RADIATA. 



sac contracts, water is forced into the corresponding tube, 

 which is thereby elongated and protruded. (Fig. 87.) 



1 



FIG. 87. Morphology of Echinoidea. i. Echmid larva, a. Mouth, b. Stomach, c. In- 

 testine, s. Skeleton. 2. Diagram of Echinus. The spines and the ambulacra are repre- 

 sented over a small portion of the test ; the vascular system is cross-shaded ; the nervous 

 system is represented by the black line. a. Anus. b. Stomach, c. Mouth. da.nAf. Vas- 

 cular rings round the alimentary canal, e. Heart, g. Test. k. Nervous ring round the 

 gullet, i. Ambulacral ring, or circular canal round the gullet, k k. Polian vesicles. /. Sand 

 canal, m m. Radiating ambulacral canal. . Secondary ambulacral vesicles, o. Ambu- 

 lacra, or " tube-feet." p. Spines, r. Madreporiform tubercle. 



The shell of the 

 Echinus is cov- 

 ered with semi- 

 globular warts, 

 or beads, each 

 of which during 

 life supports a 

 sculptured spine 

 with a hollow at 

 its base, forming 



FIG. 88. Shell of Echinus, or Sea-urchin ; on the right side, 

 and ligaments a covered with spines ; on the left, the spines removed. 



ball and socket joint, subsidiary to locomotion. (Figs. 

 88 and 89.) Pedicellarice are minute, almost microscopic, 



