CHAPTER V
ECHINODERMS (SPINY ANIMALS)
The Starfish
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Fig. 52.—Starfish on a rocky shore.
Suggestions. Since the echinoderms are aberrant though interesting
forms not in the regular line of development of animals, this
chapter may be
omitted if it
is desired to
shorten the
course.—The
common starfish
occurs
along the Atlantic
coast. It
is captured by
wading along
the shore when
the tide is out.
It is killed by immersion in warm, fresh water. Specimens are usually
preserved in 4 per cent formalin. Dried starfish and sea urchins are also
useful. A living starfish kept
in a pail of salt water will be
instructive.
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Fig. 53.—Plan of starfish; III, madreporite.
External Features.—Starfish are usually brown or yellow. Why? (See Fig. 52.) Has it a head or tail? Right and left sides? What is the shape of the disk, or part which bears the five arms or rays? (Fig. 53.) Does the body as a whole have symmetry on two sides of a line (bilateral symmetry), or around a point (radial symmetry)? Do the separate rays have
