700 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxv. 



segments are free, and the abdomen is unsegmented. On the con- 

 trary, the onl}" Phyllopod having a head shield is Apus, and even here 

 the first thoracic segment is not fused with the head. The Phyllopod 

 thorax contains many segments and the abdomen is also segmented. 



^. They have the same number and grouping of the appendages as 

 would naturally follow from the similarity in segmentation, and this 

 grouping is entirely difierent from that in the Phyllopods. 



3. They have two pairs of antenna?, one of which has been modified 

 into fixing organs. 



4. They have two pairs of maxillipeds, which in the larval state are 

 quite like those of the Siphonostoma in form and function and are 

 presumably of the same origin. That the anterior pair is afterwards 

 modified into sucking disks in no way affects their relationship. It is 

 what might be expected as a result of their parasitic habits. There 

 are no such posterior maxillary appendages in any of the Phyllopods. 



5. The mouth apparatus consists in part of a proboscis formed from 

 the lips and jaws in a very similar way to that of the Siphonostoma 

 and bearing no resemblance to that of the Phyllopods. The other 

 part, the sting with its poison glands, is a distinctive organ, found in 

 no other Crustacean group, and therefore to bo left out of account as 

 of no value in showing relationship. 



6. The ovar}^ is unpaired even from its beginning and though the 

 oviduct is at first paired, one side is afterwards atrophied. The females 

 also possess semen receptacles which differ somewhat in size and 

 arrangement from those of the Eucopepods, as would be expected. But 

 their general position, structure, and mode of operation is the same. 

 In Phyllopods the ovaries are nearl}^ always paired, at least in the 

 larval period, and the semen receptacles, when present, are entirely 

 different in position and mode of operation. 



7. The swimming legs are elongated, two-branched appendages with 

 distincth' segmented basipods, and long endopods and exopods, fur- 

 nished with plumose setffi. On the two anterior legs about half the 

 species have a flagellum attached to the basipod and pointing inward 

 dorsally. There is nothing here which corresponds even in the 

 remotest degree to the characteristic Phyllopod foot with its unjointed 

 stem, its six inner lobes or appendages (endites), and its outer flat 

 respiratory plate and sac-like branchial appendage. The Argulida? 

 breathe by means of their flattened carapace and abdomen, the Phyl- 

 lopods by means of their gill-feec. 



Such reseml)lances are conclusive, and in view of the further fact 

 that the only particulars in which the Argulidfe differ at all essentially 

 from the PLucopepods are such as would naturally be expected in two 

 parasites, even if they were closeh' related, we may confidently adopt 

 Claus\s classification as correct. 



I 



