116 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



movements, but probably by a ciliated portion of the lung cavity. 

 By the word siphon of course is understood merely the pulmonary 

 orifice. Generally speaking the word has quite another signification 

 in reference to the Mollusca ; but its use in an irregular sense has 

 been customary amongst naturalists when dealing with this genus. 

 Accompanying this peculiarity in the organs of respiration, we 

 have a Radula of a type which belongs to the pulmoniferous 

 Mollusca in both the land and fresh- water genera. In Siphonaria 

 diemenensis or denticula the buccal mass is red and fleshy, in 

 which two long, thin, rather broad, cartilaginous jaws are 

 imbedded. Amid these the broad Radula is spread, working 

 almost perpendicularly, with a very slight movement backwards, 

 as far as I could ascertain in the few opportunities which the shy 

 and sluggish animal gave me of observing. The oesophagus is a 

 bright orange-yellow, and terminates at the distance of about 

 20 mil. in a sac of the same colour. The Radula soon becomes a 

 tube enclosed in membrane. It does not follow the oesophagus, 

 but curls round and projects as a closed hyaline tube outside the 

 buccal mass into the coelom. When the animal is wounded it 

 emits a viscid milky fluid of apparently a different character from 

 the blood of Gastropods. 



The Radula with careful manipulation may be easily extracted 

 and spread out. It is not difficult to clean it from the attached 

 membranes, and when spread is is about 8 mil. long by 3 broad. It 

 is a series of curved lines of teeth diminishing in size from the 

 centre to the margin. The teeth have a broad crescentic edge, 

 which increases in width downwards, and are fixed upon the 

 membrane. The teeth gradually diminish outwardly to a mere 

 faint line of curved tubercles. The appearance of the whole is 

 more like a series of combs with long curved teeth. There 

 appears to be, properly speaking, no plate from which each tooth 

 projects, and the central tooth from which each row diverges in a 

 curved line is rudimentary. 



This correspondence between the organs of respiration and the 

 Radula, would seem to justify those naturalists who wish to make 

 the structure of the dentition a leading feature in the classification 

 of Mollusca. There can be no doubt that in this instance, the 

 dentition is an organ of far higher importance than the shell : in 

 fact the dentition goes a very long way in giving a clue to the 

 habits of the animal ; but it is not an indication in every respect. 

 Thus, if we should say that the pulmonary sac for breathing air 

 or water has a certain form of Radula accompanying it, we 

 find organs of respiration associated with almost every form. 

 The common periwinkles on our coast, Risella melanostoma, 

 Gmelin, and Trochocochlea iceniata, Quoy and Gaimard, afford 

 us illustrations of this. In both of them respiration is performed 



