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The Molluscan Radula: Its Chemical Composition, and Some Points in Its Development

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The Molluscan Radula: Its Chemical Composition, and Some Points in Its Development THE SIOLLUSCAN BADULA. 115 The Molluscan Radula: its Chemical Composition, and Some Points in its Development. By Igerna B. J. Sollas. With Plate 9. HISTORY. THE molluscan radula, or dental ribbon, has been the sub- ject of research for at least a century and a half. Aristotle (3), though he speaks of teeth in Limax, alludes apparently to the ridges on the jaw, and there is no evidence that he knew of the existence of the radula: but it is interesting to find that the great naturalist was well aware of the fact that whelks bore holes in shells with the proboscis, although he cannot have fully understood the process. Poli made a jest of the tale as a fable, but Osier re-affirmed it in 1832 without knowing of previous work, and is now credited with having been the first to observe this interesting habit. Swammerdam is the discoverer of the radula: he gives a description of both the radula and jaw of the snail (Helix aspersa), in Dutch and Latin, in his 'Biblia Naturae/ Leyden, published posthumously. His death, as we are told by Boei'haave in the Life of the author prefixed to this work, occurred in 1680. The work is now too antiquated to possess more than an historical interest. In 1757 Adanson (1) described radulae from various gastropods of Senegal: the teeth are "infinitely small,hardly 116 IGERNA B. J. SOLLAS. visible, though sometimes perceptible to the touch. Looked at with the microscope . the pointed ends of the teeth are turned towards the stomach like those of the tongue of the lion or cat." Adauson observed the regular arrange- ment of the teeth, and in some cases counted them, finding 20,000 teeth in 200 longitudinal rows in a bulimoid land- snail which the uatives call " Kambeul," and 200 in 10 longitudinal rows in Patella. Poli (13), in 1791, was the first to give a clear figure of a radula in his magnificent work ' Testacea utriusque Sicilise.' Troschel (22), in 1836, first established the radula as an organ of great systematic importance. Curiously enough, in the same year van Beneden, in a paper written in 1835, and not quoted in the literature of this subject, points out the possible value of the radula in determining the reality of doubtful species. Troschel's work attracted the interest o£ zoologists to the radula; after an interval, in which Lebert, Allman, and particularly Loven, worked along the new lines, Troschel published his ' Gebiss der Schnecken' (1856-1863) —a general and masterly work now well known. His interest was not restricted to the form of the teeth, but extended to their chemical composition. Though Troschel was the first to make the suggestion—thrown out apparently as a shrewd guess—that growth takes place at the posterior end of the radula to make good the waste going on in front, yet lie did not follow it up by closer study, nor did he investigate the development of the organ. It has been one of the chief problems of later workers, but they have arrived at some- what conflicting results. By combining a study of the chemical composition with that of the development some of the difficulties which have ai'isen may be removed. CHEMICAL COMPOSITION. In 1845 Hancock and Embleton (6), in a study of the anatomy of Bolis, state that the radular teeth consist of silica. They base their conclusion on the partly mistaken observation THE MOLLUSCAN RADULA. 117 that the teeth do not dissolve in either acetic or nitvic acid, while hydrofluoric acid coiTodes them. No particulars of their experiments are given. The same authors investigated the teeth of Buccinum and came to the same conclusion. In 1852 Leuckart (9), being interested in the distribution of chitin in the animal kingdom, examined, among many other objects, the radulse of Gastropoda and Cephalopoda, and pronounced them to be chitin. He emphasised the fact that his identification of chitin rested entirely on two characters—one its resistance to caustic alkali, the other its solubility in boiling nitric acid. He adds: " It is possible that in this sense chitin is a collective conception, and that many special modifications will be discovered later. Perhaps we may conclude this from the varying behaviour of chitin when treated with alkali," and he expresses a wish that chemists would investigate the matter. About the same time Bergh (4), without knowing of Leuckart's paper, con- futed Hancock and Embleton's view, and demonstrated the absence oE silica in three species of Prosobranchiate Gastropods. Bergh's is the first exact investigation; we are indebted to Troschel for a German translation of an extract of his paper, which is written in