ON A LITTLE-KNOWN SENSE-0EC4AN IN SALPA. 91 



tunica, and the bulb is seen not to lie strictly speaking in a 

 hollow of the tunica (for it nowhere breaks through the surface 

 of the tunica), but correctly speaking to be surrounded by a 

 deep and narrow annular depression or moat excavated in the 

 substance of the tunica around the bulb, but at a little dis- 

 tance from it [an. in the figure). The moat is not figured by 

 Ussow. Little more can be made out by study of the living 

 object. 



It may be added here that the shape of the appendage is 

 somewhat variable. It is club-shaped, inasmuch as it is 

 always somewhat thicker at the tip than lower down; but it 

 may swell out very gradually towards the tip, as in fig. 2, or 

 suddenly, as in fig. 3. These two figures represent extreme 

 cases of the relative degrees of swelling that are found. The 

 axis is normally straight, not curved as in Ussow's figure, 

 which I take to represent a pathological state. 



In fixed and stained preparations the appearances are very 

 difi'erent. As to the stem, it will be sufficient to note that the 

 lining epithelium of the tube is composed of cells similar 

 to those of the inner tunica, of which it is a process — clear 

 cells with somewhat loose-textured nuclei — nuclei in which the 

 chromatic " mitome" is readily discernible as such. 



The bulb, in good preparations, may be analysed into two 

 constituent elements — a centrally placed tuft of sense-cells, and 

 a surrounding cup-shaped wall or calyx of supporting cells. 

 These two groups of cells are readily distinguished by their 

 different optical aspect. 



The cells of the calyx are clear cells, in the sense in which 

 that appellative was just now given to the cells of the stem ; 

 whilst the sense-cells are dense, stain more deeply, and have 

 compact nuclei in which a normal chromatic mitome can only 

 be made out by careful examination with the very highest 

 powers. Fig. 3 shows these points in a sufficiently realistic 

 manner. 



The structure of the calyx is the most difficult point to eluci- 

 date in the whole of the organ. In shape the calyx is variable. 

 In Salps of not more than 3 mm. in length it is generally regu- 



