Classification of the Crustacea. 449 



The Ostracoda. 



If the derivation of the Cladocera from Estheria succeeds 

 witliout any difficulty, tlie same cannot be said of the deriva- 

 tion of the Ostracoda. Yet even in this case it is possible to 

 obtain a sufficient number of connecting-links, sucli as may 

 support the derivation of the Ostracoda from an Archi- 

 phyllopod form belonging to the Estheria-type. 



In considering this question our attention must again in 

 the first instance be directed to those forms of Ostracoda 

 \vhich appear to be the most primitive. These are to be 

 found in Ci/pridina. 



The first feature of the Ostracod body which strikes us is 

 the complete enclosure of the laterally-compressed trunk by a 

 large bivalve shell, which is closed by a muscle. On making 

 an examination of the Euphyllopods, we find the same deve- 

 lopment of shell and the lateral compression of the body 

 among the Estheridai. The number of the body-segments is 

 very small in the Ostracoda, and from reasons which have 

 already been discussed this must be regarded as an instance 

 of reduction from the number which were present in a more 

 richly segmented ancestral form. The development of the 

 posterior end of the body as a ventrally flexed furca, provided 

 with hooks directed backwards, shows the entire agreement 

 with the Estheridge. 



Among the appendages the first antenna appears to have a 

 sensory character ; yet in shape it is always similar to the 

 appendages which subserve locomotion, and is similarly 

 employed — a condition which, with reference to the original 

 significance of the first antenna as a sense-organ, must be 

 regarded as of a secondary character. The second antenna 

 exhibits in Cypridina and likewise in Eahcypris in a modified 

 degree the form of the biramous swimming-foot-antenna as 

 it persists among the Euphyllopoda in the adult condition in 

 the Estheridoe alone, and appears as the most important 

 organ devoted to the movement of swimming. 



A great difference from the Estherida3 as well as from all 

 the other Euphyllopods is seen in the shape of the mandibles 

 and in the appendages of the Ostracoda which correspond to 

 the two maxillae of the Euphyllopods. The mandible is 

 always provided with a foot-shaped palp, while this is 

 wanting in all Euphyllopods at the period of the complete 

 development of the body. As regards the appendages which 

 are the homologues of the two maxillae of the Euphyllopods, 

 in the Ostracoda only the first of these is developed as a 



Ann. & Mag. N. Hist. Ser. 6. Vol. xi. 33 