Danish. Bergh showed that in Buccinum antiquorum (Triton nodif ernm), and in Strombus gibberula most concentrated acids bring about corrosion of the raduia in the cold, and eventually complete solution on boiling, while dilute hydrofluoric acid does not alter the teeth in form, but renders them more transparent. Incinerated ribbons of Marsenia perspicua gave no silica. The raduia of Buccinum antiquorum gave the reactions of iron and calcium phosphate. Troschel was dissatisfied with what he considered the con- tradiction in the results of Leuckai't and Bergh,1 and there- fore undertook with Bergemann experiments which combined and reconciled the results of both these investigators. Helix, 1 This remark seems hardly fair to Leuckart, who nowhere states that chitin is the sole constituent of the raduia and is not interested in the ash ; of Bergh's paper I have only read the extract given by Troschel. 118 IGERNA B. J. SOLLAS. Patella, and Doliuin were chosen for study, attention being directed both to the jaw and radula. The radula of these thi'ee forms was found to behave in a similar manner and consists of an organic constituent, chitin, together with the inorganic constituents, iron, calcium, carbonic, and phosphoric acid. It was further shown that the radula of Helix nemoralis contains 5 per cent, of ash, that of Dolium gale a 6 per cent. Koehler's paper, published (7) in the same year as Troschel's fDas Grebiss der Schnecken,' deserves a word of mention, since this observer also affirmed the presence of both an organic and an inorganic constituent, and suspected the occurrence of calcium. Later workers continue to make divergent statements in describing the chemical composition of the radula. Sollas (19) in 1885, when studying the nature of the silica in organisms generally, made use of measurements of the refractive index and specific gravity; he concluded that in the molluscan radula silica was present and that, as in so many organisms, it was in the form of opal (silica hydrate), but he does not mention the species on which his observations were made. Bloch (5) and others speak of the radula as chitin, but their views do not appear to be based on original observation. Similarly some modern text-books refer to this organ as com- posed of cliitin or conchiolin, others speak of it as siliceous. Huxley and Ray Lankester, with more caution, do not commit themselves on this point. It thus seemed worth while to look once more for definite evidence of the presence or absence of silica and of chitiu in the radula. It will conduce to brevity if I state at once the general results I have obtained. I find that in all the odontophorous Mollusca the radula has an organic basis of chitin; the Docoglossa are unique among Mollusca in the composition of their teeth, of which the most important constituent is silica hydrate or opal. All the other groups, including the Rhipidoglossa, form a second type in which the radular chitin is hardened superficially by deposits containing calcium, iron, and phosphoric acid, which, together THE MOLLUSOAN RADTJLA. 119 perhaps with ati additional organic substance, form that outer covering so long known as the enamel layer but hitherto unexplained. I have not been able to confirm Troschel's statement that carbonic acid is present, and though I have made repeated attempts, I have failed to determine whether magnesium is one of the mineral constituents. These points, therefore, must still be left to chemists. The Chiton- idfe present us with a deviation from the second type and staud alone among the forms I have examined. In this family ferric oxide is the most important mineral constituent and is the cause of the dark colour of the teeth. With the partial exception of Helix aspersa, the ash of the radula preserves the form of the teeth. In the first type of radula or that of the Docoglossa the mineral matter may form as much as 27 per cent, of the whole l'ibbon, this is the case in Patella vulgata; while in the second type it may contribute only 2"4 per cent., as in Helix aspersa, though in this species it sometimes rises to 3"3 per cent.; in Dolium galea it amounts, according to Troschel's analysis, to 6 per cent. It is interesting to find that the Docoglossa, which are so well-defined a group in other respects, differ so widely, not only from the Pectinibranchiata, but also from the Rhipido- glossa in the composition of the radula. We may commence our more detailed account with the Docoglossa, and first with PATELLA. If the radular ribbon of Patella is boiled in strong nitric acid, the organic parts are completely dissolved, and there sinks to the bottom of the test-tube a coarse-grained insoluble residue. Microscopic examination shows that this consists of the dark red brown cusps or free biting ends of the lateral teeth (fig.
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