100 



INVEETEBEATE ANIMALS. 



the test IS covered with numerous tubercles of different sizes, which 

 carry longer or shorter spines (fig. 61, E). The spines are jointed to 

 the tubercle by a sort of " ball-and-socket " or " universal " joint, 

 and they are completely under the control of the animal, so as to be 

 used both in locomotion and apparently as defensive weapons. In 

 most of our British species the spines are short, but in many tropical 

 forms they attain a very great length. Besides the spines, the outer 



Fig. 61. — ^Morpiology of Ecliinoidea. A, Yoimg specimen of Sirongylocentrotus Dro- 

 TlacJiienHs, viewed from above ; B, Small portion of the test of tlie same magnified : 

 C, Summit of the test of Echinus sphcera, magnified; D, Clypeaster siitdepressuSf 

 viewed from above, showing the petaloid ambulacra ; E, Spine of Forocidarls pur- 

 piirata; F, Pedicellaria of Toxopneustes lividiLS. a a Ambulacral areas ; i i Inter- 

 ainbulacral areas ; g Genital plate ; o Ocnlar plate ; m Madreporiform tubercle ; p 

 Jlembrane surrounding the anus. (Figs. A, B, and D are after A. Agassiz.) 



surface of the test is furnished with curious little bodies called 

 " pedicellarise " (fig. 61, F), which were long believed to be parasitic. 

 They consist of two or three blades mounted upon a flexible stalk, 

 and constantly employed in snapping together like the beak of a bird. 

 They occur in many other Echinodermata, and their use is obscure. 



Locomotion is effected in the Sea-urchins by a curious system of 

 contractile tubes which are known as the "ambulacral tubes" or 

 " tube-feet,'' and which are appendages of the water- vascular system. 



