THE FREE-LIVING FLATWORMS (TURBELLARIA) 337 



'enostomiim coluber Leydig 1854. 



18 (9) Head region distinct from rest of body 



Length 6 mm. Width about one-thirtieth the length. Very 

 slender, white thread-like with snake-like movements. Head region 

 broader than the rest of the body with blunt point at anterior end. 

 Posterior end abruptly rounded. Asexual reproduction not known. 

 Brackish water, Falmouth, Mass. 



Fig. 597. Stenostomum coluber. Anterior end: wj, mouth; ^ pharynx- in 

 intestine; ov, egg (?); ns, protonephridium. X 20. (After Leydig.) ' 



A club-shaped proboscis is present. 



RhyncJwscolex. 

 Rhynchoscolex simplex Leidy 185 1. 



- (8) Ciliated pits shallow. 



Only one species. 



Length 4 to 7 mm. Color yellowish-white opaque. Anteriorly abruptly attenuated into a 

 long cylindrical clavate proboscis; anterior end abruptly narrowed, obtusely rounded Pro- 

 boscis shows longitudinal and numerous transverse marks. Mouth ventral, at the base of the 

 proboscis. Intestine straight and capacious. A small wriggling worm found among yellowish 

 fragments of vegetable matter and confervae at the bottom of clear brooks in the vicinity of 

 Philadelphia. 



Von Grafif regards the European species R. vejdovski Sekera 1888 as probably identical with 

 this American form. 



20 (5) With two lateral branches of the protonephridium. 



Family Microstomidae . . 21 



Mouth a longitudinal sUt on ventral surface, intestine occasionally with side lobes. Proto- 



nephridial branches open in anterior end. Testes and ovary either paired or unpaired, with two 



ventral sexual pores, the male posterior to the female. With or without eyes and ciliated pits. 



21 (24) The intestine extends dorsally and anteriorly beyond the junction 



with the pharynx Subfamily AIicrostominak. 



Only one genus Microstomum . . 22 



22 (23) With two reddish-yellow pigmented eye spots. 



Microstonmm lineare (Miiller) 1773. 



Length of single individuals 1.8 mm. In chains 

 up to 18 zooids with a length of 9 to 11 mm. 

 Slender. Very active. Color yellowish to grayish- 

 brown, rarely rose-colored, with the intestine 

 always darker than the body. Pre-oral portion of 

 intestine short. Two small ciliated pits. Nettle 

 cells or nematocysts in place of rhabdites. 

 Male sexual organs with {xiired testes; slender 

 chitinous spicule of copulatorj* organ with curved 

 Doint. Ovary unpaired and median in position. 

 Hi fresh and brackish water. Monroe Co. and 

 Ontario Beach, N. V.; West Twin Lake and 

 Round Lake, Mich. 



Fig. 598. Microstomum lineare. M ) anterior jxirtion 

 of a chain: e, eyes; r/>, ciliated pit; ai. pre-<>r.il por- 

 tion of intestine; m, mouth; oe, esophagus. X lo. 

 (After von Grafl.) (B) Chitinous portion of cirrus. 

 Much enlarged, (.^fter SchuJlze.) 



