BULLETIN 39, UNITED STATES NATIONAL MUSEUM. [46] 

 PRESERYATION OF THE EADULA. 



The classification of gastropod moUusks lias uo more fundamental 

 character among those upon which it is based than that of the radula. 

 This is a strip or belt of chitinous or horny material occupying the 

 place in the mouth where in vertebrates the tongue is found. Hence it 

 is sometimes referred to as the tongue. The inner end of the radula is 

 secreted by a glandular surface lining a pouch which lies under and 

 behind the cartihiginous ancf muscular buccal mass upon which the 

 radula is carried. The front end of the radula is strongly attached to 

 the cuticle. The surface is usually set with teeth disposed in rows, 

 transverse and longitudinal, each transverse row difiering from that in 

 front of it merely in maturity. The first few rows are brought into 

 active use; the others form a reserve to come in play when the anterior 

 rows are worn out. The buccal mass with the radula upon it is pro- 

 trusile, and its action may be easily studied by feeding a snail with 

 soft crumbs of bread or watching a Limnasa mowing away the green 

 confervse which gather on the glass of aquaria. 



Some few gastropods have no teeth on the radula. A few others,^ 

 like the cones, have isolated barbed teeth, to which a duct conveys 

 poison from a venom-gland. The bite of a cone is extremely painful 

 from this reason. The small round holes common in dead bivalves on 

 the beach are drilled by predaceous gastropods, like Natica or Urosal- 

 jnnx, who are able to use their teeth in a rotary manner. 



In a few forms like Aomcea, Patella, and Chiton the teeth are less 

 purely chitinous than in the land snails and. most marine gastropods. 

 Chitine is a substance very resistant to both acids and alkalies, espe- 

 cially the latter; so that, with few exceptions, such as those above 

 named, the teeth may be boiled in caustic potash without injury. The 

 horny jaw and its lateral appendages which exist in many gastropods 

 will not, as a rule, survive such treatment, and must be dissected out, 

 though in a few cases they are also chitinous. 



In large gastropods the buccal mass with the radula upon it is easily 

 recognizable, and can be dissected away without trouble. In rostrifer- 

 ous species it is in the muzzle, just within the mouth. In x)robocidifer- 

 ous forms it is to be found at the retracted tip of the inverted pro- 

 boscis. In the cones and Pleurotomoids, where the teeth are fewer and 

 less compactly set, they are often found only by the exercise of great 

 care. 



In small species dissection is often out of the question and another 

 method is to be used, which is, however, available only for such forms 

 as have a decidedly chitinous radula. The radula of chitons and lim- 

 pets subjected to boiling potash rapidly disintegrates, owing to the 

 solution of the nonchitinous cement which holds the parts together. 

 In such forms the radula must be sought with the aid of a microscope. 



In the others the process is as follows : The student provides himself 



