OCCURRENCE OF TRICLADIDA 



31 



is at once the largest and rarest. Planaria alpina (Fig. 14, B) 



Fig. 14. Forms of Triclads, with the distinguishing specific characters of certain 

 British forms. A, Planaria lactea 0. F. M., x 2 ; B, Planaria alpina Dana, x 4 

 (after Kennel) ; C, P/ucgocata gracilis Leidy (after Woodworth), x 6 ; C', the nine 

 with the pharynges (ph) extruded; D, Ounda ulvae Oer.,x4; E, Planaria 

 gonocepfiala Dug. (after Schmidt), x 4 ; F, genitalia of Ounda ulvae (after Wendt) ; 

 G, head of Polycelis cornuta Schm. ; H, head of Polycelis nigra Ehr. ; I, head of 

 Planaria polychroa Schm. K to N show the distinctive characters of the genital 

 ducts in K, Polycelis nigra ; L, Planaria polychroa ; M, Planaria alpina ; N, 

 Planaria torva Schultze (after Iijima and v. Kennel), ga, Genital atrium ; go, 

 common genital opening; mgr, " musculo - glandular organ"; mo, "mouth": 

 ovd, oviduct ; pe, penis ; ph, pharynx ; pyr, pyriform organs of unknown signifi- 

 cance ; sc, sucker ; sp, spermatophore lying in (ut) uterus ; vd, vesicals seminalis. 

 (All except C and E are found in England.) 



is characteristic of cold mountain streams, but occurs down to 



