MOTIOiN. 



433 



Fig. 227. 



d 



by a series of somersets, the tail being thrown 

 over the head ; if the head is first made the 

 fixed point at c, as before, the tail will then 

 describe the semicircle a d, which is twice 

 the 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 b. 

 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 

 having 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 Beroe Pileus, the motions are partly 

 by cilia attached to rectangular laminae, which 

 are arranged in rows along the eight costre 

 (/i /i, Jig. 32, vol. i.) of the eiliptically- 

 f'ormed body. According to Dr. Grant, each 

 row contains about forty lamina;, whose 

 transverse section presents literally the appear- 

 ance of the floats upon the paddle-wheel of a 

 steam-boat : when the whole of these laminae 

 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 ; w : ith the assistance of 

 which the animal 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 

 Agalma Okenii, and the Diphyes Campanu- 

 lifera, with most of the Acalephse, having al- 

 ready been described and figured in the article 

 ACALEPII.E, 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. III. 



their two truncated sections, which, taking place 

 alternately, gives the animal a mean uniform 

 motion of considerable velocity.* 



Cirrigrade animals. Unlike the entirely soft 

 gelatinous forms which compose the Pulmograde 

 and Ciliograde Acalephae, 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,f the orest 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 disc 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 disc 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 disc 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 Salpag, 

 and the larvae 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. 



t Vide Art. ACALEPHtf., vol. i. p. 40, fig. 12. 

 $ See Grant's Lectures, Lancet, 1833. 



2 F 



