CHAPTER XVIII 

 GASTROTRICHA 



BY HENRY B. WARD 



Professor of Zoology, University of Illinois 



AMONG the microscopic animals common in fresh water and 

 limited in distribution to that environment are certain minute 

 organisms known as the Gastrotricha. Though limited in variety 

 of species they are so abundant, so widely distributed, and so strik- 

 ing in appearance as to command the attention of every student 

 of aquatic life. They live in numbers among algae and debris and 

 in almost every bottom collection appear in 

 company with the rotifers and protozoans. In 

 movements and habits they resemble closely 

 the ciliate Protozoa, and are easily confused 

 with them. Ehrenberg, who first described in 

 detail the structure of these organisms, placed 

 them among the Rotifera and many later 

 investigators have followed this suggestion. 

 Others incline to regard them as Nematoda 

 irom which they differ most strikingly in pos- 

 sessing cilia which are not known in other 

 worms of that group. In size they are strictly 

 microscopic, varying from 0.54 mm. in maxi- E 

 mum length to only one-eighth of that. They 

 constitute a distinctly uniform group not 

 closely related to any other existing types of 

 animal life. Our knowledge of the anatomy 

 of these organisms is due principally to the 

 investigations of Stokes in this country and 

 Zelinka in Germany. 



The general structure of the group is well 

 illustrated in the figure of Chaetonotus maximus taken from Zelinka's 

 monograph (Fig. 963). While the form of the body approaches a 

 cylinder, there is usually an expanded area in front known as the 



621 



, muscles; B, brain: E, egg; 

 O, esophagus; 7, intestine; 

 Ov, ovary. X 400. (After 



