21 



showing that growth is, under favourable conditions, very 

 rapid. 



Habits of Polyclad Turbellaria. Polycladfl are exclusively 

 marine, and for the most part littoral, animals. Moreover, there 

 is no evidence of their occurrence in those inland seas where 

 certain marine animals (including one or two species of other- 

 wise characteristically marine Ehabdocoelida, p. 46) have per- 

 sisted under changed conditions. From half-tide mark down to 

 50 fathoms, some Polyclads j)robably occur on all coasts, but as 

 to their relative abundance in different seas we have very little 

 accurate information. The southern seas of Europe possess more 

 individuals and species than the northern, and probably the 

 maximum development of the group takes place on the coasts 

 and coral islands of the tropics. 1 No Polyclads have been taken 

 below 6 fathoms ; but their delicacy and inconspicuousness render 

 this negative evidence of little value. Six truly pelagic forms, 

 however, are known, 2 and these are interesting on account of their 

 wide distribution (three occurring in the Atlantic, Pacific, and 

 Indian oceans), and also from the distinct modifications they 

 have undergone in relation to their pelagic existence. 



Whatever may be the interpretations of the fact, Polyclads 

 are notoriously difficult to detect, and this fact doubtless explains 

 the scanty references to them by the older naturalists who col- 

 lected even in tropical seas. Lang, who worked seven years at 

 Naples, added to the Mediterranean fauna as many Polyclads as 

 were previously known for all Europe, in spite of the assiduous 

 labours of his predecessors, Delle Chiaje and Quatrefages. 

 Again Hallez, collecting at Wimereux at low -water, obtained 

 some twenty specimens of Leptoplana tremellaris in an hour, 

 while some other collectors working by his side could only find 

 two or three. Yet, even making allowance for the difficulty of 

 finding Polyclads, few of them appear to be abundant. 



Leptoplana tremellaris is frequently associated with colonies 

 of Botryllus, and if separated soon perishes, whereas the free- 

 living individuals are distinctly hardy (Hallez). A closely allied 

 but possibly distinct form lives upon the surface of the Polyzoon 



1 Von Stummer-Traunfels, Zcitschr. f. iviss. Zool. Bd. lx. 1895, p. 689. 



2 Planocera x>ellucida Mertens, P. simrothi v. Grff., P. grubei Grft"., Stylocho- 

 plana sargassicola Mertens, S. calif orniea "Wood worth, Planctoplana challengeri 

 Grff., all belonging to the Planoceridae. See v. Graff, " Pelagische Polycladen," 

 Zcitschrift /. iviss. Zoologie, Bd. It. 1892, p. 190. 



