AIATERIALS FOB A MONOGRAPn OF THE ASCONS. 495 



coriacea, clathrus, primordialis, cerebrum, reti- 

 culum, lacunos a, spinosa, and con tort a, besides ''Clath- 

 rina, sp. dub." Yet Leiidenfeld, after publishing a mono- 

 graph of the Calcarea, not only liad not seen this very obvious 

 structure, which Licberkiihn described with great clearness in 

 1865 (see below), but he even characterised my temerity in 

 venturing to point out its existence as showing a total lack of 

 '' Disciplin.'^ I am not quite sure that I know what " Disci- 

 pline^ means, but if it implies failure to see very obvious 

 things on account of preconceived notions, it is a quality 

 which I am very iiappy not to possess. Bidder also (1891) 

 described the oscula of cerebrum as '^ lined with collared cells 

 continuously up to the granular lip." I am unable to agree 

 with this statement, though it is true that in cerebrum the 

 oscular rim is shorter than in any other species known to me. 

 I have sections through an osculum of cerebrum where the 

 upper limit of the collar-cell layer is only separated from the 

 extreme edge of the osculum by about five epithelial cells in 

 the section. But in general, even in this species, the oscular 

 rim is deeper than that. The shortness of the oscular rim in 

 cerebrum leads, however, to one interesting deviation from 

 the more usual condition, such as has just been described in 

 con tort a. The transformation of the epithelial cells into 

 porocytes in the former does not take place wholly within the 

 oscular rim, but commences before the epithelium has turned 

 the edge, some way down on the exterior of the oscular tube. 

 In contrast to cerebrum, the specimens of reticulum which 

 I have examined were remarkable for the extreme length of 

 the oscular rim, the region devoid of collar-cells often extend- 

 ing down to the commencement of the tubes which converge 

 to open into the cavity of the oscular tube. 



Before leaving this subject there is one point to notice. 

 Both the sieve membrane,^ formerly described by me in 



' In some specimens of coriacea there is a ring-like sphincter, snch as I 

 described in clathrus. I iiave seen what is obviously a transition between 

 the two structures, namely, a sieve membrane with a very large aperture in 

 the centre surrounded by smaller apertures peripherally. In some specimens 



