48 



NATURAL HISTORY. 



In a second group of sea-urchins the rows of tube-feet, instead of 

 extending from one pole of the animal to the other,, are restricted to a 

 smaller space on the side opposite the mouth. One of these forms, the 

 common " sand-dollar " or " sand-cake " which is here illustrated, shows 

 this well, the feet being restricted to the flower-like region on the surface 

 shown. The sand-dollar is almost perfectly flat, and in life of a rich 



purple color, inclining to brown, 

 but soon after being taken from 

 the water the color changes to 

 green. The color is permanent 

 and occasionally is made the basis 

 of an indelible ink. The spines 

 and skin are rubbed off from a 

 number of sand-dollars, ground to 

 powder, and then mixed to a paste 

 with water, and with this the arti- 

 cles are marked. 



Others of these flat urchins are 

 common in the warmer waters of 

 the world, some of them attain- 

 ing considerable size. Some are 

 as regular in their shape as the 

 sand-dollar or the common sea-urchin, but in others there occurs a marked 

 division into right and left halves, and a consequent tendency to the 

 obliteration of the radial symmetry so emphasized by the older zoolo- 

 gists. In some forms the eggs do not hatch into larvae so different from 

 the adult, as in the case of the common sea-urchin. The eggs, after 

 escaping from the parent, are frequently received in a brood-pouch formed 

 as a hollow, overarched by the spines of the external surface. Here they 

 hatch, the young looking like sea-urchins, and here they live until able to 

 take care of themselves. Most of the time is spent in this brood-pouch, 

 but occasionally the young crawl out on the spines of the mother's shell 

 as if to look about and see what the great world is like. A curious and as 

 yet unexplained fact may be mentioned in this connection. In the northern 

 hemisphere a great majority of the Echinoderms pass through a metamor- 

 phosis in passing from the egg to the adult condition, but in the southern 

 hemisphere the proportions are reversed, it being the rule there for the 

 egg to develop directly into the adult condition, and only exceptionally do, 

 curious larval forms occur. 



The last group of Echinoderms to be mentioned are the " sea-cucum- 

 bers," the Holothurians of science. In these the body becomes elongate and 



Fig. 47. — Sand-dollar (Echinarachnius) . 



