CRUSTACEA. 423 



The points of agreement in the number and character of the appendages, 

 form of the abdomen, etc. between the various types of Zosea appear to me 

 too striking to be explained in the manner attempted by Glaus. It 

 seems improbable that a peculiarity of form acquired by the larva of some 

 ancestral Malacostracan should have been retained so permanently in so 

 many groups 1 more permanently indeed than undoubtedly ancestral forms 

 like that of Mysis and it would be still more remarkable that a Zosea 

 form should have been two or more times independently developed. 



There are perhaps not sufficient materials to reconstruct the characters 

 of the Zoaea ancestor, but it probably was provided with the anterior ap- 

 pendages up to the second pair of maxillipeds, and (1) with abdominal swim- 

 ming feet. The heart may very likely have been many-chambered. Whether 

 gill pouches were present on the maxillipeds and abdominal feet does not 

 appear to me capable of being decided. The carapace aud general shape 

 were probably the same as in existing Zoreas. It must be left an open 

 question whether the six hindermost thoracic appendages were absent or 

 only very much reduced in size. 



On the whole then it may "be regarded as probable that the Mala- 

 costraca are descended from Protophyllopod forms, in which, on the 

 adoption of swimming habits, six appendages of the middle region of the 

 body were reduced or aborted, and a Zoa?a form acquired, and that sub- 

 sequently the lost appendages were redeveloped in the descendants of these 

 forms, and have finally become the most typical appendages of the group. 



The relationship of the various Malacostracan groups is too difficult a 

 subject to be discussed here, but it seems to me most likely that in addition 

 to the groups with a Zorea stage the Edriophthalrnata and Cuniacese are also 

 post-zopeal forms which have lost the Zoa?a stage. Nebalia is however very 

 probably to be regarded as a pree-zoa^al form which has survived to the 

 present day ; and one might easily fancy that its eight thin thoracic 

 segments with their small Phyllopod-like feet might become nearly 

 aborted. 



Copepoda. The Copepoda certainly appear to have diverged very 

 early from the main stem, as is shewn by their simple biramous feet and 

 the retention of the median eye as the sole organ of vision. It may be 

 argued that they have lost the eye by retrogressive changes, and in favour 

 of this view cases of the Pontellidaj and of Argulus may be cited. It is 

 however more than doubtful whether the lateral eyes of the Pontellidre 

 are related to the compound Phyllopod eye, and the affinities of Argiilus 

 are still uncertain. It would moreover be a great paradox if in a large 

 group of Crustacea the lateral eyes had been retained in a parasitic form 

 only (Argulus), but lost in all the free forms. 



Cirripedia. The Cirripedia are believed by Glaus to belong to 

 the same phylum as the Gopepocla. This view does not appear to be 

 completely borne out by their larval history. The Nauplius differs very 

 markedly from that of the Gopepoda, and this is still more true of the 

 Gypris stage. The Copepod-like appendages of this stage are chiefly relied 

 upon to support the above view, but this form of appendage was probably 



1 A secondary Larval form is less likely to be repeated in development than an 

 ancestral adult stage, because there is always a strong tendency for the former, which 

 is a secondarily intercalated link in the chain, to drop out by the occurrence of a 

 reversion to the original type of development. 



