Molluscoida. 353 



motions resemble the rotation of a wheel. These disks 

 can be retracted when the animal wishes. The common 

 name for one of these animals is the " wheel animalcule." 

 As implied by the term animalcule, the animals are small, 

 usually not exceeding a thirtieth of an inch in length. 

 They occur in fresh water, and are usually to be found 

 when looking over minute water plants under the micro- 

 scope. They are transparent and are easily examined, all 

 their organs showing without dissection. Though small, 

 they are highly organized, having a complete digestive 

 system, the food being swept to the mouth by the cilia 

 bordering the disks. There is also a nervous system, with 

 one or more eyes. 



They can swim by the action of the cilia and they also 

 progress by a looping movement, attaching alternately by 



FIG. 200. WHEEL ANIMALCULE (ROTIFER). 



From Packard's Zoology, 



the two ends of the body. The posterior part of the body 

 is a segmented " tail " which can be withdrawn like a 

 telescope. It is said that after being dried for years, 

 they -will revive when placed in favorable conditions, though 

 some authors think it is contained eggs that survive instead 

 of the adults. 



BRANCH MOLLUSCOIDA. 



This group gets its name from the fact that some of the 

 forms were once supposed to be mollusks. There are two 

 principal classes. 



