1883.] NATURAL SCIENCES OF PHILADELPHIA. 229 



the term, is wanting in Spurilla^ it is hardly to be supposed that 

 the formation of the radula is exactl}- the same. 



The cells of the matrix lie, in his figure (Tab. YITI, fig. 2&), 

 behind one another, and only the most anterior one comes in 

 contact with the tooth and takes part in its formation. As is 

 easily seen, these relations are very different from the state of 

 afiairs in Helix. 



Riicker,' who docs not seem to have known of the paper by 

 Trinchese, calls these teeth the ontoginous teeth. He shows five 

 cells to be present, but not arranged in Helix pomatia as I have 

 found to be the case with H. aperta. His cell a takes the place 

 of my 4 and 5. Over his cell d is fornaed the future tooth. Then 

 "the part of the cell that lies on cell d, the future hook, is raised 

 from its bed, and the tooth passes through the arc of a quadrant 

 in order to assume the normal position." ^ 



How or by what means the tooth is raised he does not say. I 

 believe, however, that, as I have shown, the death of cell ^ (PL 

 X, fig. .5), after the tooth is formed, is a much more plausible 

 explanation. 



II. — Observations on the Nervous System. 



The nervous system of Ancylus flamatilis was first described 

 by C. V'ogt, in 1841, while that of ^4. lacuftris, as far as I know, 

 has never yet been especially described. It is, however, formed 

 on the same plan as that of the former species ; the difference in 

 the two being merely one of position. Vogt described the oesoph- 

 ageal ring in the following manner : The oesophageal ring con- 

 sists of two superior, two lateral, and one inferior ganglia.^ This 

 description is not correct. The part was better described by 

 Moquiu-Tandon^ in the j'ear 1852. 



Moquin-Tandon * found that the 03Sophageal ring consisted of 

 seven ganglia : two superior, which he called the cerebral ganglia 



1 Ueber die Bildung der Radula bet Helix pomatia. Besond. Abdruck 

 au^d. xxii. Bericht d. Oberli. Ges. f. Natur- und Heilkuiide, 1883. 



^ Dann "hebt sich der Zelle d aufliegende Theil der Zahner, der 

 zukiinftige Haken desselben von seiner Uuterlage ab, der Zahn beginnt 

 eine vierteldielung, um allmablig aus der iibergekippten in die uomaler 

 Stelliing iiberzugeheu." Ueb. d. Bildung d. Kadula, etc., p. 217. 



^ "Der bchlundring besteht aus zwei obern, zwei seitliolien und einem 

 unteni Knoten." Benieik. u. d. Bau d. Ancjlus, etc., j). 29. 



* Recher. auat. physiol. s. TAncyle, etc., p. 129, et seq. 



