640 FRESH-WATER BIOLOGY 



20 (19) Ventral margin of the branchial area without long processes. 



Dero Oken 181 5. 



D. limosa Leidy is abundant and the best known of the North 

 American species. D. obtusa d'Udekem and a species which prob- 

 ably is D. perrieri Bousefield are of frequent occurrence, but a careful 

 study of the North American representatives of this genus, as well 

 as of Nais, is necessary before we can be sure of their exact relation 

 to European species. 



Fig. 986. Posterior end of Dero limosa. X 25. (After Bousefield.) 



21 (13) First anterior dorsal setae on II 22 



22 (23) Dorsal setae of two kinds: capilliform and shorter needle-form setae 



which commonly have cleft distal ends, 



Naidium O. Schmidt 1847. 

 N. osborni Walton has been described from Lake Erie (Walton, 1906). This genus is united 

 with Pristina by some writers. 



23 (22) Dorsal setae all capiUiform, mostly with very fine teeth on convex 



side; prostomium commonly elongated into a proboscis. 



Pristina Ehrenberg 1831. 



P. longiseta var. leidyi Frank Smith has the capiUiform setae of III greatly 

 lengthened (.700 mm.) and without serrations. The typical form of this 

 species as found in Europe has extremely minute inconspicuous serrations 

 on the capilliform setae of dorsal bundles of somites other than III. In the 

 variety P. I. leidyi, which is found in the United States and certain other 

 parts of the world, the serrations are coarser and more easily seen. 



d.s. 



Fig. 987. Pristina longiseta ya-r. leidyi. r. 5., ventral seta; X 3oo- (i. 5., part of seta 

 from dorsal bundle. X 450. (After Smith.) 



P. flagellum Leidy has a very characteristic posterior end. Specimens of 

 this species have been met with by the writer but once and when there was 

 no opportunity for study beyond enough to convince him of the general accu- 

 racy of Leidy's description and that the species really belongs to Pristina. 



Representatives are sometimes found of certain species in which the dorsal 

 setae of III are not especially elongated but their exact relationship to Eu- 

 ropean species is uncertain. 



Fig. 988. Posterior end of Pm/ma^ageWMW. X 16. (After Leidy.) 



24 (2) Reproduction sexual, never by budding; clitellum ordinarily poste- 



rior to VIII 25 



25 (34i 35) Ordinarily more than two well-developed setae in each of some or 



all of the bundles; ventral setae ordinarily cleft (exc. Telma- 

 todrilus: see below); clitellum ordinarily on X or XI and 

 one or more adjacent somites; $ pores ordinarily on XI, ex- 

 ceptionally on XII; spermathecal pores on somite anterior 

 to one bearing <? pores (in North American species) ; length 

 commonly more than 25 mm.; blood vessels usually with con- 

 spicuous red contents. . . . Family Tubificidae . . 26 



^ Accurate identification of species in this family usually requires the aid of careful dissec- 

 tions or of serial sections, and depends largely on a careful study of the reproductive organs. 

 (Fig. 990.) 



