262 W. J. CROZIEK AND LESLIE B. AREY 



about the activities of the large exotic nudibranchs, and very 

 little has since then been added to the subject. We shall deal 

 with questions of natural history and behavior only in an inci- 

 dental way, paying attention specifically to the manner in which 

 Chromodoris responds when activated by various stimulating 

 agents. It was desirable to undertake a study of this kind be- 

 cause C. zebra has already provided material, of a very exceptional 

 character, for the treatment of some questions in which sensory 

 phenomena are implicated (Crozier, '15 a, '16 a, '16 b, '17 d, 

 '18 d). A knowledge of the sensory capacities and modes of re- 

 sponse in Chromodoris affords, also, some information as to the 

 comparative physiology of the nervous system in mollusks, about 

 which, particularly in nudibranchs, very little is known. 



Chromodoris zebra (fig. 1) is a large species very common at 

 Bermuda,^ with the form typical of the genus. An account of 

 its morphology w411 be found in the papers of Smallwood ('10) 

 and of Smallwood and Clark ('12). The body is elongated, es- 

 pecially in creeping, and measures up to 18 cm. in length. The 

 animal as a whole is very soft and contractile, and becomes read- 

 ily bent or twisted under appropriate conditions. Throughout 

 the year individuals of a variety of sizes are to be had by dredg- 

 ing in depths down to 10 fathoms. From September to June 

 great shoals of them, numbering thousands in all, crowd up at 

 intervals into shallow water (Crozier, '17 b). They become 

 notably concentrated in certain shallow mangrove creeks con- 

 nected with Great Sound. The cycle of events w^hich determines 

 the shoreward flocking has not yet been fully established. Its 

 coloration makes this nudibranch easy to distinguish upon the 

 bottom (Crozier, '16 b), and the migratory movements of the 

 species, owing to its lack of concealing behavior, may to some 

 extent be followed in the field. 



The animal moves with a smooth, even, gliding motion over 

 rock surfaces or on the muddy bottom, the entire surface of the 



^ As with many other marine forms found at Bermuda, it is probable that the 

 range of C. zebra is quite extensive, although so far it has been reported only 

 from Bermuda (Heilprin, '89; Smallwood, '10). I am informed by Prof. W. H. 

 Longley that a few individuals were obtained by his collectors, in seining on 

 grass-flats at Tortugas. W. J. C. 



