MICROSCOPIC AQUATIC ORGANISMS. 363 



type is seen in Salpa, where we get a pumping or squirting 

 method of locomotion, water being taken into the body and then 

 forcibly ejected. 



We now reach the most highly developed form of locomotion, 

 namely, that due to definite appendages worked by special 

 muscles. Among the Rotifers there are certain species which 

 possess what have been called skipping organs, consisting of 

 a few spines, by the sudden movement of which the animals can 

 dart about in a very rapid way. Such forms are Triarthra and 

 Polyarthra, and their erectile spines may be regarded as locomotor 

 appendages in their simplest form. There is, however, a still 

 more important example of rudimentary swimming appendages 

 to be found in another genus of the same group, viz. Pedalion, 

 where the spines are placed on distinct bumps and outgrowths 

 provided with special muscles, thus forming something very 

 closely resembling true limbs. Such a form as this serves, in 

 this respect at least, to bridge over the gap between the worms 

 and Arthropods. 



Among the simplest cases of animals possessing true appen- 

 dages, such as legs, adapted for swimming, we may instance the 

 Water-mites (Hydrachnidae). In these the only modification 

 that has taken place is that certain special tufts of hairs have 

 been developed on the legs in the forms capable of swimming 

 which do not occur in the crawling forms. Numerous aquatic 

 insects also show modifications of the legs, and very exceptionally 

 of the wings, for locomotion under water, but most of these 

 animals can scarcely be called microscopic. It is rather among 

 the little Crustaceans known as Entomostraca (some of which 

 are often referred to as Water-fleas) 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 with a kind of rhythmic swinging, the 

 resulting movement of the animals (at least, in the species not 

 wholly enclosed in shells) being exceedingly graceful. In the 

 closely related Order of the Cladocera, the swimming movements 

 are brought about not by the feet, which have been reduced to 

 five or six pairs and only possess functions accessory to feeding 

 and respiration, but by a special adaptation of the second pair 



