THE APPENDICULATE PHYLUM 51 



living animals, Nebalia and Anaspides, of especial 

 interest for us, as affording links with the ancient 

 past, and throwing some light on the course of 

 Crustacean evolution. The Crustacea fall into two 

 sub-classes, the Entomostraca, containing small animals 

 with an indefinite number of segments (from eight to 

 over a hundred), and the Malacostraca, including the 

 larger and more highly organized forms, in which the 

 number of segments, not counting the tail-piece, is 

 always nineteen. Nebalia, a small marine animal of 

 world-wide distribution, usually living on the sea- 

 shore at low-tide marks under stones or in the sand, 

 exhibits a generalized organization which might allow 

 of its inclusion in either of the two Crustacean 

 groups; it is neither decidedly Entomostracan nor 

 Malacostracan, but it exhibits the typical number of 

 Malacostracan segments, except that an additional 

 segment is present in the tail region. The limbs, 

 which are all clearly of the biramous plan, and 

 the general anatomy, indicate a primitive and un- 

 specialized condition ; the possession of two pairs of 

 excretory organs in the adult opening on the second 

 antennae and the second maxillae respectively is of 

 especial interest, since both these organs do not 

 persist in the adult of any other Crustacean, though 

 every Crustacean possesses either one or the other at 

 some period of its life. In fact Nebalia must be 

 looked upon as a Malacostracan which has retained 



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