HA RD WICKE'S SCIENCE- G OSSIF 



49 



GRAPHIC MICROSCOPY. 



By E. T. D. 



No, III. — Palate of Limpet. 



N reference to this 

 subject, the words 

 of Mr. Gosse may 

 be quoted, "Who 

 that looks at the 

 weather-worn cone 

 of the limpet, as 

 he adheres slug- 

 gishly to the rock, 

 between tide- 

 levels, would 

 suspect that he 

 carries coiled up 

 in his throat, a 

 tongue twice as 

 long as his shell ? 

 and that this 

 tongue 'is armed 

 with thousands of 

 crystal teeth, all 

 arranged with 

 the most consummate art, in a pattern of perfect 

 regularity ! " 



The tongue, or palate of the limpet {Patella viil- 

 gaia) is a typical representative of the contrivance, 

 or cutting instrument, attached to the muscular cavity 

 which contains the oral apparatus of the gasteropod 

 mollusks. It is a long, or ribbon-like tooth-bearing 

 membrane, spread open upon the floor of the mouth, 

 forming nearly a flat surface ; and then assuming a 

 more or less lengthened tubular form, the interior 

 studded with transverse rows of teeth arranged upon 

 flattened plates ; when dissected and opened, it 

 merges into the form of a consecutive or continuous 

 band (the drawing represents two portions of the 

 same band), and is in the nature of a rasp or file, 

 admirably adapted, first to graze upon, and then to 

 divide, bruise, and engulf vegetable or other nutri- 

 ment ; as the teeth, or spines, are arranged points 

 downward on the flexible cartilaginous strap ; they 

 not only collect aliment, but assist in propelling it 

 into the oesophagus ; the wear and tear are made 

 No. 231.— March 1884. 



good by constant growth and new development. As 

 the anterior prickles are worn away and absorbed, 

 another portion of the tongue is brought forward to 

 supply its place, and that there may be no deficiency 

 in its length, the apex, the point where the con- 

 tinual growth and addition are going on is soft 

 and vascular ; each principal tooth sometimes has a 

 basal plate of its own, in other instances, one plate 

 carries several. 



So strangely diversified is the character of the 

 lingual ribbon in the gasteropods, as showing generic 

 character, that the classification of the Mollusca 

 has been attempted on the basis of its structural ar- 

 rangement and disposition. In the large garden slug 

 {Limax maximtis) there are as many as one hundred 

 and fifty rows of teeth, and the rows so closely 

 packed, that the aggregate number of teeth is said to 

 amount to over twenty-six thousand. In the marine 

 gasteropods, particularly the limpet, we find the 

 teeth larger, and the tongue so long, that it is even 

 folded up in the abdominal cavity. It has been 

 urged by Dr. Gray that these structures might be an 

 important guide to the natural affinities of the 

 species, genera, and families of the group, since 

 diversities so strongly marked must necessarily affect 

 the habits, form, and character of the animal. This 

 view of the subject is full of interest, and a systema- 

 tic examination opens out to the student of micro- 

 scopy, a most genial employment. 



The actual operation of the action of the tongue, 

 in newly-hatched individuals, may be observed on 

 the stage of the microscope, and is conveniently 

 seen in an ordinary freshwater aquarium containing 

 pond snails in various stages of growth ; the intro- 

 duction of mollusks, in such arrangements, is in some 

 degree necessary to preserve an equilibrium of 

 health, as their presence suppresses the overgrowth 

 of minute confervse. And when these plants are 

 attached to the side of the tank, the snails may 

 be found continually browsing ; with an ordinary 

 lens the rolling action of the palate can be plainly 

 seen, rasping, or rather mowing the minute vegeta- 



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