20 A MONOGRAPH OF THE 



which is present in a rudimentary state in the Polycerince and a few of the Eolididce, is 

 nothing more than a fold of the dorsal skin, and as such may be looked upon as representing 

 the so-called organ in the testaceous gasteropods ; though perhaps its true homologue exists 

 in the side-lappets of the Trochidce ; in which case these latter organs must be members of the 

 dorsal skin, and not of the foot. 



Habits. As it is seldom possible to study these animals in their native haunts, the little 

 that we know of their habits is mostly gathered from the observation of individuals kept in 

 confinement, and consequently under influences more or less artificial. 



Some of the species are nocturnal, but this is not the case with many of the littoral 

 tribes. Such of these as are gregareous on small sea-weeds, as Polycera quaflrilineata, may 

 be observed in a state of activity during the day time, in tide pools left among the rocks, and 

 apparently enjoying the warm rays of the sun. Alderia modesta has a similar habit in more 

 shallow water, becoming almost amphibious ; and Boris bilamellata is frequently found 

 exposed on rocks left dry by the tide. But the greater number of the species avoid the light, 

 concealing themselves under stones and shelving rocks. Most of the littoral tribes are found 

 near to low- water mark ; though some few kinds occur much higher up among the rocks, 

 where they must remain several hours every tide deprived of water. Doris pilosa, Eolis 

 papillosa, and E. nana, are generally met with in such situations ; even the spawn of these 

 species is frequently left dry by the receding tide. 



The Nudibranchs partake largely of the sluggish character of the class to which they 

 belong. This is more especially the case with some of the Doridida and Tritoniadte, which 

 will remain for hours fixed to a spot without apparent motion, and when roused from their 

 torpor by the desire of food or some other stimulus, crawl slowly from place to place. Some 

 of the Tolycerince are an exception to this rule, and many of the Eolididce, more especially those 

 of our second section, are very active and lively in their movements. Dr. Johnston remarks re- 

 specting the Gasteropods generally, that the narrower and more elongated the foot, the quicker 

 the motion, which becomes retarded just as that organ tends more to the oval or round. This 

 observation holds good with respect to the Nudibranchs, if we except two or three genera in which 

 the foot, from its extreme narrowness, is little fitted for progression. In these genera, as Glauses, 

 Scyllcea, and Doto, the foot is formed for clasping the stems of algae or corallines, the sides 

 being very thin and flexible, anacd apble of being brought together so as to embrace the 

 stem : the foot can be completely flattened when applied to a plain surface. The Eolides with 

 a very narrow foot move slowly on a flat surface, probably for the same reason. 



Crawling is the usual mode of progression with these animals. This is effected in the 

 manner of the snail, by a series of minute undulations of the under surface of the foot, arising 

 from the alternate relaxation and contraction of the pedial muscles. None of our native 

 species have the power of swimming freely through the water; and we are not aware of any 

 means they have of reaching the surface but by crawling up any substance in contact with it. 

 This they do frequently in confinement, by ascending the sides of the vessel, and then 

 launching themselves, with outspread foot, on the surface of the water in an inverted position. 

 Like most of the other aquatic gasteropods, they are very fond of floating in this way, which they 

 do without any apparent effort. The mode of progression of the mollusca in this position has 

 been the subject of some dispute, and it is undoubtedly somewhat obscure. In the opinion of 

 M. de Q,uatrefages and some other naturalists, it is produced by the action of the vibratile 



