COPEPODA 



373 



the more primitive, pelagic copepods (Calanus, etc.) which have 

 biramous antennae and biramous palps on the mandibles, the an- 

 tennules do not take part in swimming. Such copepods feed by an 

 automatic straining of particles from the water, though their ap- 

 paratus for this purpose (see below) is very different from that of the 

 Branchiopoda. Cyclops, on the other hand, in a manner of which the 

 details are not understood, seizes its food particles from time to time. 



The alimentary canal is of much the same nature as that of Chiro- 

 cephalus but without mid gut diverticula. It possesses well-developed 

 extrinsic muscles, of which those that run from its anterior region to 

 the adjoining body wall produce rhythmical displacements of the 

 canal and so cause a movement of the blood, while the dilators of the 

 rectum draw in water which is believed to subserve respiration. Special 

 organs for circulation and respiration are wanting in Cyclops, though 

 other copepods have a saccular heart. Maxillary glands are present — 

 probably entirely mesodermal. The ventral cords of the nervous 

 system are concentrated into a single ganglionic mass. The gonads are 

 single median structures which lie above the gut in the first two 

 thoracic somites. The ducts are paired. In the female a large, 

 branched uterus adjoins the ovary on each side, communicating with 

 the lateral opening on the urosome by an oviduct which at its termina- 

 tion receives a duct from the spermatheca. The latter is median, in 

 the same segment as the oviducal openings, with a median entrance 

 of its own. The male transfers his spermatozoa to the female in a 

 spermatophore. The eggs when laid are cemented into a packet {(^gg 

 "sac") which hangs from the opening of the oviduct, and are thus 

 carried until they hatch. The possession of a pair of such packets 

 gives a characteristic appearance to the females of Cyclops, as to those 

 of many other copepods. In some genera, however, there is a single 

 median packet, and in a very few the eggs are laid into the water. 



The larva hatches as a typical Nauplius (Fig. 235). This is suc- 

 ceeded by several Metanauplius stages, and then suddenly at a moult 

 takes on the first Cyclops stage, which has the general form of the 

 adult but lacks appendages behind the 3rd pair of swimming limbs 

 and also the somites of the urosome. In five successive Cyclops stages 

 the missing somites appear, the tale of limbs being meanwhile 

 completed. 



Calanus, which is marine and pelagic in all parts of the world, often 

 occurring in enormous shoals which ^re an important item of food for 

 fishes and whales, is in several respects more primitive than Cyclops, 

 having the antennae and mandibular palps (Fig. 222 D) biramous, 

 well-developed and biramous limbs on the 6th thoracic somite, and 

 only one postcephalic somite in the cephalothorax. The 6th thoracic 

 somite is included in the mid-body, not in the urosome. The primi- 



