326 THE POPULAR SCIENCE MONTHLY. 



what is their real function. But, at last, Agassiz the younger Agas- 

 siz, I believe discovered that these curious organs, called pedicillariae, 

 are for keeping the spines clean. 



The mouth of the sea-urchin is provided with five pointed teeth, 

 which shut together on a common centre ; and these teeth can all be 

 removed together, and, thus removed, they present quite a curious ap- 

 pearance, and are known among naturalists as " Aristotle's lantern." 



The sbell, which is composed of hundreds of pieces, presents a very 

 beautiful sight when the spines are removed. It is made up of ten 

 segments, radiating from the mouth, and converging to a central region 

 on the top (Fig. 27). Every alternate segment is perforated for the 

 numerous locomotive suckers to pass out, the intermediate segments 

 being imperforate, and more prominently marked with tubercles, on 

 which spines are borne. At the termination of the five perforated seg- 

 ments there is a triangular plate with a minute opening ; here the eye 

 is situated. Alternating with these five plates are five larger ones, 

 each with a hole, through which the eggs are laid. The largest of 

 these plates is the madreporic body, corresponding perfectly to that 

 seen on the starfish, already spoken of, and which doubtless acts as a 

 sieve or water-filter. 



As sea-urchins do not shed the shell, as do crabs and lobsters, the 

 inquiring mind will naturally ask how the animal can continue to en- 

 large when once it is invested with a hard shell. The answer is, that 

 every piece of the shell grows at the same time, and in this way the 

 whole shell enlarges together, and in a perfectly symmetrical manner. 



As already indicated, the sea-urchin moves by means of its locomo- 

 tive suckers. Extending these beyond the spines, it lays hold of the 

 surface of the rock or sea-weed, and then, contracting the suckers, 

 pulls itself along. And these suckers can be extended quite a distance 

 beyond the spines. For example, a sea-urchin can extend a sucker 

 from near the top of the shell, and bend it over, and lay hold of the 

 surface upon which the animal is resting. 



The sizes and forms of sea-urchins are very numerous. The ordi- 

 nary kinds are two or three inches in diameter ; some of the elongated 

 kinds in the tropics have a diameter of five inches or more. Some are 

 nearly hemispherical ; others rise in the centre so as to be almost a 

 cone ; others are flat, and are known as cake-urchins (Fig. 28) ; others 

 are more or less heart-shaped, etc. Some of the cake-urchins are curi- 

 ously modified, as seen in Fig. 29. 



On the coast of Maine especially, but also along both shores of the 

 Atlantic, as well as in most other parts of the world, the sea-shore 

 visitor will find the "sea-cucumber" appearing not very unlike a cu- 

 cumber while it still retains the blossom upon its end. On the coast of 

 Asia it is known as trepang, and is the animal which the Chinese use 

 so extensively for food. Aristotle called it Holothuria, but for what 

 reason he does not tell us, and we can only conjecture. The dead 



