GASTEROPODA. 



385 



Fig. 182. sequently the entire 



proboscis inward. This 

 being done, a great 

 part of the inner sur- 

 face of the inner cy- 

 linder becomes a part 

 of the external surface 

 of the outer cylinder, 

 whilst the contrary oc- 

 curs when the pro- 

 boscis is elongated and 

 protruded. 



The elongation of 

 the inner cylinder by 

 the unfolding of the 

 outer, or what is the 

 same thing, the pro- 

 trusion of the probos- 

 cis, is effected by the 

 intrinsic annular mus- 

 cles which assist in 

 forming the organ ; 

 they surround it 

 throughout its whole length, and by their suc- 

 cessive contractions force it outwards; one espe- 

 cially, seen at b, placed near the junction of the 

 extremity of the outer cylinder with the inte- 

 guments of the head, which is stronger than 

 the rest. When the proboscis is protruded, 

 its retractor muscles acting separately, bend it 

 in every direction, being in this case antago- 

 nists to each other. The internal cylinder 

 incloses the tongue (f), the salivary canals (<?), 

 and the greater part of the oesophagus (d), but 

 its principal use is to apply the extremity of 

 the tongue to such objects as the animal would 

 suck or erode by its armed surface. 



In Aplysia, Akera, and others, the mouth, 

 consists of a fleshy mass of considerable 

 strength, to which are attached muscular bands 

 proceeding from the sides of the body, serving 

 for its movements, some drawing it forwards 

 whilst others retract it, but there are no jaws 

 nor anything equivalent to them, except the 

 cartilaginous hardness of the lips. 



But in such of the Gasteropoda as devour 

 vegetable matter, the mouth, instead of being 

 a proboscis, consists of a strong muscular 

 cavity, inclosing a dental apparatus adapted to 

 the division of the food. In the Snail, Slug, 

 Limnaeus, Planorbis, &c., this is a single cres- 

 cent-shaped horny tooth, attached to the upper 

 surface, and furnished along its opposite edge 

 with sharp points, separated by semicircular 

 cutting spaces, admirably adapted for the di- 

 vision of vegetable food. 



The dental organs of Tritonia and Scylloea 

 are, however, still more perfectly contrived for 

 such a purpose. The muscular mass of the 

 mouth is strong and powerful, but instead of 

 the single tooth of the Snail, it is armed with 

 two cutting blades (fig- 183, b, 6), horny in 

 their texture and exceedingly sharp, resembling 

 in every respect a pair of strong curved shears, 

 from which in fact they only differ in the mode 

 of their union, the spring of the one being 

 replaced by an articulation (c) inclosed in a 

 synovial capsule. These blades are approx- 

 imated by strong muscular fibres, and few 



Fig. 183. 



animal structures can resist their edge. The 

 lips (A), which are placed in front of these 

 teeth, are strong and very flexible, forming 

 a muscular tube, by means of which the food 

 is seized and brought within the power of its 

 formidable jaws, and then the divided morsels, 

 being seized by the horny teeth which invest 

 the tongue (d), are conveyed into the oesopha- 

 gus. 



Tongue. The tongue in these Mollusca is 

 generally a very important organ, serving not 

 only as a necessary auxiliary in deglutition, but 

 often as a means of eroding the food : in fact, 

 in one tribe only, T/iet/iys, is it found to be 

 deficient. In most of the proboscidean spe- 

 cies the tongue is short, and covered with 

 sharp, horny, and recurved spines, which, 

 seizing the morsels of food taken into the 

 mouth by a sort of peristaltic motion, push it 

 backwards into the oesophagus. In some ge- 

 nera which have no proboscis, the tongue is 

 of extraordinary length ; thus in Haliotis it 

 is half as long as the body, and in Patella, 

 Turbo, Pica, and others, it much exceeds in 

 length the entire animal. The tongue of Pa- 

 tella, which is three times the length of the 

 body, is represented at fg. 184 ; it is supported 

 by two cartilaginous pieces (6, ft) placed on each 



Fig. 184. 



side of its root; from these arise strong and 

 short muscular bands, which wield the or- 

 gan. The surface of this singular tongue, 

 a magnified view of which is given at B, 

 is armed with minute though strong teeth, 

 placed in transverse rows and arranged in 

 three series; each central group consists 

 of four spines, while those on the sides con- 

 tain but two a-piece. It is only at its an- 

 terior extremity, however, that the tongue so 

 armed presents that horny hardness needful 

 for the performance of its functions, the posterior 



