8 THE TURBELLARIA 



orders " Yers cavitaires " and " Yers parenchymateux " for the 

 two groups. 



To Ehrenberg, too, we owe the first definite attempt to classify 

 the Turbellaria in the restricted sense; and, indeed, he laid the 

 foundation for all the later systems of the class. He formed 

 two orders the " Dendrocoela " and " Rhabdocoela " in reference 

 to the character of the intestine. 



The next step in classification was made in 1844 by Oersted, 

 who divided the Dendrocoela into two families (1) marine forms, 

 with short, folded pharynx and very much branched intestine ; 

 for them he used the term " Cryptocoela " ; and (2) fresh-water 

 forms, with long, tubular pharynx and feebly branched caeca; 

 for which he retained Ehrenberg's term " Dendrocoela." 



A fourth group was erected by Uljanin (1870) for gutless 

 forms like Convoluta, for which he suggested the term " Acoela," 

 as opposed to the remainder of the Turbellaria, for which he pro- 

 posed the name "Coelata" (58). 



Our present classification is due partly to v. Graff (22), who 

 made a third group in the Rhabdocoela, viz. the Alloiocoela ; and 

 partly to Lang (42), who suggested the terms Polyclada for 

 marine, and Triclada for fresh-water and terrestrial Dendrocoeles. 



Dreparnaud (1803) may be referred to for his peculiar views 

 on the relationship of the Turbellaria ; he regarded them as inter- 

 mediate between the Mollusca and Annelida ; and Oersted held 

 a similar view, that the " Cryptocoela " (Polyclads) or " Planaria 

 molluscina " form a passage to the opisthobranch mollusca. Girard 

 and v. Jhering held somewhat similar views. 



Amongst the more important additions to our knowledge of 

 fauna, as well as of general anatomy, the following authors deserve 

 mention: 0. F. Miiller (1773-83), who gives recognisable figures 

 and diagnoses of the many new forms discovered by him ; 0. 

 Fabricius (1820-26), Duge's (1828-32), Ehrenberg (1831-36), 

 Oersted (1844), 0. Schmidt (54), and in more recent times, 

 Jensen (35). Moseley's (1874) valuable account of terrestrial 

 Triclads, v. Graffs extensive series of papers and his great mono- 

 graph on Rhabdocoelida, and Lang's valuable monograph on 

 the Polyclads, form together the best general account of the 

 group. For an account of British marine forms, see Gamble (18). 



The anatomy of these worms was first seriously studied by 

 v. Baer in 1827, who dealt with fresh-water Triclads; he was 

 followed by Duges (15) for Rhabdocoels ; by Mertens (1833) for 

 Polyclads; while Quatrefages (1845) gives an excellent account, 

 with very good figures, of the anatomy of various Polyclads. 

 These zoologists may be said to have laid the foundation of 

 our knowledge of the anatomy of the Turbellaria, more especially 

 of the generative organs, which are of so much importance in 



