ECHINODERMATA 



99 



as in the starfishes, the larva being absorbed by the growing 

 sea urchin. 



The sand dollars are greatly flattened Echinoidea, covered with 

 very numerous, short, delicate spines. The mouth is centrally 



Ap 





■->J- 



, WJJi>^ 



iS&CJ. 



J > / ■«* " V 







& • " 





FIG. 90. Shell of sea urchin with spines removed, lateral aspect. Amb, 

 ambulacral zone with perforated plates; Ap, aboral pole; Jul. amb, inter- 

 ambulacral zones, (Horn Parkei and Haswell's Manual.) 



placed and is provided 

 developed, however, as 

 margin of the disc- 

 like bodv instead' of 

 on the aboral sur- 

 face. The ambulacral 

 feet are not functional 

 in these animals, and - 

 are confined to five 

 double, radial bands 

 on the aboral surface ; 

 in structure they are 

 tentacle-like, and prob- 

 ably serve a respira- 

 tory function merely. 

 The common sea 

 urchins measure only 

 a few centimeters in 



with five teeth, which are not so highly 

 in the sea urchins; the anus is at the 



amp 



Fir,. 91. Sea urchin with oral half of shell removed and 

 mouth parts displaced to one side, ub.r.vcs, aboral ring 

 vessel of the blood system ; ali, alimentary canal; amp, 

 ampullae; int.ves, intestinal blood vessels; hint, Aristotle's 

 !. intern ; oes, oesophagus ; or.r.v, oral ring vessel of blood 

 system; ov, ovary; rect, rectum; sip/i, siphon. (Partly 

 after Cuvier, trom Parker and Haswell's Text-book.) 



