BRITISH NUDIBRANCHTATE MOLLUSCA. 21 



cilia which cover the surface of the body. Dr. Johnston objects to this, that it does not explain 

 all the phenomena : for instance, "An JEJolis crossing a basin can at once stop and remain 

 there for some time ; but during all this period of rest, the cilia are in as active a state as 

 when the creature is in motion."* This is undoubtedly the case, and would appear fatal to 

 M. de Quatrefages' theory ; and, moreover, the shell-bearing gasteropods are devoid of the 

 same extent of ciliated surface, and yet they float with equal celerity. It has also been 

 supposed that the undulating motion of the sides of the foot, acting against the water, is 

 sufficient to account for the progression of the animal when floating, but this explanation 

 appears equally unsatisfactory. We think, after carefully examining the subject, that this 

 mode of progression is not very dissimilar from that of crawling. Whilst floating along the 

 surface of the water, the sole of the foot is constantly undulating, as if moving upon the 

 ground ; and as a considerable quantity of mucus is always floating from it, the motion is 

 probably produced by the undulations of the foot acting against this mucus, which forms a 

 track on the surface of the water behind the animal. The inverted animal walks, as it were, 

 along the floating mucus, much in the same way as it glides over the mucus which it sheds on 

 its path when crawling ; but in the latter case, the mucus, becoming adherent to the ground, 

 enables the foot to act with greater effect than it can against the floating mucus : hence the 

 animal can always crawl more quickly than it can float. 



While floating in this manner the Nudibranchs occasionally drop suddenly down, 

 suspending themselves from the surface by a thread of mucus, which is fixed to the tail or 

 posterior extremity of the foot. In this way they will let themselves gradually down to the 

 bottom, or remain some time pendant in the water without apparent support ; for the thread 

 of mucus is so transparent that it can scarcely be seen. When carefully looked for, however, 

 it can always be perceived, originating in the track of mucus left on the surface by the 

 animal ; the mucus forming a small inverted cone at the point from which the thread issues, 

 and here slightly dimpling the surface of the water. This thread of mucus must not be 

 confounded with the byssus of the bivalves. Some of the species also occasionally suspend 

 themselves from the surface of the water by the hind part of the foot, which for this 

 purpose is expanded into a disc, and from this, as a fixed point, the body is suspended and 

 moved about at the will of the animal. In this case the floating mucus is the fulcrum to 

 which the hind part of the foot is attached. 



On any of these occasions, either when floating or when suspended, if alarmed, the 

 animal falls at once to the bottom. This is effected by the foot quitting its hold of the mucus, 

 when the animal, being specifically heavier than the water, of course sinks: its specific gravity is, 

 no doubt, increased by the collapse of the parts ; but the Nudibranchs are never lighter than 

 the water, even when fully expanded, though in this state many of them are nearly buoyant. 



We have seen Dendronotus arborescens float for a considerable time in the middle of the 

 water without any apparent connexion with the surface. This comes nearest to the act of 

 swimming that we have observed in any of the native species. Some foreign genera, however, 

 can swim freely through the water in any direction. Tethys and Melibe are stated by 

 M. Sander Rang to do this by means of the large veil with which they are furnished, assisted 

 by the undulating motion of the posterior part of the body; and the same means of progression 



* f Introduction to Conchology/ p. 130. 



