i] INTRODUCTORY 7 



on their surfaces (Frontispiece). Imagine a minute, 

 elongated fragment of a most delicate leaf, some J in. 

 long by i^ in. broad, and you have a picture of 

 C. roscoffensis. Imagine, further, myriads of such 

 green, filmy fragments lying motionless on moist, 

 glistening patches of a sunny beach between tide-marks 

 and you see the species in its native habitat (Fig. 1). 

 To find C. paradoxa at home it is necessary to follow 

 the receding tide, to gather handfuls of the brown 

 seaweeds (Fig. 2) which are exposed towards the low- 

 water limit of the larger tides and to allow the tips 

 of the weeds to dip into water in a white dish. Singly 

 from their hiding-places chubby, brown C. paradoxa 

 come gliding down with rounded "head" and pointed 

 "tail" to swim uneasily in the water of the dish. 

 C. roscofiensis is pre-eminently gregarious, C. paradoxa 

 by comparison is solitary. Sand from a Convoluta 

 patch scooped up in a cup contains many thousands 

 of C. roscoffensis ; a patient fishing throughout the 

 time of low tide may result in a catch of fifty, or at 

 most a hundred, specimens of C. paradoxa. 



The surface of the bodies of the plant-animals is 

 somewhat slimy ; particularly in C. roscoffensis, and 

 is covered by fine cilia (Fig. 3) which, during the life 

 of the animals, are in constant motion. The cilia, 

 which are protoplasmic projections from the super- 

 ficial cells, serve, by their unceasing movements, to 

 row the animal through the water. 



