=External structure= (figs. 18 and 19.)--In a fresh specimen or one which has been preserved in alcohol or formalin note the raying out of parts of the body from a common centre. This is characteristic of the body organization of all Echinoderms, and is known as _radial symmetry_. The lower surface of the body is called the _oral_ (because the mouth is on this surface), while the upper is called the _aboral_ surface. The central part of the body is called the _disk_. Note on the aboral surface of the disk a small striated calcareous plate, the _madreporite_ or _madreporic plate_. In the middle (or very nearly in the middle) of this surface of the disk there is a small pore, the _anal opening_. The entire aboral surface as well as a greater part of the oral side is thickly studded with the calcareous _ossicles_ of the body-wall. These ossicles support numerous short stout _spines_ arranged in irregular rows. Note that some of the ossicles support certain very small pincer-like processes, the _pedicellariæ_. In the interspaces between the calcareous plates are soft fringe-like projections of the inner body-lining, the _respiratory cæca_. Note at the tip of each arm or ray a cluster of small calcareous ossicles and within each cluster a small speck of red pigment, the eye-spot or _ocellus_. [Illustration: FIG. 18.--Dissection of a starfish (_Asterias_ sp.).] Make a drawing of the aboral surface showing all these parts. On the oral surface note the centrally-located _mouth_, the _ambulacral grooves_, one running longitudinally along each ray, and in each groove two double rows of soft tubular bodies with sucker-like tips. These are called the _tube-feet_ and are organs of locomotion. Make a drawing of the oral surface. =Internal structure= (figs. 18 and 19).--TECHNICAL NOTE.--Take a specimen which has been immersed for some time in the nitric acid solution, and with a strong pair of scissors, or better, bone-cutters, cut away all the aboral wall of the disk except that immediately around the madreporite and the anus. Now begin at the tip of each ray and cut away the aboral wall of each, leaving, however, a single arm intact. When the roof of each arm has been carefully dissected away the specimen should appear as in fig. 18.