_ bya series of somersets, the tail being thrown 
over the head; if the head is first made the 
fixed point atc, as before, the tail will then 
_deseribe the semicircle a d, which is twice 
he length of a step, or ad — cd, in the second 
_ movement ; the tail being fixed at d, the head 
_ turns upon it as the centre of a new circle, 
and takes a position in advance of d, at a dis- 
tance equal to c d; in these two actions the 
animal travels over a space equal to a 6. 
In this manner the head and tail are ad- 
vanced alternately. The third mode of pro- 
gression is by far the most speedy. The Hydra 
i javing crawled to the surface of the water, lifts 
‘its tail above it to dry in the air, whereby it 
exerts a repulsive action on the water; this 
hydrostatic apparatus, acting as a float, is ca- 
pable of suspending the body at the surface of 
the water in an inverted position ; in this pos- 
ture it rows itself along, or is drifted by the 
_ winds from point to point without effort. 
_ In the Beroé Pileus, the motions are partly 
by cilia attached to rectangular lamine, which 
are arranged in rows along the eight coste 
died Jig. 32, vol. i.) of the elliptically- 
formed body. According to Dr. Grant, each 
Yow contains about forty lamine, whose 
trarisverse section presents literally the appear- 
ance of the floats upon the paddle-wheel of a 
steam-boat : when the whole of these lamine 
‘strike the water simultaneously, the resultant of 
their combined action is in the line of the pro- 
_ jection of the axis of the animal, which 
is usually directed vertically. ‘The direction, 
_ however, may be changed by the cilia acting 
partially, by which the inclination of the axis 
in the direction of the animal is determined. 
The Physalus has the power of rendering it- 
_ Self either specifically lighter or heavier than 
__ water, by means of the inflation or contraction 
of its air-bladder; with the assistance of 
i which the aniinal is enabled either to swim upon 
the surface, or sink, if alarmed, into the bosom 
% of the ocean. The swimming-bladder of the 
_ Physalus is of considerable dimensions, and 
nearly of an elliptical figure, its longest axis being 
_ directed horizontally. The top of this bladder 
is furnished with a membranous lamina or 
_ crest, serving to increase the surface presented 
_ to the wind, before which it sails with consi- 
_derable velocity. The Rhizophysa Melon, the 
id 
4 
_ Tifera, with most of the Acalephe, having al- 
-Teady been described and figured in the article 
Aca.epux, the reader is referred to them for 
further particulars, but it may be remarked that 
the progression of the Diphyes is performed 
upon the principle of the Syringogrades, merely 
by the reception and expulsion of water by 
VOL. Tt. 
MOTION. 
_ Agalma Okenii, and the Diphyes Campanu-— 
433 
their two truncated sections, which, taking place 
alternately, gives the animal a mean uniform 
motion of considerable velocity.* 
Cirrigrade animals.—Unkke the entirely soft 
gelatinous forms which compose the Pulmograde 
and Ciliograde Acalephe, the Cirrigrade group 
have an internal solid skeleton to support their 
soft and delicate exterior tissues. In the Por- 
pita this skeleton is composed of a flat, circular, 
semicartilaginous plate, which lies horizontally 
on the surface of the water; to this plate 
are appended numerous cirrhi which perform 
the office of oars in rowing the animal on the 
surface of the sea: the Porpita is permeated 
with pores, which being filled with air render 
it of less density than the water upon which 
it floats. The Velella Limbosa has a thin 
perpendicular crest resting obliquely upon 
the horizontal plate, which being elevated above 
the water, and presenting a considerable sur- 
face to the wind, serves as a sail. In the 
Rataria cordata,t the crest is furnished with 
muscular fibres, by which the sail can be ele- 
vated or lowered at pleasure; but this does 
not take place without altering at the same 
time the centre of gravity; for the position of 
the body is nearly reversed when the crest 
is lowered, but it recovers itself on the crest 
being elevated. 
Pulmograde animals.—The umbelliform or 
mushroom-shaped dise of the Rhizostoma being 
capable of expansion and contraction at the will 
of the animal, is employed not only to keep 
the body (which is specifically heavier than 
water) at its surface, but also to propel it 
along. When the plane of the disc lies hori- 
zontally, and its whole margin contracts simul- 
taneously, the percussion given to the water is 
perpendicular to the plane, or in a vertical di- 
rection, and the animal receives an ascending 
impulse equal to the force of the reaction 
caused by the displacement of the water. In 
moving horizontally, the centre of the dise is 
turned in that direction, and the animal can 
also accelerate its descent by the assistance of 
the disc. The contractions of the disc are iso- 
chronous, and repeated about fifteen times in a 
minute,} or 15 X 60 = 900 times in an hour. 
The convex surface of the Aurelia aurita is 
directed forwards in progression ; in this posi- 
tion the whole margin of the dise is called 
into action, by which the locomotive force is 
increased, and owing to the figure of the disc the 
resistance of the water is diminished, and the 
speed is consequently accelerated. 
Syringograde animals.— Under this denomi- 
nation we shall include the Holothuria, the Salpz, 
and the larve of those insects whose progres- 
sion is effected by the alternate reception and 
expulsion of water to and from their respiratory 
organs by an action similar to that of the syringe. 
Independently of moving upon solids by means 
of its tubular feet, the Holothuria, according to 
Muller, is capable of drawing water into its 
cloacal aperture, and by means of its muscular 
system, of expelling it from its respiratory 
* See vol. i. p. 35 et seq. 
+ Vide Art. ACALEPHA, vol.i. p. 40, fig. 12. 
t See Grant’s Lectures, Lancet, 1833. 
2F 
