356 DONNELL BROOKS YOUNG 



extending more than half the length of the body. The adoral 

 zone is represented by cirrus-like membranelles which originate 

 from pockets or pits on the anterior border. The mouth is situ- 

 ated in the left posterior part of the peristome cavity. Three to 

 six membranelles are present in the gullet. The peristome is 

 bordered by preoral and endoral undulating membranes. Three 

 bow-shaped cavities are hollowed out of the posterior end of the 

 carapace, two ventral and one dorsal. Originating from the 

 dorsal pit, which is situated on the right side, are three large 

 cirri M^hich curve toward the median line. Inserted in the right 

 ventral pit are four or five straight cirri, while two sickle-shaped 

 cirri originate from the left ventral pocket. 



Two distinct types of movement are observed, one a steady, 

 forward swimming or crawling and the other a backward or 

 somewhat sidewise jumping, darting, or spinning. In the first 

 type only the cirri of the adoral zone and the undulating mem- 

 brane are at first seen to be in motion. Careful observation 

 shows, however, that there are some smaller cirri on the ventral 

 side which are used for this ordinary swimming. At the slightest 

 irritation the animal will vigorously contract one or more of the 

 large posterior cirri and dart backward with surprising rapidity. 

 Frequently these jumps are so rapid that the eye cannot follow 

 and the animal seems to have vanished as if by magic. After 

 a series of such jumps, the number and violence depending on 

 the strength of the stimulus, the animal will come to rest again 

 and resume its normal swimming. 



The nuclear complex varies. The macronucleus is usually 

 broken up into two or more fragments. One or two micronu- 

 clei may be present. 



A contracting vacuole has been described by Claparede and 

 Lachman as follows, "La vesicule contractile n'est point place 

 du cote droit mais du cote gauche, immediatement en avant 

 des deux pieds dorsaux gauches." Calkins states that in U. 

 setigera the contractile vacuole lies between the two sets of 

 posterior cirri. These observations have not been verified, 

 and it is certain that in some species, U. binucleata and U. seti- 

 gera, no contractile vacuole is present. 



