Ostracoda, Copepoda. 
29 
The Ostracoda (Fig. 10) are for the most part extremely Table-case 
minute animals, and only one or two of the larger species can be No ' 1 * 
exhibited. They occur abundantly in fresh water and in the sea, 
and their fossil remains are found in all geological formations from 
the oldest to the most recent. Nearly all the Ostracoda belong to 
two Orders, the Myodocopa and the Podocopa , of which the former 
may generally be distinguished by a notch (Fig. 10, n) in the 
anterior part of the margin of the shell which is absent in the 
latter. 
A series of enlarged drawings gives some idea of the diversity 
of form and ornamentation in the shells of these minute Crustacea. 
Fig. 10. 
Shells of Ostracoda, seen from the side. A. Philomedes brenda (Myodocopa) ; 
B. Cypris fuscata (Podocopa) ; C. Cythereis ornata (Podocopa) : all much 
enlarged. n., Notch characteristic of the Myodocopa; e., the median 
eye; ci., mark of attachment of the muscle connecting the two valves of 
the shell. A. and C. are marine species, B. is from fresh water. (From 
Lankester’s “ Treatise on Zoology,” after Brady and Norman, and Muller.) 
Sub-class III. — COPEPODA. 
There are, at most, ten free somites behind the head. The Table-case 
carapace is reduced or absent. The first thoracic limbs form No - 2 - 
maxilipeds, and are followed by four or five pairs of two-branched 
swimming feet. The posterior region of the body (the so-called 
“abdomen”) is generally narrowed and is without limbs, but the 
terminal segment carries a pair of appendages, forming the 
“ caudal fork.” 
Many Copepoda are found in fresh water, but the majority 
inhabit the sea, where they are often extremely abundant. They 
form one of the most important constituents of the “plankton,” 
the assemblage of floating organisms in the waters of the open 
ocean. Since it is chiefly on this plankton that all the other 
inhabitants of the sea ultimately depend for food, it may be said 
that the Copepoda, notwithstanding their small size, play a more 
