360 COMPARATIVE ANATOMY. 



to its function ; this function is to seize and comminute the food 

 of the animal. The apparatus, which forms the chief portion of this 

 arrangement, is a cuticular membrane which rises up from the lower 

 wall; on it there are small teeth or hooks, which are directed back- 

 wards and arranged in transverse rows. The teeth vary very greatly 

 in arrangement (Fig. 191, a b c J), form, and number; they are not 



only different in the larger di- 

 visions, but also in the orders 

 and families, and even in the 

 species, so that the genetic affi- 

 nities of the groups is indicated 

 by the form of these parts ; and 

 for this reason they have been 

 much used in classification. As 



. n -, ,. , f „ 7L a rule they consist of a median 



Fig. 191. A row or denticles from the J 



radula of Littorinalittorea. aMedian. longitudinal row (a), with which 

 b c d Lateral denticles (after Gray). symmetrically- arranged lateral 



denticles are connected. The 

 organ formed by the whole of these hooklets is known as the radula. 

 In many (Turbo, Patella) it projects some way into the ccelom, and 

 is enclosed by the sac-like sheath formed by a diverticulum of the 

 wall of the oesophagus ; it may even be longer than the body itself. 

 This organ may become very broad, and reach to the sides of the 

 pharynx. In the Heteropoda it is so far more highly developed 

 that the outermost uncini of the transverse rows may not only be 

 very long, but also be articulated in such a manner as to be movable. 

 When, therefore, the radula is protruded, these teeth are erected, 

 and when it is drawn back they come together like pincers, and so 

 act as seizing organs. 



There are special muscles for moving the radula which, with the 

 cartilage on the wall of the pharynx (p. 361), help to increase the size 

 of this organ (Fig. 200, B). The size of the radula is therefore closely 

 correlated with that of the buccal mass. This organ is found in all 

 divisions of the Mollusca excepting the Lamellibranchiata, although in 

 some (Thecosomatous Pteropoda) it is feebly developed. In rare cases 

 the radula and pharynx are altogether absent (Tethys). The radula is 

 relatively small in the Cephalopoda (Fig. 192, Or), where the oral 

 opening is marked by the presence of strong jaws. These are two 

 strong pieces (Fig. 192, C) which resemble the beak of a parrot, and 

 are provided with sharp edges ; the lower one (m) projects in front 

 of the upper one (m). The soft edges of the lips cover the bases 

 only of these jaws (Fig. 179, m m'). 



In the Gastropoda also the wall of the mouth has a firm invest- 

 ment, which forms a kind of jaw. In the Nephropneusta there is 

 a single semi-lunar piece which is beset by toothed ridges at its 

 free edge. In many of the Branchiopneusta lateral pieces are added 

 on to this unpaired one, which work horizontally on one another. 

 These paired jaws are also present in the Prosobranchiata, but are 

 best dcvelojied in the Opisthobranchiata. 



