52 



the muscle cell, and the large remaining central space is 

 occupied by the sarcoplasm. Thus in any transverse 

 sections of these muscles, the fibres are represented by 

 circular discs, the centre of which is granular proto- 

 plasm, whilst the periphery is either a dark staining circle 

 or this may have been resolved by the stain into a number 

 of segments — each a transverse section of a sarcostyle. 

 The sarcostyles do not run quite longitudinally along the 

 muscle fibre or cell, but wind spirally round it. Thus a 

 delicate double striation is present, but cannot be seen 

 unless one focusses completely through a fibre. 



The Radula, with its teeth, is being formed con- 

 tinually by the cells of the radular sac in which its 

 posterior dorsal extremity lies, and as the teeth of the 

 anterior region are broken away in action, new regions 

 move forwards and take the place of the old. The 

 radular sac is a delicate cellular caecum continuous 

 anteriorly with the epithelium of the pharynx. It ends 

 blindly at the point to which is attached the centro- 

 dorsal retractor muscle (figs. 19 and 16, C.d.m.). The 

 cells forming the wall of the blind end are known as 

 Odontoblasts. These secrete the teeth and the basement 

 membrane which bears them. In Buccinum the odonto- 

 blasts are very numerous and exceedingly narrow. 



Their length, however, varies in a regular manner 

 according to position, so that cushions of cells are formed. 

 Transverse and longitudinal sections are both required to 

 elucidate the structure of the radula sac. The transverse 

 section (Text-fig. 5, a) gives the key to the structure, for 

 it is seen that the radula near and up to the point of 

 origin is so fixed that the lateral portions with the lateral 

 teeth are turned up at right angles to the median area. 

 The cells of the radula sac are longest at the extreme end, 

 where the radula is formed. As a matter of fact, the 



