Sub-Class 2. Entomostraca. Order 4. Trildbita. 



199 



so that the structure of the appendages cannot be spoken of with 

 certainty ; they were ' probably soft and feeble like those of the 

 Phyllopods. Some Trilobites had the power of rolling themselves 

 up like Oniscus. As regards their development, it is known that 

 they possessed a smaller number of segments in the young than in 

 the adult state. 



The order of Trilobites comprises numerous species, all of which 

 are extinct. The group flourished especially in the Silurian, was 

 more sparsely represented in the Devonian, and died out in the 

 Carboniferous Period. Many specimens are of very considerable size. 



Order 5. Ostracoda. 



Prom superficial observations, tlie Ostracoda appear to be very like the 

 Daplmidse, from, which, however, they prove to be very different on a closer 

 examination. The body is short and compressed, and may, with the limbs, be 

 entirely enclosed in the very hard shell ; this is divided into two parts, which can 

 be opened and shut Uke the valves of a mussel shell. In the anterior part of the 

 animal, there is a nanplius-eye, sometimes divided into two; and, in some 

 forms, also a pair of movable lateral eyes. The first and second a n t e n n » 

 ai-e much modified, and provided with long, swimming setee ; both, but especially 

 the second pair, are natatory and locomotor. The three pairs of jaws are all well 

 developed. Besides these appendages, only two pairs of slender, jointed thoracic 

 legs are present. The most posterior part of the body is curved downwards, and 

 usually terminates in two lamellate appendages. As regards the internal 



Fig. 161. 



Fig. l^il. Cypris. oc nauplius eye, a'n}, avp first and second antennae, md mandible, 

 mo;', mx^ firat and second maxillas, j)', p^ first and second legs, c tail : enlarged. — After 

 Zenker. 



Fig. 162. Nauplius of an Ostraood. s shell, the other letters as in Fig. 161. — After 

 Claus. 



stmcture, it must be noticed that many have no heart. Sexual dimoi-phism is 

 well marked (in the structm-e of the limbs, etc.) ; the colossal size to which the 

 spermatozoa attain is worthy of note ; in the species Cypris ovum, for example, 

 the total length is 2 m/m., i.e., more than three times as long as the whole 

 animal. Many leave the egg in the nauplius state, provided only with 

 antennae and mandibles ; the shell is usually already developed. 



The Ostracoda are animals of small size, occurring both in salt and fresh 

 water. 



