THE COPEPODA 101 



parasitic species attain a greater size, the largest (Pennella) being 

 more than a foot in length. 



No fossil remains of Copepoda are known. 



AFFINITIES AND CLASSIFICATION. 



On the hypothesis that the Nauplius represents the ancestral 

 type of the Crustacea, the Eucopepoda would be regarded as the 

 most primitive existing members of the class, retaining, as they 

 do, naupliar characters in the form of the first three pairs of 

 appendages and in the absence of paired eyes and of a shell-fold. 

 As already indicated, however, it is much more probable that they 

 are to be regarded as a specialised and in some respects degenerate 

 group which, while retaining, in some cases, a very primitive 

 structure of the cephalic appendages, has diverged from the 

 ancestral stock in the reduction of the number of somites, the loss 

 of the paired eyes and shell-fold, and the simplified form of the 

 trunk-limbs. The prevalence of parasitism and the great structural 

 changes associated therewith render the classification of the Euco- 

 pepoda a matter of peculiar difficulty, and none of the schemes 

 hitherto proposed is altogether satisfactory. In Claus's system 

 (1880) the distinction drawn by most of the older authors between 

 free-living and parasitic forms is still maintained in the two chief 

 divisions of Gnathostomata and Siphonostomata, the latter includ- 

 ing forms in which the mouth -parts are more or less distinctly 

 suctorial. Giesbrecht has shown, however, that this arrangement, 

 and also that proposed by Canu, based on the copulatory pores of 

 the female, are quite unnatural, since forms with biting and suc- 

 torial mouth-parts and with paired and unpaired copulatory pores 

 may occur within the limits of the same family. Giesbrecht's own 

 classification of the Eucopepoda, which is given below, marks a 

 distinct advance, especially as regards the separation of the more 

 primitive pelagic families to form the group Gymnoplea. His 

 arrangement of the remaining families, which he groups together as 

 Podoplea, is, however, less convincing, and he does not attempt to 

 define the position of many of the parasitic forms. The system is 

 therefore incomplete and can only be adopted as a temporary 

 expedient pending further investigation. 



Some modern writers follow Zenker and Thorell in referring 

 the Branchiura to the Branchiopoda, although the only character 

 which can now be referred to in support of this arrangement is the 

 presence of paired compound eyes. On the other hand, the com- 

 parison instituted by Glaus between the appendages of Branchiura 

 and Eucopepoda shows a general similarity of structure which can- 

 not be disregarded. The only serious difficulty in the way of this 

 comparison is the difference in the position of the maxillary gland, 



