TECHNICAL NOTE.--Remove the spines from the underlying shell or test (fig. 21) and wash the test until perfectly clean, or place in a solution of lye for a short time and then wash. [Illustration: FIG. 20.--A sea-urchin, _Strongylocentrotus franciscanus_. (From specimen from Bay of Monterey, Calif.)] Note the characteristic radial symmetry of the _shell_ or _test_. Note on the aboral aspect, diverging from the medial anal aperture, five double rows of pores. What are these for? Each of the five divisions set with pores is called an _ambulacral area_, while the intervening segments which support the long spines are called the _interambulacral areas_. Note on the aboral surface, surrounding the median-placed anal aperture, a series of small plates. Those which are located in the interambulacral areas are the _genital plates_. Through these plates the ducts from the reproductive organs open by small pores. Note a very much enlarged plate with a striated appearance. This is the _madreporite_, which, as in the starfish, is the external opening of the stone canal and water-vascular system. Note the small _ocular plate_ at the tip of each ambulacral area. The ocular plates contain small pigment-cells and communicate with the nervous system. [Illustration: FIG. 21.--"Test" of sea-urchin, _Strongylocentrotus franciscanus_, with spines removed. (From specimen.)] From a general inspection of the sea-urchin's shell the Echinoderm characteristics, namely, radial symmetry and the presence of the water-vascular system, are readily seen. While at first glance there is apparent little similarity between the starfish and sea-urchin, nevertheless careful examination shows that the two animals are alike in their fundamental structure. Both are radially symmetrical. The position of the anal opening makes both starfish and sea-urchin slightly asymmetrical. In both the madreporite and anus are on the aboral side, while the mouth is centrally located on the oral side. In the starfish we noted five ambulacral areas, one on the under side of each arm; similarly we find five in the sea-urchin. In both cases also we find the ocular spots at the tips of the ambulacral areas. The genital apertures are situated interradially in the starfish. In the sea-urchin they are similarly placed. The dissimilarity between the two forms is largely due to the very much developed outer spines and the dorso-ventral thickening of the disk in the sea-urchin. The starfish is carnivorous, while the sea-urchin lives on vegetable matter consisting for the most part of green algæ and the red sea-weeds. Correlated with this difference in food-habits there are certain differences in the structure of the internal organs. For example, the alimentary canal in the sea-urchin winds in about two and one-half turns within the body-cavity before it reaches the anus. OTHER STARFISHES, SEA-URCHINS, SEA-CUCUMBERS, ETC.