ANNELIDA 239 



colony of Tubifex nearly always has assoacited 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. 

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



Class II. Hirudinea. — Annelida in which there are secondary external rings 

 which tend to obscure the metameres, inasmuch as the external and internal seg- 

 mentations 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 or terrestrial. Predatory or parasitic in habit. 



277. There are several other groups of "worms" of considera;ble interest to the 

 zoologist, sometimes classed with the Annelida, 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 (flephyrea). 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 Annelida. 



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 Oligochasta do you find reproduction by 

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



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



