MORPHOLOGY OF MELIBE 525 



laboratory it moves along the side of the glass aquarium as 

 other midibranchs do, e.g. Aeolidia. Such movements 

 seem to be caused by the ciliary action of the foot. In this 

 respect my observations differ decidedly from those of Pease 

 (1 860) on M . p i 1 o s a , who writes : ' Their foot cannot be 

 used for creeping on a flat surface, but it is well adapted for 

 clasping sea-weed ; ' and O'Donoghue (1921 : 194), who says 

 in part : ' It does not creep about on the eel grass but only 

 seems to adhere for the purpose of laying its eggs. In the 

 laboratory, too, it does not creep on the sides of the aquaria 

 and only partly clings to them. It has not been observed 

 creeping on anything after the manner of other nudibranchs, 

 and if not entirely a pelagic form like Phyllirhoe it is 

 beyond doubt very nearly so and is a most interesting form.' 

 Although O'Donoghue thinks Melibe is mainly pelagic, it 

 is quite evident, judging from its habitat, that the pelagic 

 habit is periodic at the most, i.e. its recurrence is spasmodic 

 (Agersborg, 1916, 1919, 1921, 1921a, 1922, 1922a, 1923). 

 M. leonina occurs not only as a pelagic form, but may 

 be found at a considerable depth, which is perhaps its habitat 

 the greater part of the year. Gould's specimen, 133 mm. long, 

 17 mm. high, and 32 mm. wide, was dredged at about 5| metres 

 depth; Cooper's, 70mm. long and 17mm. high, at a depth 

 of 38 metres. Presumably, when it is at the bottom, it crawls 

 on the bottom, for it has a well-developed foot not only 

 for clinging to sea- weeds but also for actual creeping and, 

 indeed, for ' galloping ', to use a term employed by previous 

 writers for other forms (Agersborg, 1923). 



On one occasion, as I was trying to feed M. leonina, it 

 dropped to the bottom of the aquarium and commenced 

 gliding g,long the bottom. I continued the feeding experi- 

 ment, when to my astonishment this nudibranch, Melibe 

 leonina, suddenly elongated to nearly twice its normal 

 length, showing a method of creeping similar to that described 

 by Parker (1917) for the sea-hare, Aplysia californica 

 Cooper (PI. iv, fig. 9). When elongating the body, the anterior 

 one-third of the foot was lifted above the substratum and 



