. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. XVI. THE STARFISH AND ITS ALLIES Stricture of a Star fish.\—A g^nce at the body of a starfish shows us that the name is rightly given. The body is called the disk; the five racU- ating structures the arms or rays. The term echinoderm (leaning spinv- us^ll sir^^^Thesl^elt^^^ ^^ examination of the dried specim/n b^fo?e the animal is literally em- bedded in the skin and pro- trudes as many thousands of little spines. Closer examina- tion reveals the fact that the skeleton is composed of very m
. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. XVI. THE STARFISH AND ITS ALLIES Stricture of a Star fish.\—A g^nce at the body of a starfish shows us that the name is rightly given. The body is called the disk; the five racU- ating structures the arms or rays. The term echinoderm (leaning spinv- us^ll sir^^^Thesl^elt^^^ ^^ examination of the dried specim/n b^fo?e the animal is literally em- bedded in the skin and pro- trudes as many thousands of little spines. Closer examina- tion reveals the fact that the skeleton is composed of very many tiny plates, all articu- lated together in such a way as to give great flexibility as well as strength to the frame- work of the starfish. Notice that the thin skin covers the skeleton, but that in a ray broken in cross sec- tion the interior of the ray is hollow. In the starfish the arms have the position of radii of a circle; hence the animal is said to be radially symmet- rical. Notice the differences be- tween the dorsal and ventral surfaces. The latter is called the oral surface because of the position of the mouth, which can be seen as a hole in the center of the disk. In living specimens the baglike stomach is frequently found projecting from this hole. The five grooves, which lead outward along the rays from the area around the mouth, are known as the amhulacral grooves because they contain the ambulacroB or tube feet. In the dried specimen the tube feet may be found as very small dried projections in the grooves. There are four rows of tube feet in each groove. Estimate the number of tube feet in a single row and thus figure out the number of tube feet in a starfish. Do you believe the number to be exactly the same for every starfish? Give reasons for your answer. Locomotion in the starfish is performed by the movement of hundreds of the little suckerlike feet. The process of movement of a single tube foot is a complicated one. It is performed partly by means of
Size: 1474px × 1696px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., boo, bookpublishernewyorkamericanbookcompany, booksubjectbiology
