STRUCTURE OF COMMON STAR-FISH. 179 
body over the mussel-beds, or rocks, or weeds, the arms 
being capable of slightly bending ; after observing the red 
eye-spot at the end of each arm or ray, and the movements 
of the numerous spines which are attached to the separate 
plates forming the calcareous framework of the body- 
walls, and examining the movements of certain modified 
spines called pedicellarie, which are pincer-like bodies situ- 
ated among the spines, the student will be ready to study 
the external and internal anatomy. 
First, as to the calcareous framework of the star-fish. 
In order to study this, a transverse section should be made 
through an arm, and a vertical one through the body and 
along the middle of a single arm, and finally the anime: 
should be divided into two halves, an upper and lower. It 
will then be seen that the calcareous framework or so-called 
skeleton consists of a great number of limestone plates or 
pieces attached by a tough membrane and covered by the 
skin. Between the plates are spaces by which the water enters 
the body-cavity through the skin. These plates are arranged 
so as to give the greatest strength and lightness to the body. 
‘There-is also to be seen an oral (actinal) side on which the 
mouth igs situated, and an aboral (abactinal) side, the re- 
spective limits of which areas vary greatly in the different 
groups of Echinoderms. Each arm or ray is deeply chan- 
nelled by the ambulacral furrow containing four rows of 
suckers or ‘‘ambulacral feet,’’ which are tentacle-like 
protrusions of the skin growing out through orifices in 
the ambulacral plates, and are a continuation of the water- 
sacs or ‘“‘ampulle’’ within. The madreporic plate is a 
flattened hemispherical body situated on the disk between 
two of the arms. It is perforated by canals. 
The nervous system of Echinoderms consists of a plexus of 
cells and fibres overlying the surface of the shell. The oral 
ring and radial nerves may be seen without dissection. By 
closely examining the mouth, a pentagonal ring is seen sur- 
rounding it, each angle slightly enlarging* and sending off 
* Owfsiannikoff states that the nervous ring is a flat band, con- 
taining no swellings or ganglia, and not differing in structure from the 
ambulacral nerves, which latter possess nerve-cells as well as fibres. 
