488 MR. P. GEDDES ON THE MECHANISM 



that the radula in Buccinwn has muscular attachments, hoth dorsal and ventral (figs. 24 

 & 28), to the cartilages over which it is bent, has argued that the radula " travels over the 

 ends of the cartilages just like a band over its pulley, the cartilages being entirely passive 

 in the matter," and, further, that " the tongue has the same chain-saw-like mode of 

 operation throughout Cephalopoda and Gasteropoda." The anatomical facts, however, 

 above described appear to me to render these propositions, on the whole, no longer 

 tenable, though an observation remains. In the transparent bodies of some Heteropoda, 

 Prof Huxley' describes a chain-saw movement; so, if the framework remains quite 

 stationary, I can only suggest that the sliding of the radula over its support, which we 

 saw as a secondary factor in the Limpet, though impossible in the Cuttlefish and highly 

 improbable in the Whelk, may in these animals have acquired greater importance. 



A remaining argument, however, of high importance in confirming the view here 

 taken is the embryological. In developing embryos of Limncea, days before they 

 leave the egg, the action of the incipient odontophore may be observed with a low 

 power. Neither radula nor muscles are yet to be seen ; but the future cartilages rock 

 steadily, licking upwards and forwards with deepening sulcus, quite as described above 

 in the adult mechanisms of Patella. The fully developed structure in Pulmogasteropods 

 has the usual type ; and a transverse vertical section, say of _ the apparatus in the 

 common Snail-, is sufficient to prove its use in licking ; while in some cases, e. g. the 

 adult Limncea, this may easily be proved by direct observation. In Aplysia, too, the 

 same structure may be observed. 



Though not coming strictly, perhaps, within the limits of the present paper, a by- 

 product of til e inquiry may here be noted. Dr. Herman Fol''' recently published some 

 interesting observations upon Ascidians, from which it appears that the endostyle secretes 

 an inverted hollow cone of mucus lining the pharynx, which gradually descends into 

 the gullet, bearing attached to it the numerous nutritive particles which enter the 

 pharynx in currents of water. An odd analogy to this mode of feeding is furnished 

 by the small Limnma stagnalis, which, when creeping about, foot uppermost, at the 

 surface of the water in a slightly stagnant aquarium, covered here and there with a 

 film of resting Bacteria, is wont cautiously to curve the middle of its foot downwards a 

 little, thus producing a shallow boat-shaped depression below the surface, but free from 

 water. A very active secretion of mucus now commences over the whole surface of 

 the foot; and part of the adjacent film of Bacteria is permitted to slide gently down- 

 wards into the hollow, over the anterior edge of the foot, which is not depressed enough 

 to let any appreciable quantity of water enter as well. When the cup is nearly full 

 the mollusc raises its head and laps up the mixture of mucus with Bacteria, &c. in 

 large mouthfuls. For such a purpose a tongue is necessary, a chain-saw would be 

 unavailable. 



' Loc. cit. ' Bronn's ' Thier-Reich,' Malacozoa, plate xcv. 



•* Gegenbaur's Moiphologisches Jahrbuch, Ed. i. 



