300 



ZOOLOGY 



cilia : this is the trochal disc and is one of the most characteristic 

 organs of the class. By the action of the cilia the animal is 

 propelled through the water, and, as in Vorticella, their successive 

 flexion gives an appearance of rotation to the disc or " wheel- 

 organ " whence the name of the class is derived. Within the circlet 

 of cilia arise three prominences (c.L) covered with cilia of large 

 size. The trochal disc is not perfectly symmetrical, but has at one 



part of its circumference a depression in which the mouth lies : 

 this marks the ventral surface. The anus (.) is dorsal in position, 

 and is placed at the junction of the tail with the body-trunk. 



The body- wall consists of an epidermal layer covered by a 

 chitinoid cuticle : it is by a thickening of the latter in the region 

 of the trunk that the lorica is produced. There is no continuous 

 muscular layer, but several bands of unstriped muscle (???,) pass 

 from the lorica to the trochal disc in front and to the tail behind, 

 and act as retractors of those organs. 



