62 THE OPEN BOOK OF NATURE 
fossils in rocks, such as limestones, which have been 
formed in the sea. 
We have not done with the spiny-skinned animals 
yet. There are the Sea-urchins—sea-merchants I 
heard a small boy call them. Their form, too, will 
be familiar to you. They have globe-shaped, heart- 
shaped, or flattened, rounded bodies, with no arms. 
They are enclosed in a shell or skeleton made up of 
plates of carbonate of lime. Tube feet, similar to 
those of the starfish, are arranged in five bands, 
which are well marked on the surface of the shell, 
and the latter is covered with a great number of 
socketed spines, forming a protection against 
enemies. ‘The mouth is central to the under surface, 
and the rows of tube feet pass from it in regular 
lines upwards. Fossil sea-urchins are found in sea 
deposits from the Ordovician Period upwards. Some 
beautiful species occur in the English White Chalk, 
which is of the Cretaceous Period. 
The Echinodermata also embrace the Crinoids, or 
sea-lilies. These were animals something like star- 
fish, with long, often subdivided, arms, attached to 
long stalks. Living species of crinoids are rare, 
but fossil forms are found quite large in size, and the 
animals they represent must have been very beauti- 
ful. The commonest modern descendant of the 
ancient sea-lilies is known as the Feather-star, 
which in the early part of its career lives attached — 
to seaweed, and has the appearance of a small sea- 
lily ; but later it gets rid of its stalk, and swims 
about by means of its five arms. I have seen lime- 
stones of Carboniferous age composed almost entirely 
of broken stalks and arms of fossil crinoids. Fossil 
