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the Mediterranean, and by the medusoids of certain Hydrozoa. 

 In the case of Nematodes and many insect larvae, locomotion 

 is obtained by throwing the body into a series of waves or even 

 figures-of-eight. Another type is Salpa, where water is taken 

 into the body and suddenly ejected. Such a method has been 

 used for the propulsion of small vessels. In more advanced 

 organisms we find spines worked by muscles, which almost 

 correspond to limbs, and by the sudden movement of which 

 the organisms can dart about in a very rapid way. Forms such 

 as these, in this respect at least, serve to bridge over the 

 gap between the Worms and Arthropods. It is amongst the 

 little crustaceans known as Entomostraca that we must look 

 for the highest development of swimming appendages among 

 microscopic aquatic organisms. The most primitive of these, 

 the Phyllopods, possess very many pairs of foliaceous feet, of 

 elaborate and beautiful construction, which beat the water 

 rhythmically. In the related order Cladocera the swimming 

 movements are brought about by a special adaptation of the 

 second pair of antennae. The direction of the stroke, at least 

 in the genera Daphnia and Simocephalus, has been found to 

 be slightly oblique to the axis of the body, having a slight 

 inclination towards the back — a circumstance which gives 

 rise to a tendency for the locomotion to take place in a curved 

 path. This, however, is counteracted partly by the action of 

 gravity on the body between the strokes, and also probably 

 by the resistance offered to the turning movement by the long 

 shell-spines which most of the more rapid swimmers possess. 

 But it is among the Copepods that we find the most noticeable 

 swimming organs. The four or five pairs of feet are usually 

 broad, two-branched, paddle-like organs, provided with fringing 

 hairs and feathered spines. The tail, also, has a set of feathered 

 setae, and probably assists in movement. In the case of Cyclops, 

 the antennae do beat ; but it is not certain that they assist 

 in locomotion, being probably chiefly balancing organs. It 

 will be noted that in certain forms we may find one 

 individual possessing four different methods of locomotion — 

 the antennae, the mouth-organs, the feet, and the tail. In 



