ROTIFERS AND GASTROTRICHA 



Chapter 4 



In the year 1703 the Dutch scientist, Leeuwenhoek, saw through his home- 

 made microscopes many pecuHar Httle animals which appeared to have tiny 

 wheels rotating at their anterior ends. Later workers confirmed his report, 

 and these little animals, multicellular but often no larger than many of the 

 Protozoa, were called Rotifera, or "wheel-bearing" animals. It is now known 

 that this wheel effect is an optical illusion caused by the successive motions of 

 cilia which form a single or double ring around the mouth. The chief purpose 

 of this corona or ring of cilia is to create a little whirlpool which will drag 

 unfortunate passers-by into the ever ready mouth of the rotifer. Once en- 

 gulfed there is no escape, for in the throat of the animal and plainly visible 

 through its transparent body is a mastax or set of jaws which soon disposes 

 of any morsel which may reach it. Some of the rotifers have reduced the 

 corona and developed the ability to project a forceps-like mastax out at the 

 mouth and so grab their prey. Others have become vegetarians and have 

 adapted the mastax into a sucking pump, highly efficient in extracting the 

 contents of plant cells. 



Some of the Rotifera are sac-like forms, but many have a characteristic 

 posterior extension of the body called the foot, which may fork at the end 

 into two or three toes. This foot serves as a rudder in swimming and as an 

 anchor when the animal rests. Frequently the foot shows several joints and 

 is capable of folding up or of extending like a telescope. 



Most rotifers are solitary nomads, but a few species have developed colonial 

 habits and, like Conochilus, may be found in spherical, floating colonies, all 

 hooked together by their feet. Some appear to feel the need for protection and 

 secrete thin and usually transparent tubes around themselves. In time of 

 danger the head can be withdrawn into this protective sheath, which is called 

 the lorica. Other rotifers have settled down and built themselves homes. One 

 of the most beautiful of these, Floscularia, puts out a finger-like projection 

 from the region back of the head and makes up tiny bricks wath which it builds 

 itself a delicate, chimney-like house. 



Rotifera, like Protozoa, are found in almost all standing water, even in 

 temporary puddles and the cavities of pitcher-plant leaves. They are com- 

 monly associated with small aquatic plants, especially with algae, or with decay- 

 ing vegetation. Several species are often found on Spirogyra, the alga popularly 

 called "pond scum", upon which they feed by sucking out the cell contents. 

 One rotifer, Ascomorpha, spends most of its life inside the spherical protozoan 

 colony, Volvox. 



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