PREHENSILE ORGANS OF ECHINIDA. 45 1 



so that the animal gradually ascends the smooth surface, as it 

 would the face of a submarine rock. In the animals of this 

 form, too, we have abundant evidence of the use of the tubular 

 feet in conveying food to the mouth. If the Asterias were 

 entirely dependent on the slight hold which it can take by 

 bending its rays, many animals upon which it preys would 

 easily escape ; but in this beautiful apparatus of suckers at the 

 extremities of the feet, is a provision for retaining with great 

 force whatever has been once entrapped, and for carrying it 

 towards the central mouth. In the Echinus these tubes are 

 much longer, and would seem to act like the tentacula of the 

 Polypes, floating loosely in the water, ready to attach their 

 suckers to any animal that in its active movements may cross the 

 field which they command. When a single foot has gained an 

 attachment, others are speedily brought to its assistance ; and 

 the unlucky victim is soon secured. That the Echini use their 

 tubular feet for progression also, is evident from the fact that, if 

 placed in a basin of water, they are not unfrequently found 

 within a short time perambulating its margin. 



1008. Besides these tubular feet, there exists another series 

 of appendages, so remarkable in their character, that they have 

 been supposed to be separate animals parasitic upon the Star- 

 fish and Echinus, and have been described as such under the 

 name of Pedicellaria. Each consists of a stem, bearing at its 

 summit a sort of forceps of calcareous matter, not unlike a crab's 

 claw ; except that the two (sometimes three) blades are equal and 

 similar. When the point of a fine needle is introduced between 

 the blades, which are for the most part open in a fresh and 

 vigorous specimen, they instantly close and grasp it with con- 

 siderable force. The particular use of these prehensile organs is 

 not evident. They are not confined to the neighbourhood of the 

 mouth ; but, where they exist at all (for it is not in every 

 species that they are found), they are disposed over the greater 

 part of the surface, often forming dense groups around the 

 spines. 



1009. In all the Echinida the alimentary canal forms a long 

 tube, possessing two orifices. We shall presently find that the 



