140 MARVELS OF FISH LIFE 
ing how often negatives taken at totally different times 
and places will piece together to illustrate the life of 
any particular animal—or you may adopt the more 
interesting method of devoting your attention to some 
individual form of life, say, for example, the sea-urchin. 
Sea-urchins are to be found everywhere, but I have 
never seen the common sea-urchin (Echinus esculentus) 
in greater numbers, more beautiful in colour, or larger 
in size, than on the old breakwater at Port Erin. Here 
the sea is generally clear, and on a still day these urchins 
can be watched as easily under ten feet of water, as 
when they are only a foot or two below the surface. 
It may seem strange that the sea-urchin should be 
classified by zoologists with star-fishes, brittle stars, 
and sea-cucumbers, but close examination will show 
that in their structure they are very similar. 
The common star-fish (Astertas rubens) with which 
most of us are quite familiar, has five fingers, and in a 
groove along the under-surface of each finger are 
numerous semi-transparent structures known as tube- 
feet, each of which is capable of expansion and con- 
traction and terminates in a sucker. By means of 
these tube-feet the star-fish attaches itself to rocks, 
and also moves about. If an attempt is made to pick 
a star-fish off a rock under the water, considerable 
force may have to be exerted, and many of the tube- 
feet will be torn away. 
In the sea-urchin nature has bent up these five 
fingers and made them meet on the top. The flat 
