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*. 



DQaoMQDOQgQDQDDSaaaOOiJ 



ftfc^ 



Tongue of Whelk ; with separate teeth t- Magnified 

 10 diameters, the teeth 40 diameters. 



pleuree. 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**. 



Row of teeth of the tongue of the Periwinkle. 

 Magnified 120 diameters. 



They may be easily examined m the lim- 

 pet {Patella), the whelk [Buccinuni), or in 

 the freshwater snails, Lyimiceus, Planorhis, 

 Sec. 



In the tongue of many terrestrial Gastero- 

 poda, as the snaU (Helix) and slug {Limax), 

 the number of plates in each row is veiy 

 considerable, amounting to 180 in the large 

 garden-slug (Lwiax tnaximus) ; whilst in 

 many marine Gasteropoda, sucli as the com- 

 mon whelk (Bucciniwi 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 the 

 nearly globular head; but the closely-set 

 rows of teeth are usually very numerous, 

 frequently more than 100, and in some 

 species as many as 1(50 or 170; so that the 

 total number of teeth may mount up, as in 

 Helix jwmatia, to 21,000, and in Limax 



maximus to 26,800. The transverse rows 

 are usually more or less cm'ved, whilst the 

 longitudinal rows are quite straight; and 

 the cm-vatm'e arises on each side from the 

 central longitudinal row, the teeth of which 

 are 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 degi'ee as we pass fi-om the central line 

 towards the edges. The tongue of the 

 marine Gasteropoda is generally longer, and 

 its teeth larger ; and in manj' 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 beuig fre- 

 quently smaU and smooth at their edges, 

 whilst those of the lateral are large and 

 serrated. The tongue of Trochus zizyphimis 

 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 diflerent species of 

 the same genus. Thus in Doris jiHosa the 

 central baud is almost entirely wanting, and 

 each lateral band is formed oi a single row 

 of very large hooked teetji, set obliquelj' ; 

 whilst in JJoris tnberculata the central band 

 is most developed, and contains a number of 

 rows of perpendicidarly conical teeth, like 

 those of a harrow. In Dendronotm, 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 GOO rows of teeth. 



The tongues may be preserved either in 

 the dry state, or in glycerine or in balsam . 



