408 Prof. H. James-Clark on the Anatomy and 
body of the Hydra, and at the next instant assuming the reverse 
form (fig. 10), and embracing some projecting group of enide, 
or as it were wrapped around the parietes of an extremely elon- 
gated tentacle. As a further extension, the base is margined 
by an annular membrane, or velum (f, f*), and a single row of 
cilia (9 ); both of which serve to render it more conspicuous, and 
give to this region of the body the appearance of greater weight 
and firmness. 
§ 4. THe Prenensite OrGans.—The motory organs appear 
to be divided into two groups, of which one is very active in 
character, and the other is comparatively passive and resistant. 
The members of the former group are the vibratile cilia and 
velum; and those of the latter constitute the ‘ adherent organ.” 
The vibratory crown.—The vibratile cilia occupy two widely 
separate parts of the body, in one place fulfillmg the office of 
purveyors of food, and in the other acting as organs of locomo- 
tion in the strictest sense. The former are the true prehensile 
organs, and, with the margin to which they are attached, con- 
stitute the so-called ‘vibratory crown” (6, b', b?).. This organ 
lies, in the form of a nearly flat spiral, at the anterior end of 
the body, and borders the edge of the cup, which forms the 
principal part of the front. It therefore rests on the periphery 
of the disk (c, c!, c?), so that a delineation of the one defines the 
contour of the other. The spiral commences (5) at the extreme 
right of the front, and, sweeping around ventrally and just before 
the edge of the mouth (m) of the vestibule (v), passes to the 
extreme left, and thence along the dorsal edge of the cup, whence 
it passes toward its starting-point on the right, but a little ex- 
terior to it, so as to overlap it. Thus far it follows the edge of 
the cyathiform disk, and forms a distinct border throughout its 
circumference ; but in passing to the termination of its course it 
runs along the extreme brink of an inclined plane (figs. 11, 12, c*) 
which rests on a cornice-like projection that extends obliquely 
across the body, from the right, slightly backwards, toward the 
left, as far as the aperture (m) of the vestibule, and then rapidly 
narrows and vecomes blended (fig. 13, d*) with the body beyond. 
In fact the vestibule (v) is buried for its major part im this 
oblique projection, and opens at the widest or terminal part of 
the inclined plane which forms the anterior face of the latter. 
Consequently the vibratory crown, when followmg the bor- 
der (d°) of this plane, passes exterior to, and along the ventral 
side of, the aperture of the vestibule, but, instead of going beyond 
it, gradually approximates to it, and finally entering at its left 
side, and taking an oblique course toward the right, plunges to its 
very bottom, in one unbroken, single line (PI. ane fig. 13, 5°). 
In the true Vorticellide the disk is a prominently marked 
