170 INVERTEBRATE ZOOLOGY 



Observe the macronucleus ; it is a narrow elongated structure 

 and is easily seen ; near it is the small spherical micronucleus. 



Exercise 2. Make a large semidiagrammatic drawing of a Vor- 

 ticella, showing these details, and label all. 



Vorticella has no special vegetative organs except the pulsat- 

 ing vacuole. The food particles which are ingested into the 

 endosarc are there digested, and waste matters are egested 

 through a temporary anus in the upper portion of the body. 

 Respiration is carried on through the external surface of the 

 body. Organs of locomotion are present in the cilia, by which 

 the animal can swim about if it is broken from its stalk. The 

 axial fiber in the stalk is a delicately striated muscle fiber. 

 Sensation is exercised through the external surface. 



Vorticella reproduces asexually, by a longitudinal division. 

 The process begins at the upper end of the body and proceeds 

 to the base, so that finally there are two individuals upon a 

 single stalk. One of these now separates itself from the stalk, 

 assumes a cylindrical form, and, having developed a band of 

 temporary cilia near one end, swims away to find a place for 

 itself. It soon attaches itself, loses the temporary cilia, and 

 develops a stalk. 



In the case of the colonial Vorticellidse both of the individ- 

 uals produced by the process of division remain on the stalk. 

 In Zoothamnium the colony is dimorphic ; it contains nutritive 

 individuals which are similar to Vorticella, and reproductive 

 individuals which are large and globose and do not feed. The 

 latter separate themselves from the parents and swim off and 

 found new colonies. This dimorphism and division of labor 

 remind one of similar phenomena in the Hydromedusse. In 

 Vorticella, as in Paramecium, reproduction is largely a matter 

 of sufficient nutrition. Animals which are not surrounded by 

 the proper conditions of nutrition gradually lose their vitality 

 and reproductive powers, and from time to time must perform 



