TONGUE. 



[ 772 ] 



TONGUE. 



It exists in the Gasteropoda, the Ptero- 

 poda, and the Cephalopoda. 



It consists of a central strip or band, 

 called the rachis, and two lateral bands, the 



Fig. 755*. 



Tongue of Whelk ; with separate teeth f- Magnified 

 10 diameters, the teeth 40 diameters. 



pleurse. In the rachis, the teeth often rather 

 resemble overlapping toothed plates, the 

 teeth being straight, while the pleural teeth, 

 or uncini of the lateral bands, are usually 

 curved and more or less serrate. 



Fig. 755**. 



Kow of teeth of the tongue of the Periwinkle. 

 Magnified 120 diameters. 



They may be easily examined in the lim- 

 pet (Patella) , the whelk (Bucdaum), or in 

 the freshwater snails, Lymnceus, Planorbis, 

 &c. 



In the tongue of many terrestrial Gastero- 

 poda, as the snail (Helix) and slug (Limax) 

 the number of plates in each row is verj 

 considerable, amounting to 180 in the large 

 garden-slug (Limax maximus) ; whilst ii 

 many marine Gasteropoda, such as the com 

 mon whelk (Buccinum undatum), the tongue 

 has only three plates in each row, one bear 

 ing the small central teeth, and the two 

 others the large lateral teeth. Generally 

 speaking, the tongue of the terrestrial Gas 

 teropoda is short, and contained within. th 

 nearly globular head; but the closelyr'se 

 rows of teeth are usually very numerous 

 frequently more than 100, and in som 

 species as many as 160 or 170 ; so that th 

 total number or teeth may mount up, as i 

 Helix pomatia, to 21,000, and in Lima 



maximus to 26,800. The transverse rows 

 are usually more or less curved, whilst the 

 ongitudinal rows are quite straight; and 

 he curvature arises on each side from the 

 central longitudinal row, the teeth of which 

 ire symmetrical ; whilst in those of the late- 

 ral portions of each transverse row, the 

 prominences on the inner side of each tooth 

 being suppressed, those on the outer side 

 are increased, this modification augmenting 

 in degree as we pass from the central line 

 towards the edges. The tongue of the 

 niarine Gasteropoda is generally longer, and 

 its teeth larger ; and in many instances it 

 extends far beyond the head, which may, 

 indeed, contain but a small part of it. Thus 

 in the common limpet (Patella) the princi- 

 pal part of the tongue is folded up, but per- 

 fectly free, in the abdominal cavity, between 

 the intestines and the muscular foot ; and 

 in some species its length is twice or even 

 three times as great as that of the entire 

 animal. In a large proportion of cases the 

 tongue exhibits a very marked separation 

 between the central and the lateral portions 

 the teeth of the central band being fre- 

 quently small and smooth at their edges, 

 whilst those of the lateral are large and 

 serrated. The tongue of Trochus zizyphinus 

 is one of the most beautiful examples of 

 this form not only the large teeth of the 

 lateral bands, but the delicate leaf-like teeth 

 of the central portion having their edges 

 minutely serrated. A yet more complex 

 type is found in the tongue of Haliotis, in 

 which the central band of teeth has nearly 

 straight edges instead of points, with on each 

 side a lateral band consisting of large teeth 

 shaped like those of the shark, and beyond 

 this, again, another lateral band on either 

 side, composed of several rows of smaller 

 teeth. Very curious differences also present 

 themselves" among the different species of 

 the same genus. Thus in Doris pilosa the 

 central band is almost entirely wanting, and 

 each lateral band is formed of a single row 

 of very large hooked teeth, set obliquely ; 

 whilst in Doris tuberculata the central band 

 is most developed, and contains a number of 

 rows of perpendicularly conical teeth, like 

 those of a harrow. In Dendronotus, the 

 central and lateral teeth are very finely and 

 beautifully serrated. 



In Littorina, the periwinkle, the tongue is 

 2^ inches long, coiled like a watch-spring, 

 and contains about 600 rows of teeth. 



The tongues may be preserved either in 

 the dry state, or in glycerine or in balsam . 



