54 INVERTEBRATE ANIMALS. 



becomes a larva with an internal calcareous skeleton, 

 shaped like a painter's easel (Pluteus). 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 Salmacis, Echinocidaris, Ambly- 

 pneustes, Garelia, Trichodiadema. 



Group 147. — Genus ECHINUS (Linnaeus) and allies. Ixlvoj, 

 a hedgehog. In the shell of Echinus sphcera, the com- 

 mon Egg-urchin of British coasts, are combined upwards 

 of 600 plates, bearing more than 4,000 movable spines. 

 At the apex is an anal opening, surrounded by plates 

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

 larger than the rest, is of a spongy character, and admits 

 water for the supply of the water- vascular system. Five 

 smaller adjacent plates have orifices answering to the 

 eye-specks of Star-fishes. On the under side of the 

 Echinus may be seen the points of five projecting teeth, 

 forming the extremities of a marvellously complicated 

 dental apparatus, which has been termed Aristotle's 

 Lantern. Estimated number of species : recent, 19 ; 

 British, 3 ; fossil, 26. Cretaceous and Tertiary. 



IT Examples of Sphcer echinus, Lytechinus, Psam- 

 mechinus. 



Group 148.— Genus TOXOPNEUSTES (Agassiz) and allies. 

 To^ov, an arc ; Trveuo-ij, respiration. Echinus lividus, the 



