GASTROTRICHA 47 



The nervous system consists of a dorsal ganglion with which 

 are connected one or more eye-spots. There are peculiar tactile 

 organs which consist of " rod-like structures tipped with deli- 

 cate sensory hairs." There are excretory and reproductive 

 organs. 



They are dimorphic (of two forms). The sexes are separate. 

 The males are rarer, much smaller, and less highly developed 

 than the female. The female lays thin-shelled summer eggs 

 of two sizes — the larger developing into females, the smaller into 

 males — and thick-shelled winter eggs, which in the spring de- 

 velop into females. 



The majority are free swimming, being propelled by the 

 trochal disk, but the Bdelloida also have a looping movement 

 like that of the leech. 



The rotifers may be dried up in the mud for several months, 

 and upon being brought into contact with water they revive, or, 

 some think, their contained eggs bring forth live animals. 

 When in the dry condition they may be carried long distances on 

 the feet of birds or by the wind. 



CLASS n. DINOPHILEA 



These, like the rotifers, are modified trochospheres. They are minute 

 and worm-like. They have a prostomium or head, a body of five to eight 

 segments, and a short tail. Both the body and the head are ciliated. The 

 Dinophilea are marine. In the arrangement of the nephridia in pairs, 

 corresponding to the imperfect segments, and in the tendency to seg- 

 mentation, they resemble the Annulata. 



CLASS m. GASTROTRICHA 



This class resembles the Rotifera, though the relationship is not close. 

 The class comprises a small number of minute fresh-water forms with 

 spindle-shaped bodies, flattened ventrally. The dorsal surface bears 

 several rows of cuticular processes, while the ventral surface has two rows 

 of cilia. 



Classification. — 



Class. Examples. 



I. Rotifera. Brachionus. 



II. Dmophirea. Dinophilus. 



III. GastrSt'richa. Ichthydium. 



