British Species of Polycells (Platyhelminthes)

Abstract

IN 1884 Ijima¹ described Polycelis tenuis Ijima, a species which closely resembled the well-known P. nigra Ehrbg. More recently, interest in the legitimacy of this species has been shown by Continental workers, particularly Komarek² and Sekera³. These two authors agreed that both P. nigra and P. tenuis are good species; but they differed on the external features whereby the species might be distinguished. Lender⁴ concluded that they could only be distinguished on the basis of internal anatomy and discounts such differences as shape of head, number of eyes and colour ; he also regarded them as ecologically similar. Latterly, Tu⁵ has questioned their separation as true species, but, like Lender, he found them in the same habitat on the Continent.