STRUCTURE AMD DEVELOPMENT OF LOXOSOMA. 
265 
tentacles, ciliated on their inner sides, are borne on the oblique 
lophophore, and can be extended to the exterior (fig. 1), or, 
when the animal is irritated, can be completely sheltered 
within the vestibule (fig. 2). In the latter case, the vestibular 
aperture is reduced by means of sphincter muscles to a small 
circular hole situated on the anterior side (fig. 2, va .) . 
The body is covered by a delicate, transparent cuticle, 
secreted by an ectoderm composed of a single layer of cells. 
The study of these cells is immensely facilitated by the use 
of nitrate of silver applied in a manner suggested to me by Dr. 
W. H. Ransom, of Nottingham, the essential feature of the pro- 
cess consisting in washing the tissues in a solution of a neutral 
salt (KN0 3 ) which gives no precipitate with nitrate of silver, 
the solution having the same specific gravity as sea water [i. e. 
about 5 per cent. KN0 3 in distilled water). By means of 
this process a precipitate of silver chloride can be completely 
avoided, whilst the tissues suffer practically none of those 
changes which are brought about by washing with distilled 
water (No. 44). PI. XIX, fig. 3, represents the appearance 
of the posterior side of the calyx treated in the above manner, 
these preparations being in nearly all instances so successful 
that it was possible to draw, by means of a camera lucida, 
the outline of every ectodermic cell. 
These cells are of the following kinds : 
(i) Ordinary epithelial cells, which are large and polygonal, 
with a conspicuous nucleus (fig. 7 ) ; they occur over the whole 
of the calyx and stalk, on the outer and part of the inner 
surface of the vestibular fold, and on the dorsal and lateral 
portions of the tentacles. The inner sides of the latter are 
covered with a much higher epithelium, which is composed of 
cylindrical cells provided with large active cilia. In most 
regions of the ectoderm it is not possible to distinguish any 
definite arrangement of the epithelial cells, which, however, on 
the stalk of L. Tethyae occur in eight definite longitudinal 
rows, of which three are represented in fig. 9. 
(ii) Sense Cells. — In fig. 3 are indicated, scattered at 
irregular intervals, certain small round areas, which occur 
