LOCOMOTION 73 



size at different ages and seasons. According to the sex 

 of the individual these glands produce either eggs or 

 sperms, which are discharged into the water. 



LABORATORY STUDY 



Dried specimens of starfish serve well for general study. These may 

 be compared with specimens which have been preserved in alcohol or 

 formalin. Work out the several parts such as disk, arms, madreporic 

 plate, spines, groove of the feet, and position and form of the mouth. If 

 skeletons of sea urchins are available, they are interesting for comparison. 



59. Life History. — The eggs and sperms fuse outside 

 the body. In their development into adults they pass 

 through a series of striking changes. The young or 

 larval forms do not resemble the adults at all. This de- 

 velopment through a series of marked changes is as 

 striking as that seen in the insects and is likewise called 

 a metamorphosis. 



60. Food Taking. — The starfish takes its food in an un- 

 usual manner. Most animals move the food to the mouth, 

 swallow it or engulf it, and digest it within the body 

 cavity. In the case of the starfish we find that the 

 stomach is projected through the mouth and made to 

 surround its food. In this position it digests and assimi- 

 lates the food and then withdraws its stomach through 

 the mouth and moves on slowly to some other place. A 

 common food of the starfish is the clam. The arms or 

 rays surround the clam, and the "hinge ligament" which 

 holds the shell together is tired out, thus causing the 

 protecting clam-shells to separate. The stomach is then 

 pushed out, enveloping the clam. The digestive fluid is 

 secreted and the dissolved clam is absorbed as food. 



61. Locomotion, — The animal moves chiefly by means of 

 the tube-like feet found in the groove on the under surface 

 of the rays. These so-called feet make little sucking disks. 



