The Ostracoda ([Fig. 13]), which form the second subclass in the system of classification here adopted, are nearly all microscopic animals, and are found abundantly in fresh water as well as in the sea. The carapace forms a bivalved shell, which completely encloses the body and limbs, and is often sculptured in an elegant fashion. The Ostracoda are remarkable for the very small number of their appendages. There are not more than two pairs of limbs behind the maxilla. Most of the species are included in two orders, the Myodocopa and the Podocopa, of which the former may generally be distinguished by a notch in the anterior part of the margin of the shell ([Fig. 13], A, n). In the Podocopa the margin is entire.

Fig. 13—Shells of Ostracoda. Much enlarged. (From Lankester's "Treatise on Zoology," after Brady and Norman, and Müller.)

A, Philomedes brenda (Myodocopa); B, Cypris fuscata (Podocopa); C, Cythereis ornata (Podocopa). n, Notch characteristic of the Myodocopa; e, the median eye; a, mark of attachment of the muscle connecting the two valves of the shell. A and C are marine species; B is from fresh water

[View larger image]

Fig. 14—Cyclops albidus, a Species of Copepod found in Fresh Water. (After Schmeil.)

Female specimen carrying a pair of egg-packets. The actual length is about one tenth of an inch