THE SEA-URCHIN. 



Lesson I. 



Kot the little ones asnally seen on the shore after a Btorm or in 

 the tide-pools, bat large ones that have been taken from their 

 hiding places nnder stones below low-water mark. The beat prep- 

 arations for stady are made b; drying them with the spines on, and 

 then sawing them in two horizontally. Besides these every shell 

 and piece of a shell without the spines will be of nse. Perfectly 

 bleached shells are often oast up on the shore by the waves, bat if 

 enough of these cannot be found, the spines may be removed by 

 placing them for a time in a dilute solution of potash, then clean- 

 ing them with » tooth-brush. Oare must be taken not to leave 

 them in the potash too long, or it will cause the plates to drop 

 apart. It will add greatly to the lesson if the teacher can also have 

 a few large sea-nrchins in alcohol, and a Mediterranean Echinus, 

 or sea-egg, either with or without the spines. 



The sea-urchin (Fig 1) is also sometimes called the 

 sea-egg. It is a green or purple ball. -It is not a perfect 

 ball, but flattened on both sidep, more so on the mouth 

 side than on the back. It is bristling all over like a 

 small hedgehog with spines longer than those of the 

 star-fish. 



We try the spines on the alcoholic specimens and 6nd them 

 movable. They are held to the shell by tiny muscles. We care- 

 fully pull off a large spine, and see the knob upon the shell and 

 the socket in the base of the spine that fits over the knob, thus 

 making a ball-and-socket joint. 



We know the mouth on the under side (Fig. 2) by its 

 little star made of the five white teeth. Though the 

 teeth appear small when seen from the outside, on exam- 

 ining them from the inside we find that with the jaws. 



