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Minute Innabitants of Aquaria 

 PERRY BRUCE CLARK 



San Francisco Aquarium Society 



An infusorian of interest is the Vorti- 

 cella, or bell animalcule. This little ani- 

 mal is found attached to aquatic plants, 

 generally in old water, sometimes ap- 

 pearing like a fine white fringe. As the 

 name implies, it is bell-shaped, and it 

 attaches itself to a solid object by a long, 

 slender, flexible stalk. This stalk con- 

 tains a long muscle fiber, and when the 

 Vorticella becomes disturbed, the stalk, 

 by means of the fiber, is coiled with a 



Vorticella 



jerk, the movement pulling the animal 

 close to the object to which it is attached. 

 When everything is quiet again the stalk 

 gradually relaxes and the little animal- 

 cule once more reaches out in search 

 of food. The bell-shaped body is not 

 hollow, but is composed of protoplasm. 

 Across the mouth of the bell is a disk, 

 which is slightly smaller than the mouth, 

 leaving a space for the opening of the 

 gullet. Both around the mouth and the 

 disk are placed a row of cilia, which con- 

 stantly vibrate to and fro, setting up cur- 

 rents in the surrounding water, which 

 bring small particles of food to the gul- 

 let. This food, which is composed of 

 minute plants and animals and frag- 

 ments of larger forms, collects at the 

 bottom of the gullet, forming a food ball 



in the same manner as in Paramecium. 

 At the outer end of the gullet is a space 

 called the vestibule, and into this is 

 passed any undigested residue, which is 

 swept out by the outgoing currents of 

 water maintained by the cilia. There 

 is also a contractile vacuole near the ves- 

 tibule, which empties into it. It has a 

 C-shaped nucleus. Vorticella multiplies 

 by longitudinal division. Sometimes a 

 number of these infusoria are found in 

 a group where they have multiplied and 

 stayed together, but generally after mul- 

 tiplication has occurred the different in- 

 dividuals separate and move to other 

 places, where they may have a better 

 chance to obtain their food. 



We will now proceed to the Metozoa, 

 or many-celled animals, one of the low- 

 est of these which is of interest to us 

 is the fresh-water polyp, or Hydra. 

 These little animals, which are generally 

 found attached to aquatic plants, have 

 a cylindric body about half an inch long, 

 and the thickness of a pin, and are gen- 

 erally colorless, but sometimes brown or 

 green. At the free end are a number of 

 fine, thread-like tentacles, which gently 

 sway to and fro. If the Hydra is dis- 

 trubed these tentacles are quickly drawn 

 in and the whole body contracts until it 

 is nothing more than a small ball. The 

 mouth is at the free end in the centre of 

 the tentacles. The body is hollow and 

 is composed of two layers of cells, the 

 outer layer serving as a protective cover- 

 ing and the inner layer digesting the 

 food that, is taken in. In the process of 

 evolution this is one of the first exam- 

 ples of the division of labor among cells. 



