118 



ZOO LOOT. 



Fi§. 80. — Schematic figures of a Sea-urchin. A. from 

 the oi-al end ; B, from one side. Ambulacra indicated 

 by rows of dots. 7', ambulacra!; ir, iuterambnlacral 

 areas; o, mouth; a, vent. — After Gegenbaur. 



In order to examine the external anatomy, the shell 

 should be deprived of its spines in part, meanwhile observ- 

 ing the mode of attachment of the spines, of which micro- 

 scopic sections 

 should be made. 

 The solid mouth- 

 parts, the oral 

 membrane sur- 

 rounding the fiva 

 sharp conical teetli 

 or "pyramids," 

 and their mode of 

 attachment to the 

 " auricles " in the 

 shell, should be thoroughly investigated, as well as their re- 

 lations to the mouth-ojjening and the digestive canal. The 

 shell consists of five double rows of ambulacral plates^ 

 perforated for the exit of the 

 feet, and a series of five dou- 

 ble rows of interambnlacral 

 plates to which the spines 

 are attached, and of such 

 form and arrangement as to 

 give the greatest possible 

 strength and lightness to the 

 shell (Figs. 80, 81, 82). The 

 outlet of the alimentary canal 

 is situated on the aboral 



(abactiual) or upper end of ^g. 8,._Abon,l end of the .heU of an 



the shell, while the madre- ■E>:A"'!«.w«h the upper end of the rows or 



' . . , , plates, a, ambulncraJ area; i, interambu- 



poriC plate is situated upon '"?"'' f''^; ?. genital plates; i<j, inteigeni- 



,1 „ I 1 J- J.1 V n i^ P.lates; m, one of the genital pTates 



tne top or end OI the shell forming a madieporfc plate ;a!aiialopeimi(; 



/ ,1 1 ,, in the aboral area em-rounded by the geiiitiS 



(as the animal moves mouth plates. The tubensles to which the spmS 



dnwTiwirrl^ lioinn- n m/^,q1•fi^n are attached ate Duly drawn On one ambula- 



UOWnwaraj, Oemg a moamca- cral and one iuterambulacral area ; on the- 



tion of one of tliA o-pnifnl f"™J'' a^e also di-awu the pores throngh 

 uiuu oi one OI ine genual wWch the suckers protrude. -After Gegeu- 



plates (Fig. 81, m). There are ''''"■ 



five large plates, one at each end of the iuterambulacral 

 zones meeting on the aboral end of the body ; in them are 

 the ovarian openings through which the eggs escape ; these 



