AETHBOPODA — CBUSTACEA. 
93 
in which they are united, in the limbs which they bear, and 
in the extent to which some of them are covered by a cara- 
pace, the Crustacea manifest mncli diversity. They may 
be divided into five Sub-Classes: Branchiopoda, Ostracoda, 
Copepoda, Cirripedia, Malacostraca. 
The BRANCHIOPODA (gill-feet) are represented among 
fossils by only one Order, the Phyllopoda (leaf-feet), which 
owe their name to their numerous leaf-shaped gill-bearing 
limbs. In the large number of body-segments and the 
uniform nature of the limbs, as well as in some points of 
internal structure, modern phyllopods are considered to 
approacli the supposed ancestors of Crustacea. The carapace 
may be absent, as in Branchipus, or may form a large shield 
over the fore-part of the body, as in the living Ajms, or may 
be folded down the middle line, and the two halves held 
together by a muscle crossing them inside, as in the little 
JSsthcria. All recent phyllopods are essentially inhabitants 
of fresh water, but since they survive the change of tliis into 
sale water and even brine, as in the Great Salt Lake, their 
presence in any rock does not necessarily imply its fresh- 
water origin. Protocaris from the Cambrian of Georgia 
resembles Ap^is, and the doubtful Anomalocaris from the 
Cambrian of British Columbia has been compared to the 
abdomen of Branchipus. In Devonian shales Estheria and 
its relatives are common fossils. In Triassic and a few later 
rocks Apus is met with, and from tlie Oligocene of the Isle 
of Wight there is a Branchipodites, here exhibited. 
The OSTRACODA (Fig. 46, 5) resemble Estheria in 
having a carapace of two valves united along the back by a 
membrane, and capable of being tightly closed by an internal 
muscle. Tlie appendages, which differ from those of the 
Phyllopoda, can be thrust out between the lower edges of 
the valves, and used for creeping or swimming. Appearing 
already in Cambrian times, the Ostracoda rapidly became 
numerous in both individuals and species, and still abound 
in tlie seas and lakes of to-day. They live usually in shallow 
water, and occur in such crowds that their remains some- 
times form considerable beds of rock. Leperdiiia, Beyrichia, 
Thlipsura, Entomis, and the four families which they repre- 
sent, are all Palaeozoic. Cytherella, Cypridina, and the 
relatives of Cypris, range from Onlovician to modern times. 
Cythere and its family are Mesozoic to Eecent. Most ostra- 
cods are too small for satisfactory exhibition, so that only 
a small selected series is shown. 
Gallery 
VIII. 
Wall-case 
13c. 
Table-ease 
23. 
Table-ease 
23. 
Wall-case 
13c. 
