100 



INVEUTJililfATE ANIJIALS. 



the test is c.i\'croil with numerous tubercles of tliffereut sizes, which 

 carry lunger or shorter sjiiues (tij,'. Gl, E). The K]>ines are jointed to 

 the tuljercle by a sort of " Ijall-anil-socket " or " universal " joint, 

 and they are com2)letely under the control of the animal, so as to be 

 used both iu locomotion and apparently as defensive weapons. In 

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

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



Fig. 61. — Morphc'o^' of Ecliinriidea. .\, Tnmi^' .siiecimen of .^troiujylocenirottis Drb- 

 taclii^nsis, viewed frnm alii'\p ; B, Siiinll pniticui nf 1!r' ti^st nf the sntne magnified: 

 C, KiiiiiTiiit of tlie test of Krhiiiii.-i sjihirnf, iiiauniified ; D, l_'l!/p>'.(isl.n- siih'lrprf^^us, 

 viewi'il fn.iri .-[liove, sliowiie,' (lie prl.ilniil anilinliiriM ; E, S]iiiie of I'omriitirris pvr- 

 p,tnil<i; K, I'l-iliC'U.'iria of r„.n-,i„n'iislis lirnhix, e «. Aiiil.ulacriil areas; / i Inter- 

 .Tiil.iilaeral aivas ; .Mieiiilal plate; ,:(]cular ].late ; m M.elreporilonii tnliercle ; p 

 Jlrijil.rane surrouij.lin;^ tie- amis. (Im:,'s. .\, B, auit D iue after A. Agassiz.) 



surface of the test is furnished M'ilh curious little bodies called 

 "pedicellariie " (lig. (il, F), whicli wei'e lo]ig believed to be j)arasitic. 

 They consist fif two or tliree blades mounted u]ion a flexible .stalk, 

 ;ind const;iii(ly employed in sn.-ip|iiiig (o'^'etlier like the beak of a bird. 

 'J'heV <iia'Ui' in ]u:iuy either Et'hi inulcnii'if't^ ;ind their use is obsciu'e. 



Tjoroiiiution is eH'crted iu tile S";i- u I'oh ills 1)\- ;i i-iii'ious system of 

 rii;itt;ietil(.' fiilies which are' known ;is the '' andiiil;ici-;d tubes" or 

 " lube-feel," ;iiid w liich are appendages of the w;iler-vascular system. 



