OSTRACODA AND COPEPODA. 
283 
organs. Ostracods occur both in fresh water and the sea; the best known 
forms being Cypris and Cythere. The former contains species found in ditches 
and ponds in England. When -the waters in which they live dry up, the 
species of Cypris bury themselves in the mud until rain falls; the eggs, which 
are spherical, being attached to aquatic plants. The species of Cythere are mostly 
marine, haunting rocky pools on the coast, and crawling amongst the seaweed. 
In Cypridina, on the contrary, which is also marine, the animals dart about 
with velocity; the females carrying their eggs between the valves of the shell 
attached to their feet. 
Oar-Footed Group,—O rder Copepoda. 
In the free-living members of this group, the body is elongate and segmented; 
the thorax bears four or five two-branched swimming-feet, and the abdomen is 
without appendages. A common fresh-water form is Cyclops, the structure of 
which serves as a type of that of the order. The body is broad in front and 
tapering behind, being thus pear-shaped in outline. The normal live pairs of head- 
appendages are well developed, the first pair of antennas being long and acting as 
oars. The dorsal elements of the head are fused to form a carapace, which bears 
a single eye in front and is behind united to the first 
thoracic segment, the remaining live of this region 
being free. The abdomen consists of four narrow 
cylindrical limbless segments; but the last bears a pair 
of processes severally tipped with a tuft of four long 
bristles. The eggs are carried by the mother in a 
couple of oval sacs attached to the last segment of the 
thorax, and so prolific are these creatures, that a 
female, it has been calculated, will in a year produce 
over four thousand million young. The young when 
*q ° J & COPEPODS. 
hatched is an oval NaupliuS (b), which gradually a, Female Cyclops, with egg-sacs; 
acquires the characters of the adult. Closely resem- c , Nauplius and later larva 
bling the preceding is the marine Cetocliilus, which 
is devoured in large quantities by whale-bone whales. These crustaceans are of 
a bright red colour, and when seen in myriads give the sea the appearance of being 
stained with blood. 
The Copepods hitherto noticed are spoken of as the Eucopepoda, but we now 
come to a number of genera which have taken to a parasitic life, these Epizoa, 
or Parasitica, being strangely unlike the higher forms. As one of the least 
modified types, may be mentioned the carp-louse ( Argidus ). Of the more 
degenerate types, the structure is exemplified in the annexed cut. In these the 
body may be broad and flat, as Caligus (e), which is frequently found upon the cod¬ 
fish and the brill, or long and worm-like, as in Lernceonema and Pennella {a and c), 
the former being a common parasite on the herring and sprat, while in Lerncea, the 
gill-sucker, to which Hcemobayhes (d) is allied, the body is swollen and twisted in 
the form of the letter S. The two long processes represented in the figures 
projecting from the posterior end of the body are the egg-sacs. The appendages 
