152 INVERTEBRATE ZOOLOGY 



Tube Dwellers. — Members of the order Sedentaria are tube 

 dwellers. Some live in burrows in the sand (Arenicola), others 

 form membranous tubes (Myxicola, Manyunckia), and still 

 others live in calcified tubes within which they withdraw (Hy- 

 droides, Spirorbis) and close the opening with a modified tentacle, 

 the operculum. The Sedentaria lack the jaws characteristic 

 of the free-moving Errantia and display much greater diversity 

 in structure of the anterior and posterior regions of the body. 

 Numerous filamentous gills and tentacles frequently adorn the 

 head and anterior body somites which usually protrude from the 

 tube or burrow (Fig. 77), while the parts constantly encased 

 have weakly developed parapodia. Bright colors frequently 

 occur on the gills and anterior region of some of the tube dwellers. 

 When such forms stick their heads from their tubes, they have 

 all the brilliance of full-blown flowers. 



Subclass Oligochaeta 



The oligochaetes are as characteristically terrestrial and fresh- 

 water inhabitants, as are the polychaetes marine. In many 

 ways, they bear evidences of degeneracy as an accompaniment 

 of the change from marine to fresh-water or land habitat. Pe- 

 lagic larvae and parapodia are entirely lacking in all members of 

 the subclass, while gills occur in only a few forms and the sensory 

 apparatus represents a very low stage of specialization. Setae 

 occur in pairs, rows, or bundles but never have parapodia associ- 

 ated with them. The sexes are never separate. The male and 

 female gonads occur in different segments. In the Naididae 

 and Aeolosomatidae, asexual as well as sexual reproduction 

 occurs. 



Near the anterior end of the worm, usually not far removed 

 from the openings of gonoducts, the body wall of a number of 

 somites is supplied with numerous glands which in the height of 

 sexual development form a thickened collar-like band over the 

 dorsal and lateral surfaces of the body known as the "clitellum." 

 This clitellum produces secretions which harden to form a capsule 

 or cocoon for containing the eggs after they are laid. In the 

 earthworms, fertilization is reciprocal. During copulation (Fig. 

 78) a spermiducal pore of each individual is opposite the opening 

 of a receptaculum seminis of the other so that sperm cells of each 

 individual pass into the receptaculum of the other, thereby 

 accomplishing cross-fertilization. 



