AETHROPODA — CRUSTACEA. 



93 



segments form the abdomen, at the end of which is the Gallery 

 telson. In the number of the body-segments, in the manner V"III. 

 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 much 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 

 approach the supposed ancestors of Crustacea. The carapace 

 may be absent, as in Branchiijus, or may form a large shield 

 over the fore-part of the body, as in the living Ajpus, or may 

 be folded down the middle line, and the two halves held 

 together by a muscle crossing them inside, as in the little 

 Estlieria. All recent phyllopods are essentially inhabitants 

 of fresh water, but since they survive the change of this into 

 salt 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 Apus, and the doubtful Anomalocaris from the Wall-case 

 Cambrian of British Columbia has been compared to the 

 abdomen of Branchipus. In Devonian shales Estheria and Table-ease 

 its relatives are common fossils. In Triassic and a few later 

 rocks Apus is met with, and from the Oligocene of the Isle 

 of Wight there is a Branchiioodites, here exhibited. 



The OSTRACODA (Fig. 46, 5) resemble Estlieria in 

 having a carapace of two valves united along the back by a 

 membrane, and capable of being tightly closed by an internal 

 muscle. The 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 the 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, Beyricliia, Table-case 

 Thlipswra, Entomis, and the four families which they repre- -v^afl-case 

 sent, are all Palaeozoic. Cytherclla, Cypridina, and the i3c. 

 relatives of Cypris, range from Ordovician to modern times. 

 Cytherc and its family are Mesozoic to Eecent. Most ostra- 



