SEA-LILIES AND FEATHER-STARS 459 



and giving origin to five radial vessels of which the tube-feet are 

 branches, these vessels, however, running within the test. The 

 nervous system, too, is of the same general kind, but there are two 

 nervous net-works, one outside the test and the other within it. 



Sea- Urchins are grouped in two divisions: I. Regular forms, of 

 which the just-described species is typical, and II. Irregular forms, 

 in which there is a strongly - marked bilateral symmetry, and the 

 body is often flattened, while the upper parts of the ambulacra 

 are broadened out so as to collectively make up a flower-shaped 

 figure, the tube-feet of which are definitely specialized into gills. 

 These irregular forms again are divided into Shield- Urchins and 

 Heart- Urchins. 



Shield -Urchins are sometimes very much flattened, as the 

 name implies, and though the mouth has the normal position, the 

 aperture of the intestine is shifted back into an interradial position. 



Heart-Urchins are distinguished by the shape under which the 

 heart is conventionally represented, the notch of the heart being in 

 front. The opening of the intestine is displaced to the edge of the 

 disc, and the mouth, instead of being central, is shifted forwards. 

 The Heart -Urchins are devoid of the special chewing apparatus 

 known as " Aristotle's lantern ". A common British species is the 

 Purple Heart- Urchin (Spatangus purpiireus). 



CLASS IV. SEA-LILIES AND FEATHER-STARS 



(CRINOIDEA) 



This is a decadent group, once of great luxuriance, but of 

 which the most typical modern representatives are rare, and con- 

 fined to the very deep sea. Taking such a typical Sea- Lily, for 

 example, as Pentacrinus (fig. 282), it may roughly be compared to 

 a star-fish with branching arms placed on the top of a long stalk, 

 and having its mouth surface turned upwards. A certain vague 

 sort of resemblance to a flower upon its stem has suggested the 

 name. 



Examining Pentacrinus more closely, there will be seen a 

 relatively small central disc, or cup, as it is usually termed, from 

 which five arms project. These are of the nature of appendages, 

 and begin almost at once to branch in a forking manner. Each 

 branch bears a double series of short filaments termed pinnules, 

 the name having reference to the arrangement seen in some plants, 



