152 AN INTRODUCTION TO THE STUDY OF FOSSILS 



soft tissues of its prey. Then the great digestive glands in the 

 upper part of the stomach and in the arms pour out their secre- 

 tion through the tubular passage remaining in the center of the 

 sheet, and the food is rendered fluid and absorbed. The stomach 

 then contracts and is rolled back through the mouth into the 

 body. The starfish may live for months practically without 

 food, but when opportunity offers it will eat many mollusks, one 

 immediately after another. It opens gastropods (periwinkle 

 or conch) in a similar manner by attaching the tube-feet to the 

 operculum and shell. 



The upper, narrow portion of the stomach has long divisions, 

 one extending to the end of each arm. These divisions secrete a 

 digestive fluid which converts starch into sugar, proteids into 

 peptones, and emulsifies fat ; the resultant, chyle, is taken 

 by osmosis into the veins. A vein runs along the digestive tube 

 from the mouth to the arms and forms a circle around each end 

 of the tube ; branches from these traverse the body and each 

 arm. The rings around the mouth open at one side into the 

 general body cavity, the coelome ; hence the "blood" of the 

 coelome and of the blood vessel is the same, a thin fluid consisting 

 of sea water, chyle, and some amoeboid corpuscles. From the 

 upper portion of the stomach, the waste products are conducted 

 out through the anus, w^hich is situated nearly in the center 

 of the dorsal surface of the disk. 



Respiration takes place (i) through the short, hollow, thread- 

 like processes (dermal branchiae) which extend out through 

 microscopic pores between the plates over the whole body and 

 open directly into the coelome, and (2) by the ambula- 

 cral system, the oxygen entering through the tube-feet and 

 madreporite. 



The nervous system has three divisions, (i) the epidermal, 

 — a pentagonal nerve ring around the mouth, from each of the 

 five angles of which one radial nerve fiber extends below the 

 radial ambulacral vessel to each eye-spot ; (2) the deep portion, 

 a double pentagon around the mouth, of which each angle sends 



