CRUSTACEA. 509 



more permanently indeed than undoubtedly ancestral forms like that of 

 Mysis and it would be still more remarkable that a Zoasa 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 appen- 

 dages up to the second pair of maxillipeds, and (?) 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 and general 

 shape were probably the same as in existing Zoseas. 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 Malacostraca 

 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 Zoaea form acquired, and that subsequently 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 Zoaea stage the Edriophthalmata and Cumaceas 

 are also post-zoaeal forms which have lost the Zoaea stage. Nebalia is 

 however very probably to be regarded as a pras-zoaeal 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 Pontellidae and of Argulus may be cited. It is however 

 more than doubtful whether the lateral eyes of the Pontellidas are related to 

 the compound Phyllopod eye, and the affinities of Argulus 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 Claus to belong to the 

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

 borne out by their larval history. The Nauplius differs very markedly from 

 that of the Copepoda, and this is still more true of the Cypris stage. The 

 Copepod-like appendages of this stage are chiefly relied upon to support the 

 above view, but this form of appendages was probably very primitive 

 and general, and the number (without taking into consideration the doubtful 

 case of Cryptophialus) does not correspond to that in Copepoda. On the 

 other hand the paired eyes and the bivalve shell form great difficulties in the 

 way of Claus' view. It js clear that the Cypris stage represents more or less 



