112 THE STORY OF LIFE IN THE SEAS. 



parent. The uniform oval shape which it has 

 when first hatched becomes changed as it 

 develops by the formation of a certain number 

 of short blunt processes or arms, and it was 

 the presence of these which caused the older 

 naturalists to call this larva the Brachiolaria. 



If one of these minute Brachiolaria larvse 

 be caught and examined with a microscope it 

 is not difficult to see that it has a little round 

 mouth leading into a short digestive canal 

 which opens to the exterior by a vent. It is 

 therefore clearly capable of feeding itself and 

 leading a perfectly independent existence. In 

 the older larvae there will be noticed an appear- 

 ance which has, under a low magnifying power, 

 the form of an incomplete and rather opaque 

 ring round the stomach. This opaque ring 

 becomes larger and larger, it exhibits five projec- 

 tions radiating from its centre, and at last gives 

 rise to all the organs of the fully formed Star-fish. 

 As the ring develops the .larva sinks from the 

 surface and loses the power of independent 

 feeding, and then, when all is ready, the skin is 

 cast off and a small but perfectly formed 

 Star-fish emerges. 



The Trepangs, the Brittle-stars, the Sea-urchins 

 and other Echinoderms have, as a general rule, 

 life-histories similar to that of the Star- fish, but 

 there is one point of difference in detail which 

 is of sufficient interest to be mentioned before 

 passing on. The larva of the Brittle-stars and of 

 some of the Sea-urchins has a number of arms 

 which are much longer, in proportion to the 

 whole size of the larva, than they are in the 



