GENERAL ANATOMY. 21 



stage is oval, has a single frontal eye, three pairs of 

 limbs arranged round the mouth, and no frontal 

 appendages ; the mouth-organs proper are entirely 

 absent, and the posterior part of the body has no 

 appendages except a couple of setae in the neighbour- 

 hood of the anus. The anterior portion of the body 

 is equivalent to the three anterior cephalic somites, 

 its three pairs of limbs becoming eventually antennae 

 and mandibles. At the first moult the body becomes 

 elongated and new limbs appear in the following 

 order : a fourth and fifth pair representing respec- 

 tively the maxillae and foot-jaws ; a sixth and seventh 

 which become the two anterior pairs of swimming- 

 feet. At this stage the larva still resembles a 

 Nauplius, and does not take on a Cyclopoid appear- 

 ance until after the next moult.* It then resembles 

 more closely the adult Cyclops as to the antennae and 

 mouth-organs, but the number of limbs and somites is 

 smaller ; the body in this condition is composed of an 

 oval cephalothorax, three thoracic and one long 

 terminal segment, which in succeeding moults be- 

 comes forked. In the Cyclopidce the posterior 

 antennae and the mandibles lose their accessory 

 branches, but in other families these parts are usually 

 retained. All the free, and many of the parasitic 

 species pass through a further series of moults, in the 

 course of which the still-wanting limbs and body- 

 segments appear, the limbs attaining, by successive 



* In some Nauplii, if not in all, the terminal part of the intestine is 

 subglobular, and contracts periodically like the " contractile vesicle " 

 of a Rotifer. 



