34 THE VOYAGE OF H.M.S. CHALLENGEE. 



form branchial legs in Nehaliopsis and the thoracic legs of any Schizopocl or other 

 Malacostracan. In both these genera, however, these limbs are constructed upon the 

 same general plan, the chief agreement being found in their adaptation for solely respir- 

 atory purposes, and in so far they very materially differ from the thoracic legs in the 

 Malacostraca. In Nehalia the branchial legs hold an intermediate position and 

 consequently exhibit the most typical structure, whereas in the two above named generic 

 types they represent, as it were, the extremes of a series of modifications, apparently 

 standing in some connection with the very difi'erent conditions of life in the two, the 

 one being a shallow- water form, the other a very marked deep-water form. That these 

 limbs in Nehalia are much more closely related in structure to the branchial legs in 

 other Branchiopoda, than to the thoracic legs in the Malacostraca, cannot in my opinion 

 be disputed. It is true that they somewhat differ from the branchial legs in the 

 Phyllopoda by the want of the so-called endites or lateral lobes of the endopodite. But 

 it must be remembered, that these endites are only peculiar to the Phyllopoda, whereas 

 in other undoubted Branchiopoda, as the Cladocera, they are more or less completely 

 obsolete. Nor can it properly be adduced in support of the assumption of the nearer 

 relationship of Nehalia to the Schizopoda, that the " thoracic " legs in the former exhibit 

 the same principal parts as in the latter, since these parts, viz., the endopodite, exopodite, 

 and epipodite, may easily be distinguished in Crustacea belonging to very different 

 groups, and in most other Branchiopoda are quite so well definable as in Nehalia. The 

 number of these limbs in all the known recent Phyllocarida is eight pairs, and if we 

 consider the anterior pair as homologous with the maxillipeds in the Malacostraca, the 

 number will certainly answer to that of the thoracic legs in these Crustacea, whereas 

 their number in other Branchiopoda is very variable and at least in the Phyllopoda is 

 much greater. But on closer examination we shall find that the anterior pair of branchial 

 legs in the Nebaliidse evidently belong to the trunk and not to the cephalic part, as do 

 the maxillipeds in all the known Malacostraca, and even in the Euphausiidse, where the 

 maxillipeds are constructed upon the very same type as the succeeding legs, the 

 development shows them clearly to belong to the head and not to the trunk.^ It thus 

 results that the maxillipeds are wanting in the Nebaliidse, and that the number of the 

 limbs belonging to the trunk is in reality greater than in the Malacostraca. In the 

 Copepoda these limbs are only represented by the so-called maxillipeds, which according 

 to the suggestion of Professor Claus may properly be regarded as the outer and inner 

 branches of a single pair of limbs. 



Homology of the Pleopoda and Caudal Limbs : — The great agreement both in form 

 and composition between the two posterior divisions of the body in the Nebaliidse and 

 the so-called " thorax " and " abdomen " in the Copepoda, has been stated above. This 

 agreement becomes still more evident by a comparison of the respective appendages. 



1 See my Report on the Challenger Schizopoda, Zool. Chall, Exp., pt. xxxvii. 



