21 



where no Periostracum is formed. Against this, however, 

 he adds that the gland is too large for this function alone 

 and that it would be peculiar to find a special gland for 

 the secretion of the Periostracum, whilst the shell itself 

 is formed by the general epithelium of the mantle. 



In my opinion the objections that Tullberg brought 

 forward are not important. In the lamellibranchs the 

 Periostracum arises in a groove from a very definite 

 pad of cells, certainly epithelial in position but still 

 differentiated enough to form a special organ. Hence 

 there is no reason why the thick Periostracum of the 

 whelk should not be formed by this gland. In any case 

 no other function has been ascribed to it. The compara- 

 tive anatomy of this organ is being followed up by the 

 author. 



The shell side of the mantle from the opening of the 

 gland inwards is faced by epithelial cells differing from 

 those already noted in the absence of cilia. These are the 

 shell secreting cells. They are marked, particularly near 

 the gland, by the possession of granules of yellow 

 pigment. 



RESPIRATORY ORGAN (CTENIDIUM). 



There is only one ctenidium present in Buccinum, as 

 in most of the higher Gastropoda. This is the morpho- 

 logical right gill, but is situated now on the left side of 

 the pallial cavity. It has already been referred to as 

 being visible through the thickness of the mantle. This 

 ctenidium extends from a point, in line with the anterior 

 limits of the osphradium and mucous gland, as far back 

 as the pericardium (fig. 8, Ct.). It is composed of a large 

 number of flattened leaflets which are packed parallel to 

 one another and vary in size, so that they become 



