups. The recent discovery of the very remarkable genus Anaspides, 

 which has stalked eyes but no carapace, and the closely allied 

 Koonunga with sessile eyes, makes the retention of the old arrange- 

 ment quite impossible. 



