52 INVEKTEBRATE ANIMALS. 



and the sea-urchin might to some extent be thus illus- 

 trated. In the sea-urchin five spaces with ambulacral 

 pores alternate with five spine-bearing segments, all 

 vertically disposed, the mouth being always on the under 

 side. Estimated number of species : recent, 17 ; fossil, 

 424. Chiefly from the Oolitic and Tertiary formations. 

 H Cldaris paplllata (Fleming), the only representa- 

 tive of the group in the British seas, was 

 formerly supposed to be found exclusively near 

 the Shetland Islands, where it is called the 

 Piper, its spines being compared with the 

 drones of a bag pipe. 



Group 146. — Genus DIADEMA (Gray) and allies. e;<aSv;ju,a, 

 a diadem. The metamorphosis of an echinus has no 

 parallel out of its own class. The egg of the sea-urchin 

 produces a free-swimming ciliated embryo, which be- 

 comes a larva with an internal calcareous skeleton, 

 shaped like a painter's easel (Plutcus). On the interior 

 lining of its stomach is formed a germ (blastema), which 

 assumes a radiated form, absorbs in part, or casts off, 

 the larval body with its skeleton,, develops a new mouth, 

 and grows into an Echinus. The adult form is therefore 

 constructed on a lower type than that of its larva, from 

 which the sea-urchin seems only to borrow materials 

 necessary for its growth. Estimated number of species : 

 recent, 39 ; British, ; fossil, 107. 



IT Examples of Salmacls, Echinocidaris, Amhly- 

 lyneustes, Garclia, Trlchodiadema. 



Group 147. — Genus ECHINUS (Linn^us) and allies. Ixlvoj, 

 a hedgehog. In the shell of Echimis spluera, the com- 

 mon Egg-urchin of British coasts, are combined upwards 

 of 600 plates, bearing more than 40C0 movable spines. 

 At the apex is an anal opening, surrounded by plates 

 perforated to admit the passage of the ova. One plate, 



