WORMS, ROTIFERS, LEECHES,^POLYZOA 79 



The Rotifera, or Wheel Animalcules as they are often called, 

 are a group of microscopic animals which bear certain general 

 features of resemblance to the Trochosphere, a characteristic larval 

 form of a great group of worms, the Annulata, to which the Earth- 

 worms, the marine Segmented Worms, and the Leeches belong. 

 Minute as they are in size, the Rotifers are relatively highly 

 organised animals, occurring abundantly in fresh water, less numer- 

 ously in the sea, while they have a world-wide distribution. The 

 majority are free-swimming, movement being effected by cilia 

 confined to the front or oral extremity of the body and borne on 

 what is called the trochal disk which is a very characteristic 

 organ of the group. The motion of the stout cilia is such as to 

 give to the trochal disk the appearance of a rapidly rotating wheel, 

 and it is from this appearance that the animals have derived 

 their name of Rotifera, or " Wheel-bearers." The trochal disk is 

 not always circular in form, but may be divided into almost petal- 

 like lobes, or be drawn out into long processes. 



The body of a Rotifer is usually distinguishable in two parts, 

 a body and a tail, the latter being frequently divided by a series 

 of freely movable joints into a number of tubular segments, and 

 provided at its extremity, in many forms, with a pair of rather 

 forceps-like processes, by which the Rotifer can temporarily anchor 

 itself to any object. The head is crowned by the trochal disk, and 

 the mouth, which is a funnel-shaped cavity, may be in the centre 

 or on one side of the disk. The walls of the funnel-shaped mouth 

 are ciliated, and lead down to a muscular pharynx provided with 

 peculiar movable jaws for the grinding up of the food particles 

 before they enter the stomach. Food, in the shape of very minute 

 organisms and particles of animal and vegetable matter, is carried 

 to the mouth by the action of the cilia and the currents they 

 produce. The sexes are distinct, the males being nearly always 

 much smaller and degenerate in structure ; the females are usually 

 viviparous, or carry about the eggs until they are hatched. The 

 eggs are of two kinds, and are termed summer and winter eggs ; 

 the former may develop without fertilisation, 1 while the fertilised 

 eggs are thick-shelled winter or resting eggs, and can resist con- 

 siderable heat, cold, and draught. 



The Rotifers are sufficiently familiar objects to every one who 



1 Development without fertilisation is known as parthenogenesis. 



