UNSEGMENTED WORMS. 195 



Phylum Trochelminthes {wheel-worms or rotifers'). The Rotifers 

 or wheel-animalcules are microscopic animals. They are usually bilater- 

 ally symmetrical. The anterior end possesses a retractile disc supplied 

 with cilia variously arranged, the rhythmic motions of which often give 

 the appearance of a rotating wheel. From this the name of the group 

 comes. This organ assists in locomotion and produces currents in the 

 water by which food is brought within reach of the mouth. There is a 

 digestive tract with both mouth and anus. The pharynx into which the 

 mouth opens is provided with a chitinous grinding apparatus (mastax). 

 Usually a pair of digestive glands open into the stomach. The nervous 

 system is usually limited to a single ganglion dorsal to the pharynx. 

 Eye-spots and other sense organs, called tactile rods or antennae, are pres- 

 ent. There is a true ccelom communicating with the exterior by means 

 of excretory tubules. For a diagrammatic view of these structures see 

 Fig. 93- 



The sexes are distinct and are frequently very different in appearance. 

 The males are often much smaller than the females, are much less numer- 

 ous, and are often degenerate. The summer eggs are of two kinds 

 large and small and develop parthenogenetically. The large eggs pro- 

 duce females and the small, males. The winter eggs have a thick shell 

 and are believed to require fertilization in order to develop. They rest 

 during the winter and in the spring develop into females. Development 

 is direct. The adult cofftlition in the Rotifers suggests the larval (trocho- 

 phore} condition in some Annulata. There are some traces of external 

 segmentation in the tail or foot region in some species and for these 

 reasons some authors class the Rotifers near the Annulata. Rotifers are 

 aquatic, being more common in fresh water than in the sea. They are 

 abundant in water-troughs, gutters, ponds. They are capable of resuming 

 activity after having been dried up in the mud for a year or more. This 

 power must be of great value in preserving the species as well as in 

 spreading it. 



Phylum Molluscoidea (mollusk-like) . The two groups' included here 

 are quite diverse in general appearance and habit. Their larval stages 

 have more points in common than the adult. There is in the adult a 

 variously-shaped tentacle-bearing ridge (lophophore) about the mouth. 

 The central nervous system consists of one or two ganglia about the 

 oesophagus. They have often been grouped with the mollusks but authors 

 are agreed that much of the seeming resemblance to mollusks is super- 

 ficial. 



Class i. Polysoa (Bryozoa; sea-mats; corallines). The Polyzoa are 

 colonial animals which resemble in general appearance some of the com- 

 pound hydroids. The individual animals however are very different in 

 their structure. They are found both in salt and fresh water. In Poly- 

 zoa (Fig. 94) the digestive tract is sharply bent, the anus opening close 

 to the mouth either within or outside the circle of tentacles (lophophore) . 

 A distinct coelom is typically present. There are no blood vessels. 

 An exoskeleton is formed by the ectoderm, by means of which the indi- 



