TROCHELMINTHES. 



Minute animals whose adult structure seems to be related to 

 that of the trochophore larva. Mouth usually surrounded by a 

 circlet of cilia. Three classes (Rotifera, Dinophilea, and Gastro- 

 tricha) are referred to this phylum, but the}' may not be geneti- 

 cally related. 



ROTIFERA. 



Mostly fresh-water forms, but a few are marine. All are of 

 microscopic size. The pharynx is provided with a masticator}^ 

 apparatus, and the anterior end bears a trochal disk. Most 

 rotifers are free, but a few are permanently attached, and some, 

 as Melocerta, live in tubes of their own formation. 



BRACHIONUS (A Rotifer). 



These animals are frequently quite abundant in ponds and 

 aquaria. They are not very active, and spend most of their 

 time near the bottom among the plants and debris. Owing 

 to their minute size, they must be studied with a high power 

 of the microscope. 



1. The body is divided into a trunk, which is inclosed in a 

 transparent cuticular lorica, and a movable tail or foot. The 

 tail is tipped with two processes which form forceps, by means 

 of which it attaches itself to plants. Can you see how these are 

 used? Why does the animal need to attach itself? 



2. Projecting anteriorly from the lorica is the retractile 

 trochal disk. Notice the cilia on the margin of this disk. Is 

 the disk used in locomotion? Does the animal always move 

 when the cilia are active? What other use has the disk? Is the 

 animal entirely dependent upon the cilia of the disk for loco- 

 motion ? 



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