2O2 



ZOOLOGY 



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 VII: 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 ten- 

 tacle-bearing ridge (lophophore} about the mouth. The central nervous system 

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

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

 resemblance to mollusks is superficial. 



FIG. 96. 



FIG. 96. A fresh-water polyzoan, Plumatella. From Parker and Haswell, after Allman. a, 

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

 intestine; m, mouth; o, esophagus; r, reproductive gland; rt, retractor muscle; si, stomach; stat, 

 statoblast; t, 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 



