GENERAL BIOLOGY 



1. SIZE: measure. 



2. SHAPE: fusiform, rounded at the anterior end, bluntly 

 pointed at the posterior end. 



3. LOCOMOTION : due to cilia uniformly distributed over the 

 whole surface. Note also movements of flexion (bending). 



4. STRUCTURE. The two protoplasmic layers: (ectoplasm and 

 endoplasm). 



a. Ectoplasm (Cortex) : the firm elastic outer layer; its deeper 

 part marked by oblique myophan striations. 



( i ) . The cuticle, a delicate superficial layer differentiated from 

 the underlying protoplasm. 



(2). Cilia, delicate vibratile filaments arising from the ecto- 

 plasm and protuding through openings in the cuticle. These open- 

 ings can be seen on a specimen from which the water is allowed 

 to evaporate. 



(3). Trichocysts : minute oval sacs in deeper part of the ecto- 

 plasm arranged perpendicular to the surface; when the animal is 

 irritated, e. g. by iodine, a stiff thread can be shot out and pro- 

 jected beyond the cilia. They are probably defensive organs. 



(4). Two contractile vacuoles in the ectoplasm of the dorsal 

 side about l /3 of the animal's length from each end. While dilating 

 they are nearly spherical, but at the moment of contraction sep- 

 arate canals can be seen radiating from them. 



(5). The oral groove begins at the anterior end of the left 

 side and runs back to the mouth near the middle of the ventral 

 side. The cilia of the groove drive food particles into the mouth. 

 Run some Chinese ink in water under the cover glass and note that 

 some of it is carried into the groove. 



(6). The mouth is an aperture in the ectoplasm at the posterior 

 end of the groove through which food passes into a narrow tube, 

 the gullet. Watch the ink collect at the inner end of the gullet into 

 a ball which is suddenly passed into the endoplasm. Watch the 

 course of the food ball within the endoplasm until it is finally 

 ejected through the anus. 



(7). The anus is a temporary aperture between the mouth and 

 the hinder end of the body visible only at the moment of ejection 

 of fecal matter. 



b. The Endoplasm (Medulla) is the more fluid protoplasm 

 filling the central portion of the body. In it observe : 



[38] 



