960 CRUSTACEA 



number of ' fin-feet ' swim with an easy gliding movement, sometimes 

 on their back alone (as is the case with Branchipus) and sometimes 

 with equal facility on the back, belly, or sides (as is done by Artemia 

 salina, the ' brine-shrimp '). Some of the most common forms of 

 both tribes will now be briefly noticed. 



The first group contains two orders, of which the first, Ostracoda, 

 is distinguished by the complete inclosure of the body in a bivalve 

 shell, by the small number of legs, and by the absence of an external 

 egg-sac. One of the best known examples is the little Cypria, which 

 is a common inhabitant of pools and streams ; this may be recognised 

 by its possession of two pairs of antennae, the first having numerous 

 joints with a pencil-like tuft of filaments, and projecting forwards 

 from the front of the head, whilst the second has more the shape of 

 legs, and is directed downwards, and by the limitation of its legs to 

 two pairs, of which the posterior does not make its appearance outside 

 the shell, being bent upwards to give support to the ovaries. The 

 valves are generally opened widely enough to allow the greater part 

 of both pairs of antennae and of the front pair of legs to pass out 

 between them ; but when the animals are alarmed, they draw these 

 members within the shell, and close the valves firmly. They are 

 very lively creatures, being almost constantly seen in motion, either 

 swimming by the united action of their foot-like antennae and legs, 

 or walking upon plants and other solid bodies floating in the water. 

 Nearly allied to the preceding is Cy there, whose body is furnished 

 with three pairs of legs, all projecting out of the shell, and whose 

 superior antennae are destitute of the filamentous brush ; this genus 

 is almost entirely marine, and some species of it may almost in- 

 variably be met with in little pools among the rocks between the 

 tide-marks, creeping about (but not swimming) amongst Conferva' 

 and Corallines. There is abundant evidence of the former existence 

 of Crustacea of larger size than any now existing, for in certain 

 fresh-water strata, both of the Secondary and Tertiary series, we find 

 layers, sometimes of great extent and thickness, which are almost 

 entirely composed of the fossilised shells of Cyprides ; whilst in 

 certain parts of the chalk, which was a marine deposit, the remains 

 of bivalve shells resembling those of Cythere present themselves 

 in such abundance as to form a considerable part of its substance. 1 



In the order Copepoda there is a jointed shell forming a kind 

 of buckler or carapace that almost entirely incloses the head and 

 thorax, an opening being left beneath, through which the appendages 

 project ; and there are five pairs of legs, mostly adapted for swim- 

 ming, the fifth pair, however, being rudimentary in the genus Cyclops, 

 the commonest example of the group. This genus receives its name 

 from possessing only a single eye, or rather a single cluster of ocelli ; 

 which character, however, it has in common with the two genera 

 already named, as well as with Daphnia, and with many other 

 Entomostraca. It contains numerous species, some of which belong 



1 On the recent British Ostracoda see the monograph by G. S. Brady in vol. xxvi. 

 of the Transactions of the Linnean Society of London ; compare also Zenker, 

 ' Monographic der Ostracoden,' Arcliiv fur Naturg. xx. 1854. Glaus has an essay on 

 the development of Cypris, Marburg, 1868 ; see also Dr. Brady's ' Challenger Report.' 



