ON STYLOCHUS PELAGICUS. 29 



the genus Lepto'plana , but as ouly very young specimens 

 were obtained the evidence is insufficient. 



The species of Pelagic planarian at present known seem 

 to be eight in number, viz. : — 



Stylochus Mertensi, Mertens. 



— pellucidus, ibid. 



— pelagicus, Mihi. 



Leptoplana striata, [Schmarda, ' Neu Wirbellose Thiere/ 



Th. i, 1, 17, Taf. ii, 34. 



— sp. (?) Mihi. 



Stylochoplana tenera, Stimpson, ' Prodromus Proc. Acad. 



Nat. Sci. Philad.,' 1857, p. 29. 

 Eurylepta oceanica, DarAvin. 

 Cenh'ostomuin gigas, Schmarda, 1. c, 17, Taf. iii, 36. 



The two species of Mertens, living as they do on and 

 amongst the Gulf-weed, are not thoroughly pelagic in their 

 habits; but they are excellent swimmers, and are often to be 

 found free in the region of the weed, and form a transition 

 to the entirely pelagic species. Probably Schmarda^s Cen- 

 trostomum gigas may also be only occasionally free swimming. 



Larva of Thysanozoon (-^olidiceros, Q-uatrefages) . The 

 remarkable Planarian Larva of Johannes MUller. 



A planarian larva, closely resembling that described by 

 Johannes Miiller, in a paper entitled " Ueber eine eigen- 

 thumliche Wurmlarve aus der Classe der Turbellarien und 

 aus der Familie der Planarien" ('Miiller's Archiv,^ 1850, 

 pp. 485-500, Taf. xii, xiii), was found in great abundance 

 on the sea surface at the harbour of Zamboangan Mindonao, 

 Phillippines, on January 30th, 1875. The same larva was 

 also obtained in the open sea on February SOth, 1875, at a 

 point about 140 miles due north of Point d'Urville, New 

 Guinea, but where reefs were not far distant. 



The press of other work prevented much time being given 

 to the study of these larvae, and Johannes Miiller's paper 

 was not on board the ship. The larvae are very difficult of 

 observation, moving, as described by Miiller, with great 

 rapidity, and a frequent gyration of the body on the long 

 axis. 



The present figures were taken from specimens kept at 

 rest by being squeezed under a covering glass. Hence the 

 exact form of the body, which resembled much more closely 

 Miiller's figures, is not maintained. Two stages of the larva 

 were observed. 



