334 THE INVERTEBRATA 



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 sper- 

 matophore. The eggs when laid are cemented into a packet (egg 

 "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. 224). This is succeeded 

 by several Metanauplius stages, and then suddenly at a moult takes on 

 the^zr^^ 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 are an important item of food for 

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

 having the antennae and mandibular palps (Fig. 211 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 primitive 

 custom of feeding by the automatic straining of food particles from 

 the water is retained : the feeding current eddies from the swimming 

 current which the antennae, mandibles, and maxillae set up, and is 

 strained through a fringe of bristles on the maxillae (Fig. 240). 



The parasitic habit has been adopted by members of very different 

 families of copepods, and to very various degrees even by members 

 of a single family. Every stage may be found between normal, free- 

 living forms and the most degenerate parasites. Parasitic forms often 

 have a suctorial proboscis, which is formed by the upper and lower 

 lips enclosing mandibles adapted to piercing. Such a proboscis is not 



