300 JELLY-FISH, STAR-FISH, AND SEA-UKCHINS. 



logically separated from the rest of the organism. 

 If the two nerve-divisions are so placed as to in- 

 clude two ailjacent rays — i.e. if one cut is on one 

 side of a ray and the other on the further side of 

 an adjacent ray — then these two rays remain in 

 physiological continuity with one another, although 

 they suffer physiological separation from the other 

 three. When a Brittle-star is completely divided 

 into two portions, one portion having two arms and 

 the other three, both portions begin actively to turn 

 over on their backs, again upon their faces, again 

 upon their backs, and so on alternately for an in- 

 definite number of times. These movements arise 

 from the rays, under the influence of stimulation 

 caused by the section, seeking to perform their 

 natural movements of leaping, which however end, 

 on account of the weight of the other rays being 

 absent, in turninor themselves over. An entire 

 Brittle-star when placed on its back after division 

 of its nerve-rino^ is not able to rig^ht itself, owinor to 

 the destruction of co-ordination among its rays. 

 Astropecten, under similar circumstances, at first 

 bends its rays about in various ways, with a pre- 

 ponderant disposition to the tulip form, and keeps 

 its ambulacral feet in active movement. But after 

 half an hour, or an hour, the feet generally become 

 retracted and the rays nearly motionless — the 

 animal, like a Brittle-star, remaining permanently 

 on its back. In this, as in other species, the eflect 

 of dividing the nerve-ring on either side of a ray 

 is that of destroying its physiological connection 

 with the rest of the animal, the feet in that ray, 



