MOLLUSCA. 527 



and smooth on the surface, or consist of an enlargement 

 closely covered with short hairs. These, by their elasticity 

 and resistance, aid the animal in climbing and leaping. In 

 some animals they are single in each j(nnt, in others double, 

 and either extending to all, or confined to a few of the 

 joints. The suckers vary greatly in number, shape, and 

 position. In all, they are capable of being applied to the 

 surface of a smooth body. In some, the whole disc is ap- 

 plied, in others only the margin. These suckers are ei- 

 ther sessile or seated on footstalks ; sometimes they occupy 

 the tibial joints of the tarsus only ; in other cases they are 

 confined to the last joint, and ai'e from one to three in num- 

 ber *. 



The clatv (unguis) is attached to the distal edge of the 

 last joint of the tarsus. In a few species it is single, or 

 capable of being opposed to a projection, serving the 

 purpose of a thumb. In others, the claws are double, and 

 either move in the same plane, or act in opposition. The 

 muscles of the different parts are all included in the limb, 

 except those of the coxae, which originate in the trunk. 

 The former gives origin to those of the tibia, the latter to 

 the first joint of the tarsus, and this again to the one which 

 succeeds. These muscles are chiefly flexors and extensors, 

 the former placed on the ventral, the latter on the dorsal 

 surface of the limb. 



These divisions of the limbs are frequently subdivided 

 by sutures, but seldom or never by the ordinary method of 

 articulation which is by Ginglymus. 



The animals we are now considering are qualified for 

 executing different kinds of motion. The best walkers are 



• Accurate representations of these remarkable organs are given by Sir 

 BvBRARD HoMK, in the Philosophical Transactions for the Year 1816, Plates 

 18 — 21, from the beautiful drawings of Mr Baurr. 



