2 Prof. J. Miiller on the Structure of the Echinoderms. 



an ambulatory region ; many Holothuria having nothing of the 

 kind. The ambulatory region, or the surface turned towards the 

 ground, sometimes includes an equal portion of all the radii or 

 ambulacra, so that the mouth, which is situated in the midst of 

 them, occupies its centre, as in the regular Echini and in the 

 Asterid(2, or the mouth is placed at the end, and the ambulatory 

 region, as in those HolothunadcB which possess one, is not con- 

 stituted by all the radii, but is formed by only three out of 

 the five ambulacra. The abdominal and dorsal surfaces there- 

 fore are not constant in relation to the radial form. The inquiry 

 into the bilateral symmetry of a radial form becomes, partly from 

 this reason and partly from the predominating radial symmetry 

 of the regular forms, exceedingly complicated, and it is even 

 necessary to avail ourselves of the instinct manifested by the 

 Echinoderms in their movements. 



All the forms provided with one, exhibit the most distinct in- 

 stinctive impulse to direct the ambulatory region, however con- 

 stituted, whether the mouth be placed in its centre or at one end, 

 towards the ground, and if laid upon their backs they endeavour 

 to turn round as an insect would do under like circumstances. 

 Even the radiated rudiment of the Echinoderm, in the larva, 

 behaves in this way as soon as the first ambulacral feet have 

 made their appearance. The primitive rudiment of the Echinus 

 or of the Starfish, dragging its larval framework about with it, 

 opposes by the movement of its suckers every position in which 

 they are not directed towards the containing glass, and thus 

 jendeavours to reinstate itself in its natural ambulatory posture. 



It is undoubtedly no part of the instinctive tendencies of the 

 animal to turn its ambulatory or abdominal side in accordance 

 with gravitation : for insects run upon the under side of fixed 

 surfaces when they are able to adhere to them, and the Echinidcs 

 creep actively up perpendicular surfaces by the aid of their 

 suckers ; the instinct of these animals, it may rather be said, di- 

 rects them to turn the ambulatory portion of their ambulacra to 

 whatever firm supports present themselves. 



The regular forms, whose ambulatory region includes equal 

 portions of all the radii and whose mouth lies in its centre, as 

 the regular Echinida, the Asteridae, and Ophiuridce, do not creep 

 in any particular direction, with one radius constantly anterior, 

 but it is sometimes one, sometimes the other radius or inter- 

 radius which is directed forwards as the animal progresses. 

 What Tiedemann says about the Asterida holds equally good of 

 the Ophiuridce and Echinida>. These animals move exactly as if 

 they were not aware of any diff'erence between back and front, 

 although they distinguish the ventral from the dorsal sides in 

 the most marked manner. The very young Holothuria with 



