232 



ZOOLOGY. 



back and forth in the water. They may occur so thickly that thousands 

 may be seen in the space of a few feet. When their home is jarred they 

 speedily withdraw from sight. A colony of Tubifex nearly always has 

 associated with it one or more genera of smaller worms, as Dero or Nais, 

 a species similar to Dero but with eye-spots. Dero has an interesting 

 respiratory apparatus at the posterior part of the body (Fig. 99, />.), one 

 of the few instances where Oligochaeta possess such organs. 



FIG. 1 06. 



FIG. 1 06. Cirratulus grandis, from Vet rill. 



Questions on Figs. 105 and 106. Are these Chaetopods? What are 

 your evidences? What is the -nature and function of the numerous out- 

 growths (branchial cirri) ? In what respects are they differently arranged 

 in the two types? Are these Oligochaeta or Polychaeta? Your reasons? 



Class II. Discophora (bearing suckers}. Annulata in which there are 

 secondary external rings which tend to obscure the metameres, inasmuch 

 as the external and internal segmentation do not coincide. There are no 

 bristles. The body cavity is much reduced by the growth of muscles 

 and connective tissue. The remaining spaces contain blood and are in 

 communication with the vascular system. Two sucking discs are present 

 and are powerful organs of attachment. The anterior sucker embraces 

 the mouth ; the posterior is near the anus. Sexes are united in one indi- 

 vidual ; testes numerous, ovaries a single pair. Development direct. 

 Marine, fresh water, terrestrial, or parasitic in habit. 



