RECENT BRITISH OSTRACODA. 



355 



Mr. D. O. DreAYett, of Jarrow. My thanks are also especially due to Dr. Baird, of the 

 British Museum, Professor T. Bupert Jones, the Bev. Alfred Merle Norman, Dr. Alcock, 

 of Manchester, and Mr. David Bohertson, of Glasgow, for much valuable advice and 

 assistance, and for the kind and liberal manner in which they have placed their collec- 

 tions at my disposal ; lastly I owe similar thanks to M. Bosquet, of Maestricht, Dr. 

 Oscar Speyer, of Hesse Cassel, and Herr G. O. Sars, of Christiania, for the good service 

 which they have done me by the communication of their admirable memoirs and series 

 of illustrative specimens. I am fully conscious that whatever value this monograph may 

 possess is owing in great measure to the generosity of these naturalists, and their kind 

 interest in the progress of my work. 



The recent Ostracoda are divided by Sars into four great groups or sections, namely : — 

 (1) Podocopa, including the two families Cypridse and Cytheridse; (2) Myodocopa, 

 including the families Cypridinidse and Conchoeciada? ; (3) Cladocopa, containing one 

 family, Polycopidse ; and (4) Platycopa, containing also one family, Cytherellidse. 



The characters of these four sections may (after Sars) be stated as follows : — 



1. Podocopa. — This is by far the most extensive of the four sections, including all the 

 freshwater, and a vast majority of the marine Ostracoda, and embracing all the forms 

 classed by the earlier writers under the two great genera Cypris and Oytliere. The 

 lower antennae are here simple, pediform, geniculate, armed at the apex with sharp claws, 

 and are used for swimming (as in Cypris), for Avalking (as in Cy there), or as prehensile 

 organs. The first pair of appendages following the mouth is always a distinct maxilla, 

 bearing a large halfmoon-shaped branchial plate, which is bordered with numerous 

 ciliated setse. The next pair of appendages forms in the Cypridse a jaw of somewhat 

 similar shape, but in the Cytheridas becomes pediform, owing to the atrophy of the jaAV 

 proper and the greater development of the palp. Of the two following pairs of limbs, 

 the last is found, in the Cypridse, to have lost its use as a locomotiA^e organ, and is doubled 

 up between the valves, whilst in the Cytheridse it is used, like the foregoing pair, as a 

 walking limb. The postabdomen is, in the Cytheridse, rudimentary, but in the Cypridae 

 is mostly well developed, consisting of two elongated laminar processes, lying close 

 together, and each armed at the extremity with two long claws. The eyes are simple, 

 often so close together as to appear single. The heart is always wanting ; the digestive 

 cavity has two dilatations, of which the foremost is (in the Cypridse) provided with two 

 lateral blind sacs. In this last family the organs of generation are produced between 

 the two laminae of the shell. These animals live for the most part an idle life, crawling 

 leisurely on the mud, on plants, or swimming through the water. Their motions are 

 effected by the two pairs of antennae, which move synchronously, the upper pair moving 

 up and down, the lower backwards and forwards, thus propelling the whole animal in a 

 straight line. 



2. Myodocopa. — This group comprises the forms of which the genus Cypridina is the 

 type, the characters indicating a higher organization and presenting well-marked dif- 

 ferences, which show an approach to the higher order Branchiopoda. As a rule the 

 lower antennae are here the only true locomotive organs, whilst the upper antennae (as 

 in the Branchiopoda) wholly lose their importance in this respect, and become the seats 



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