UNSEGMENTED WORMS 



203 



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

 reasons some authors class the Rotifers near the Annelida. Rotifers are aquatic, 

 being more common in fresh water than in the sea. They are abundant in water- 

 troughs, gutters, ponds. Some species 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. They are probably not as closely 

 related as this classification would suggest. 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 con- 

 sists of one or two ganglia about the esophagus. The Brachyopoda have often 

 been grouped with the moUusks, but authors are agreed that much of the seeming 

 resemblance to moUusks is superficial. 



Pig. 96. 



Fig. p6. A fresh-water polyzoan, Plumatella. From Parker and Haswell, after AUman. a, 

 anus; fu., funiculus, a band of tissue anchoring the intestine to the body wall; g, ganglion; int., 

 intestine; m, mouth; 0, esophagus; r, reproductive gland; rt, retractor muscle; st, stomach; stat, 

 statoblast; I, tentacles. 



Questions on the figure. — Is this an individual or a colony? What is the 

 function of the retractor muscles? To what degree are the polyps capable of 

 contraction as shown in the figure? The value of this power? What are the 

 statoblasts? 



Class I. Polyzoa {Bryozoa; sea-mats; corallines). — The Polyzoa are colonial 

 animals which resemble in general appearance some of the compound hydroids. 

 The individual animals however are very different in their structure. They are 

 found both in salt and fresh water. In Polyzoa (Fig. 96) 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 individuals of the colony are held together. Each member of the colony 

 may retire into its own particular portion of the exoskeleton, when disturbed, by 



