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 ■'^°' ^' 

 exhibited. Tliey occur abundantly in fresh water and in tlie 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 tlie shell which is absent in the 

 latter. 



A series of enlarged draw^ings gives some idea of the diversity 

 of form and ornamentation in the sliells of tliese minute Crustacea. 



Fig. 10. 



Shells of Ostracoda, seen from the side. A. Philomedes brenda (Myodocopa) ; 

 B. Cijpris fitscafa (Podocopa) ; C. Cythereis ornata (Podocopa) : all much 

 enlarged. n., Notch characteristic of the Myodocopa; e., the median 

 eye; a., 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 

 Laukester's " Treatise on Zoology," after Brady and Norman, and Miiller.) 



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 narrow^ed 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 \vaters 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 



