30 Lessons in Zoology, 



Lesson III. 



The hard parte of the back ware noted tht spinea, the forks, 



and the beama of the skeleton, — now those of the month side are 

 to be examined. 



On the cross-section of each ray (Fig. 1) may be seen 

 two rows of narrow plates (Fig. 3, a) forming the roof 

 of the groove, with small openings between them 

 through which the tube-feet pass. These are the per- 

 forated plates. On each side of these is a single row 

 of irregularly shaped, somewhat thickened plates (Fig. 

 3. b), called in distinction from the others, unperforated 

 plates. On the alcoholic specimens we observe that 

 the slender spines borne on these olates are movable, 

 the only movable ones, in fact, on the body of the star- 

 fish. 



Inside the ra;a are the brovn masses of the liver and the grape- 

 likfl clusterB of the ovaries. The eggs pass ont at minnte openings, 

 difficalt to find, in the angles of the rays. 



N^ot only is the shape of the body radiate, bat all the organs 

 show a radiate arrangement. From the oentral water-system ran 

 five radial water-tnbes ; from the stomach a lobe to each ray ; liver- 

 lobes are found in each ; the oral nerve-cord sends ont five 

 branches, and each ray ends in an eye. The attention of the 

 class may be drawn to as many of these points as they have ob- 

 served, in order that they may get the idea of the symmetrical 

 arrangement of parts aronnd the common center. 



Some of the specimens will show the convenient power 

 that the star-fish has of replacing lost rays (Fig. 5). He 

 seems not to mind the loss of two, three, or even four, at 

 a time, and grows them again with wonderful rapidity 

 More than this, if he is torn in pieces, and the parts 

 thrown into the sea, each ray will become a star-fish. 



