JOTTS8EAUMIA. 253 



Owing to the very small size of Jousaeaumia and the minuteness of the elements 

 composing the gill filaments, I had some difficulty in making out the details of the 

 gill structure, but as some few of my specimens were well preserved and the very 

 minuteness of the objects was of assistance in enabling me to study optical sections 

 under a high power, I have been able to make out some interesting points not 

 hitherto recorded. The individual filaments are slender, and, except for the fact that 

 their interlamellar edges are broader than their frontal edges, they have the usual 

 lamellibranchiate structure. The central cavity is lined by the usual chitinoid layer, 

 thickened at the sides. I could not determine from my sections whether the cavity 

 is divided by a transverse partition into an afferent and an efferent canal, but the 

 appearances seen in optical section lead me to think that it is. The greater number 

 of my specimens when mounted whole and viewed in optical section seemed to possess 

 a large number of closely set ciliated discs, and the late Professor WELDON to whom 

 I showed my preparations was of the opinion that there could be no doubt that 

 ciliated discs were present. Further investigations led me to modify my first opinion, 

 but disclosed an arrangement of the ciliated cells that merits a detailed description. 



Figs. 20 and 21 are transverse sections through the gill filaments, the former of a 

 somewhat young and the latter of an adult individual. The triangular shape of the 

 section of the filament with the narrower frontal edge and broad interlamellar base is 

 seen to be due to the great size and thickness of the cells marked I.e. Following the 

 usual terminology, the short cilia on the narrow frontal edges may be called the 

 frontal cilia ; they are borne on two or three wedge-shaped cells with small nuclei, 

 and the more laterally disposed frontal cilia are longer than the others, so much 

 longer that I was disposed to regard them as latero-frontal cilia, but I do not think 

 that they can be identified as such. The true latero-frontal cilia are very long and 

 rather stiff and are borne on very definite longitudinal rows of columnar cells arranged 

 in single series. These cells are large, with conspicuous round nuclei at their bases, 

 and can be very clearly seen in optical section when the surface of the filament 

 is brought into focus, fig. 2 2 A. Their position and shape is clearly shown in the 

 sections figs. 20 and 21, l.f. Following on these are one or two non-ciliated interstitial 

 cells, and tbe sides of the filaments just above their basal angles are occupied by 

 longitudinal rows of very large oblong cells with flattened elongated nuclei. These 

 cells are best seen in optical section by focussing below the latero-frontal cells, as in 

 fig. 22B, but they are clearly distinguishable in transverse section, though their 

 elongate shape is, of course, not shown in this case. These cells bear a large number 

 of very fine cilia, which interlock with those of adjacent filaments, and the interlocking 

 is so effectual that when the tissues are contracted by the action of reagents the 

 limiting membranes of the cells are torn off and remain adherent to the cilia in the 

 interfilamentar spaces (figs. 20 and 21). The interlamellar bases of the filaments are 

 covered by a few flattened noil- ciliated cells with small nuclei. The false appearance 

 of ciliated discs observable in so many of my specimens is due to the fact that in 



