190 INVERTEBRATE ZOOLOGY 
Observe the macronucleus; it is a narrow elongated structure 
and is easily seen; near itis 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 
entosare 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 delicate 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 Vorticellide both of the individ- 
uals produced by the process of division remain on the stalk. 
In Zoéthamnium 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 the Hydromeduse. In Vorticella, as in Parame- 
cium, reproduction is largely a matter of sufficient nutrition, 
well-nourished animals reproducing faster than poorly nourished 
ones. Conjugation also occurs; it is brought on by the same 
conditions as in Paramecium and is highly important to the 
