238 ZOOLOGY 



common genera are Dero (Fig. 101), a beautiful, almost transparent worm which 

 often forms a temporary tube for itself of particles cemented by its own secretion, 

 and Tubifex, a longer worm which burrows in the mud at the bottom of streams; 

 a portion of the body protrudes from the mud and waves gently 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 eyespots. Dero 

 has an interesting respiratory apparatus at the posterior part of the body (Fig. 

 101, p.), one of the few instances where Oligochaeta possess such organs. 



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 segmentations 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 individual; testes numerous, ovaries a single pair. Development 

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



277. There are several other groups of "worms" of considerable interest to the 

 zoologist, sometimes associated with the Annulata, which it seems necessary to pass 

 by with mere mention. -(See also 236). 



Class: Archi-annelida ; a few primitive forms, as Polygordius (Fig. 106). 

 Class: Sipunculoidea (Gephyrea). With traces of segmentation in the embryo, 

 but not in the adult. 



Class: Chaetognatha (arrow worms). 



Some authors would place here also the Rotifers (see 235). 



278. Suggestive Studies for Library and Laboratory. 



1 . Look up the characteristics of the Archi-annelida, the Gephyrea, or Sagitta 

 and report on their likenesses to the types studied. 



2. On what grounds might the rotifers be associated with the Annulata? 



3. Compare the "segments" in cestodes and Annulata. 



4. In the Chaetopoda which sets of organs pass through all the segments, which 

 are repeated in essentially all, and which are limited to a few? 



5. Examine and report on the habits of the earthworm. (Study in its natural 

 haunts or in box of moist earth in laboratory.) What are its haunts? Method 

 and rate of burrowing? Does it avoid water? What is its food? How taken? 

 Does the animal prefer light or darkness? 



6. If near the sea-shore select other forms and report in a similar way. 



7. Investigate parasitism among the Annulata. 



8. What is the economic value of the earthworm? Of other worms? 



9. Make a study from the text-books of the reproductive organs in any of the 

 hermaphrodite Oligochaeta. 



10. In how many species of aquatic Oligochaeta do you find reproduction by 

 fission ? In what particulars does the process seem to differ in the different species ? 



11. Outline the life-history of Autolytus, including the origin of the sexes. 



