102 NEW ZEALAND NATURE-STUDY BOOK 
II.—THE SEA-URCHIN. 
The common Sea-urchin, or sea-egg, found in abundance 
in the rock-pools along our coasts, belongs to the sub- 
kingdom known as Echinoderms, which includes Star-fish, 
Sea-cucumbers, and other allied forms. 
It lives in the rock pools, and feeds on the minute forms 
of animal life with which these abound. 
The body of the animal is protected by a hard disc-like 
shell, convex above and flattened and slightly concave 
below. The surface of the shell is covered with a number 
of movable spines carried on small rounded elevations. On 
the under surface is an opening (the mouth) through which 
the teeth can be thrust. The teeth, five in number, are 
packed closely together, and present a sharp beak-like 
appearance. 
If the animal with its shell-covering be buried in. sand 
or in the ground for some time, the spines will drop off 
and the soft parts will disappear; the shell can then be 
examined more closely. | 
In its living state the spine-covered test is dark in colour, 
but, when prepared for examination as described, the colour 
changes to light green, or reddish brown. The elevations 
on which the spines are supported are plainly visible, 
radiating from a small circular area on the upper surface of 
the test. . 
If held up to the light and looked at through the opening 
on the lower side, the perforations in the shell can be 
easily made out ; these will be found to occur along definite 
areas. The outer surface of the shell shows five well- 
marked perforated areas, wedge-shaped at either end, and 
lying between these five similarly shaped spaces where no 
perforations occur. Through these minute openings the 
animal is able to thrust out numerous small tubes, each of 
