SEA-URCHINS, STARFISH, AND BRITTLE-STARS. 141 



something like a flower. The odd ray is to the front, and is 

 more conspicuous than the others because it is placed in a 

 deep groove. On the sides of this groove there are rows of 

 spines bent inwards until they nearly meet. Place a living 

 specimen before you with the grooved region towards you, 

 and you will notice that the slope of the test, the position 

 of the groove, and the arrangement of the spines, are all so 

 adjusted as to form a definite canal, which leads from the 

 crest of the shell straight towards the mouth with its spout- 

 like process. Notice also that the tube-feet of the petaloid 

 area are extensile and well developed, and so arranged as to 

 serve to catch hold of food-particles and sweep them down- 

 wards into the groove and so to the mouth. Notice the 

 anus near the middle of the vertical posterior region of the 

 shell, and the peculiar rounded sub-anal area beneath it, 

 which is liable to be mistaken for it. You will also notice, 

 what is even more obvious in dissection, that the apertures 

 of mouth and anus are very small indeed, showing that the 

 animal cannot live upon particles of considerable size, as do 

 the regular urchins. 



Having made these observations on the external aspect 

 of the living animal, you may proceed to study some of the 

 details of anatomy. To do this you should provide yourself 

 both with fresh specimens and with a considerable number 

 of dried tests, in the condition in which they are to be 

 found on every sandy beach. Dissection in the strict sense 

 is of course impossible; but a good idea of the anatomy 

 may be obtained by cutting open the shells with a strong 

 pair of scissors in different directions, so as to get different 

 views of the interior. 



Let us consider first the function of nutrition. What 

 does the heart-urchin feed upon? The first one you open 

 will show, even if you had not previously come to con- 

 clusions on the subject from the habitat. It feeds on the 

 minute particles contained in sand, and the alimentary canal 

 is always filled with sand, which is swept into the mouth 

 down the groove in the way of which we have already 

 spoken. As sand is abundant, the urchin does not need 

 to go and seek its food, but remains more or less passively 

 within its burrow, and uses its tube -feet and spines in 

 directing the food-supplies to the mouth. The food requires 



