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, — Order 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 antennae 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 five 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 

 hatched is an oval Nauplius (b), which gradually 

 acquires the characters of the adult. Closely resem- 

 bling the preceding is the marine Cetochilus, which 

 is devoured in large quantities by whale-bone whales. 

 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 (Argulus). 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 Lemceonema and Pennella (a and c), 

 the former being a common parasite on the herring and sprat, while in Lerncea, the 

 gill-sucker, to which Hcemobaphes (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 



COPEPODS. 

 a, Female Cyclops, with egg-sacs ; 

 b, c, Nauplius and later larva 

 of same. 



These crustaceans are of 



