20 PLATYHELMINTHES TURBELLARIA CHAP. 



Appearance and Size of Poly clad Turbellaria. Polyclads 

 are almost unique amongst animals in possessing a broad and 

 thin, delicate body that glides like a living pellicle over stones 

 and weeds, moulding itself on to any inequalities of the surface 

 over which it is travelling, yet so fragile that a touch of the 

 finger will rend its tissues and often cause its speedy dissolution. 

 The dorsal surface in a few forms is raised into line processes 

 (Planocera villosa), or into hollow papillae (T/iysanozoon brocchii}, 

 and in very rare cases may be armed with spines (Acantkozoon 

 armatum, 1 Enantia spinifera} ; in others, again, nettle-cells (nema- 

 tocysts) are found (Stylochoplana tarda, Anonymiis virilis). Some 

 Polyclads, especially the pelagic forms, are almost transparent ; 

 in others, the colour may be an intense orange or velvety 

 black, and is then due to peculiar deposits in the epidermal cells. 

 Between these two extremes the colour is dependent upon the 

 blending of two sources, the pigment of the body itself and the 

 tint of the food. Thus a starved Leptoplana is almost or quite 

 white, a specimen fed on vascular tissue reddish. Many forms 

 are coloured in such a way as to make their detection exceed- 

 ingly difficult, but this is probably not merely due, as Dalyell 

 supposed, to the substratum furnishing them with food and thus 

 colouring them sympathetically, but is probably a result of 

 natural selection. 



The largest Polyclad, the bulkiest Turbellarian, is Leptoplana 

 gigas (6 inches long and 4 in breadth), taken by Schmarda, 

 free-swimming, off the coast of Ceylon. The largest European 

 form is Pscudoceros maximus, 3^- inches in length and stoutly 

 built. A British species; Prostheceraeus vittatus, attains a length 

 of from 2 to 3 inches. These large forms, especially the- 

 Pseudoceridae (pre-eminently the family of big Polyclads), are 

 brightly coloured, and usually possess good swimming powers, 

 since, being broad and flat, they are certainly not well adapted for 

 creeping rapidly, and this is well shown by the way these 

 Polyclads take to swimming when in pursuit of prey at night. 

 The size of any individual is determined, amongst other factors, 

 by the period at which maturity sets in, after which prob- 

 ably no increase takes place. Polyclads apparently live about 

 twelve months, and mature specimens of the same species vary 

 from -|- inch to 2-^- inches in length (Thysanozoon Irocchii}, 

 1 Collingwood, Trans. Linn. Soc. 2 ser. vol. i. pt. 3, 1876, p. 83. 



