166 HALF AN HOUR WITH SEA-URCHINS 



the young are bi-lateral; that is, have two equal sides. 

 Singularly enough, the adult sea-urchin is developed 

 only out of a part of the young " Pluteus." 



Dredged up from the same depths as the Echinus, 

 are specimens of a heart-shaped animal, covered with 

 dark purple spines like the true sea-urchin. This 

 is the Spatangus, which may readily be recognised 

 from its shape, and its depressed height. Both the 

 sea-urchins and the spatangus are sad gluttons, and 

 may usually be found in the neighbourhood of oysters 

 banks, on which they commit great depredations. 

 The sea-urchins and the star-fish are connected by 

 an intervening series, commenced by the spatangi 

 on the one hand, and the " Cushion-stars " (Clypeaster, 

 &c.) on the other. The latter are five-sided, and do 

 not possess arms. They are peculiar, however, to 

 warmer climates than ours. 



In the true star-fish, of which we may take our 

 common British species, the "Five-finger" (Uraster 

 rubens, Fig. 90), as a familiar example, the skin 

 which covers the upper surface, as seen in the 

 illustration, has the power, in varying degrees, of 

 secreting carbonate of lime, to strengthen it in the 

 shape of spines, &c. On the under side, we find each 

 arm deeply furrowed, and along the furrows are 

 four rows of ambulacral, or sucking feet, like those 

 described in the sea-urchin, and hydrostatically 

 worked in a similar manner. When a star-fish has 

 been turned over so as to bring the grooved surface 

 of the arms uppermost, the student sees the mode in 



