206 



ZOOLOGY. 



movable. The spines are useful in defense and locomotion. 

 Special forms of spines known as pedicellarice often occur 

 (asteroids and echinoids). They consist of two- or three- 

 pronged pincers moved by muscles. They may be mounted 

 on short stalks. It is suggested that they help clear the body 

 of foreign objects which lodge among the spines. 



239. Digestive System. The mouth and anus usually 

 open at opposite poles of the principal axis (asteroids, holo- 

 thuroids, and some echinoids). When the axis is vertical the 

 mouth is usually directed downward, in the centre of the oral 

 surface, and the anus occupies a more or less central position 

 on the upper or aboral surface. In some of the echinoids the 

 mouth or anus, or both, have vacated their central position and 

 have come to occupy opposite margins of the body. The diges- 

 tive tract is a simple tube, in the holothuroids running spirally 

 through the body. In the echinoids a similar condition is 

 found except that it begins in a complex masticating apparatus 



FIG. 96. 



FIG. 96. Vertical (sagittal) section through an arm and an interradius of a Starfish 

 (diagrammatic), a, anus; amp., ampulla; c.b., circular blood vessel; c.w., circular 

 water canal; co., ccelom; co.e., ccelomic epithelium; d.b., dermal branchiae; e, position 

 of the eyespot; ect., ectoderm; ent, entoderm; f, arnbulacral foot; g, ambulacral 

 groove; h, hepatic caeca or liver; i, intestine; i.e., intestinal caeca; mes, mesoderm; 

 mo"., mouth; m.p., madreporic body; n.r., nerve ring; os., ossicles in mesoderm; 

 r.n., radial nerve band; r.b., radial blood vessel; r.p., reproductive pore; r.w., 

 radial water canal; s.c., stone canal; sp., spines; z, lacunar spaces in the mesoderm. 

 (Adapted from various sources.) 



