Prof. J. Aliiller on the Stmcture of the Ecbinoderms. 3 



fi\re saekers round its moutb, but without any upon its body, 

 does not creep upon its future ambulatory region, but acts ex- 

 actly as if the oral suckers constituted that region ; taking the 

 position of an Echinus with its mouth directed downwards and 

 its body upwards, it adheres by the oral suckers and stretches 

 them about in various directions. When, however, the first 

 ventral sucker emerges at the posterior extremity of the body, 

 immediate use is made of it, as if it at once belonged to the am- 

 bulatory region and availing itself, sometimes of the oral, some- 

 times of the lateral sucker for adhesion, and as fixed points 

 whence to stretch itself, the young animal forms a sort of trans- 

 ition towards a Holothuria with an ambulatory region. In this 

 condition also we see that the young Holothuria does not creep in 

 any particular direction ; on the other hand, feeling about on the 

 glass, it moves hither and thither indiscriminately. I have not 

 yet seen the adult HolothuriadcR with ambulatory regions creep- 

 ing head foremost ; I have no doubt however that they do, as 

 they are completely adapted for this mode of progression. The 

 extant information upon this point is obscure ; they have been 

 seen creeping, but it is not stated in what direction. Johnston 

 says of Thyone papillosa (Forbes's British Starfishes, p. 236), 

 that it has a slow progressive motion, more gradual than that of 

 the shadow on a sun-dial, effected by the suckers being elongated 

 and fixed to some spot, and then contracting so as to drag the 

 body forwards ; the suckers, however, are said to be used more 

 frequently as anchors than as feet, since these animals are slug- 

 gish and indolent in disposition. The Synapta do not move in 

 any particular direction, but simply twist and wind about, unless 

 they are upon a sandy bottom ; thus placed however in vessels 

 of water, Quatrefages has seen them bury themselves in the sand 

 with their oral tentacles. 



The elongated Sea-urchins occupy an important position in 

 this inquiry. In the Spatangidce the ambulatory region consists 

 of segments of all the five ambulacra, but the mouth is situated 

 nearer the one extremity of the ventral surface and the anus at the 

 other, between two radii and opposite to the azygos radius, which 

 has thence been called the anterior radius ; in fact, as its suckers 

 are more particularly made use of in locomotion, it would really 

 seem to deserve this name. Accordingly it has been sought for 

 also in the regular forms, in which the anus either lies in the cen- 

 tre of the dorsal surface (regular Echinida and most Asterida), 

 or as in other Asterida is wholly absent. That the position of the 

 madreporic plate does not determine the posterior extremity was 

 rendered evident by the circumstance that in many elongated 

 Echinida it is lateral, while in others it is an expansion of the 



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