118 THE HETEROPODS THEIR MOTIONS. 



niirans delectansque adspexi." It can move in a retrograde 

 manner. Wlien weary with rowing, or when touched, the 

 little boatman contracts its oary fins, and drawing itself 

 within the shell, sinks to the bottom, where it rests a short 

 space either upon the keel, or the prow, or the vertex, but 

 never on the umbilicus. Then again it rises upwards, 

 rowing obliquely until the surface has been gained, where its 

 course is held in a straight line* o'er the trackless surge." 



The small Gasteropod order which Cuvier has called Hete- 

 ropods, are fully as pelagic as the Pteropods, and, like them, 

 have no other way of changing their place than by natation. 

 The foot, instead of forming a flat horizontal sole, you will 

 remember, has a vertical direction, and assumes the figure of 

 a compressed semicircular fin, which being moved by its own 

 muscles from right to left, propels the animal forwards, — 

 like a sculler who works his boat with a single oar. The fin 

 is ventral, but, on a hasty glance, you might mistake it for a 

 dorsal crest ; for the Heteropods — as indeed all pelagian 

 mollusca do — swim in a reversed posture, the foot to the 

 surface and the back looking downwards. In the Carinaria — 

 beautiful creatures ! clear as crystal and painted with the 

 liveliest colours, — and in the Firola, this ventral fin is aided 

 in its office by some subsidiary membranes situated upon 

 the neck or near the tail, but whose powers of propul- 

 sion are inferior to its own. Combined, however, they give 

 to these genera a velocity superior to what has been noticed 

 in any other tribes of mollusks ; being, indeed, very remark- 

 able for the quickness of their movements, propelling them- 

 selves in a forward or a backward course, in a straight or a 

 curved line, with equal facility, and without any retardation 

 of their pace. The Atlantes, which are destitute of these 

 secondary fins, and whose body, compressed in the small 

 space of their spiral shell, presents less resistance to the cir- 

 cumfluent medium, are slower of foot ; and instead of moving 

 in an even line, they advance after the manner of the Hyales, 

 with alternate periods of activity and rest. 



For the Pteropods, as we have seen, there is no organ of 

 rest ; they float or swim unfixedly in the abyss of waters, 

 realising in their persons the fable of the paradise-birds, 

 which ever hovered in the heavens, too aerial and spiritual 

 to require the support of our gross earth ; but the Hetero- 

 pods need occasional repose and a cessation from activity, — 

 and how admirably is the foreseen want provided against ! 

 Where are they to rest — where fix their anchor in the world 



* Fauna Gioenlandica, 388. 



