INTRODUCTION. 



The Ostracoda form a very sharply defined order of Crustacea, the most 

 conspicuous feature of which is the presence of a bivalved shell, into which 

 the body with its several appendages may be wholly withdrawn, the valves 

 admitting of being closed by the aid of an adductor muscle traversing the body 

 and attached to the inside of the valves at about their centres. They thereby 

 at the first sight look rather like small mussels, and their popular name "mussel- 

 shrimps" is indeed derived from that character. The only other Crustacea in 

 which a somewhat similar feature is met with, is a group of the Phyllopoda 

 (Conchostraca). The Ostracoda are however very essentially distinguished from 

 these Crustacea by the want of any distinctly defined cephalic part or head, 

 as also by the considerable reduction of the postoral limbs. Whereas in the 

 conchostracous Phyllopoda more than 20 pairs of limbs may be found behind 

 the mouth, the number of these limbs in the Ostracoda never exceeds 4 pairs 

 and may be reduced to 3 (Cytherella) and even to only 2 pairs (Polycope); 

 nor do these limbs in any case assume the uniform foliaceous appearance 

 characteristic of the Phyllopoda. As to the preceding limbs, the anterior an- 

 tennas are far less rudimentary than in the above-named Crustacea, and the 

 mandibles are always provided with distinct, in some cases very powerfully 

 developed palps, whereas such palps are wholly absent in the Phyllopoda as 

 also in the nearly allied group C/adocera, at least in the adult stage of the 

 animal. The body, as a rule, terminates in 2 juxtaposed procurved rami, in some 

 cases very movable and strongly spinous or claved at the edges, in other cases 

 however much reduced. These rami evidently ansver to the so-called "post- 

 abdomen" in the Cladocera and the conchostracous Phyllopoda, but are 

 scarcely at all homologous with the "furca" of the Copepoda, though gene- 

 rally so termed by recent authors. 



1 Crustacea. 



