PHYLUM ANNELIDA. SEGMENTED WORMS 



189 



species in 10 genera; they have originated in 

 various ways, and are thus a heterogeneous 

 assemblage and not to be regarded as a 

 unit. 



Another aberrant group of polychaetes in- 

 cludes the family Myzostomidae. All are 

 parasites of echinoderms, notably sea lilies 

 (crinoids); in size they range from 0.5 to 9 

 mm. The body is oval and depressed with 

 few segments. Individuals are protandric, 

 that is, the smaller younger ones function as 

 males, and later, with increase in size and 

 age, become females; cross-fertilization is 

 thus insured. The egg gives rise to a swim- 

 ming trochophore. 



Oligochaetes 



The members of the class Oligochaeta are 

 mostly terrestrial, but some inhabit fresh 

 water; no parapodia, and few setae are pres- 

 ent, and the head has no distinct appen- 

 dages. They are hermaphroditic, but no 

 trochophore larva develops from the egg. 

 The earthworm is the best-known species. 

 Among the interesting species of oligo- 

 chaetes are those of Aeolosoma (Fig. 101), 

 which are only 1 mm. long and spotted with 

 red oil globules in the integument. They live 

 among algae, consist of from 7 to 10 seg- 

 ments, and reproduce asexually by trans- 



FiGURE 104. A giant earthworm is shown being pulled from its burrow in the wet river slopes 

 of Gippsland, Australia. Although the giant earthworm, in popular accounts, is said to be 12 

 feet long, scientific reports give 7 feet as the length. (Courtesy of Australian News and In- 

 formation Bureau.) 



