126 GASTEROPOD MOLLUSCA. 



genus is, in fact, a numerous colony of little mollusca, every 

 one in its own cell, distinct, yet inseparably connected with 

 its fellows. Collected into the figure of a gelatinous cylinder, 

 open at one extremity and closed at the other, and rough- 

 ened externally by a multitude of tubercles disposed some- 

 times in rings and sometimes irregularly, they float in the 

 Australian seas like stars of this lower world, shedding 

 around them a halo of light, brilliant indeed, but surpassed 

 in beauty by those other colours of the creatures which it 

 serves to disclose ; colours which come and go at pleasure, 

 glorying, as it were, in their subtle changes, passing rapidly 

 from a lively red to aurora, to orange, to green, and to azure 

 blue ; a magic scene, compelling more than the admiration 

 of every beholder. 



II. CRAWLERS. I have already mentioned that the octo- 

 pod cuttles drag themselves along the ground with a speed 

 for which they seem ill adapted : " It is a difficult task," 

 says the Rev. Mr. Guilding, " to secure them without a 

 spear, so quickly do they hobble away under the rocks ;" * 

 and you have just read, that the shelled Cephalopods are 

 normally creepers. The other molluscans of this division 

 belong to the classes of Gasteropods and bivalved Acephala ; 

 but there is a very considerable difference between them in 

 the mode of their progression. The Gasteropods move 

 forwards with an even, gliding motion and continuously ; the 

 Bivalves drag themselves along, halting at every step ; and 

 those with a compressed elbowed foot, making often and 

 short leaps. The difference is determined by the structure 

 of the foot. In the Gasteropods this forms a flat, oblong, or 

 linear-oblong sole, constituting the inferior plane of the 

 body, and composed of muscular fibres, which have a longi- 

 tudinal direction down the middle, where they are collected 

 into two bands, while the fibres on each side are mostly trans- 

 verse. The figure of the foot in Bivalves has been compared 

 to that of the tongue of a quadruped, and the comparison 

 is equally illustrative of its structure, which is a close tissue 

 of interwoven muscular fibres : the organ is free, project- 

 ing like an arm from near the centre of the body, and is 



* Mag. Nat. Hist. ix. 194. Blainville shortly and graphically describes 

 the motions of the Octopods on land : "Being thrown, with a number of 

 other live creatures, upon the bridge, they moved very nimbly in all direc- 

 tions, a little after the manner of crabs, at the same time elevating their 

 backs, so that the tube might not touch the ground ; that is to say, raising 

 the point of junction of the head and trunk, crawling backwards upon the 

 lower surface of the mantle or sack, and forwards with the help of the four 

 arms on each side, the upper ones before, and the lower ones behind, a little 

 like the Ophiurse." CHARLESW. Mug. N. Hist. i. 400. 



