118 



ZOOLOGY. 



Fig. 80. Schematic figures of a Sea-urchin. A. from 

 the oral end ; B, from one side. Ambulacra indicated 

 by rows of dots, r, ambulucral; ir, interanibulacral 

 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 five 

 sharp conical teeth 

 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-opening and the digestive canal. The 



shell consists of five double rows of ambulacra! plates,, 



perforated for the exit of the 



feet, and a series of five dou- 



ble rows of interambulacral 



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 



(abactinal) or upper end of Fif ,. 8l ._ Ahoral end of thc phol] of nn 



thp shpll whilp flip marlrp- Echinus, with thc upper end of the rows of 



plates. , ambulacra] area; i, interambu- 



plate is Situated Upon iu cral area; y, gemtal plates; i?, intergeni- 



* tal plates; m, one of the genital plates 



top Ol* end Ot the Shell f orming a madreponc plate ;x, aual opening 



, ., . , in the aboral area surrounded by the genital 



(as the ailimal moves mouth plates. The tubercles to which the spines 



i j\ i j-e are attached are only drawn on one ambula- 



dOWHWard), being a modlfica- cral and one interambulacral area ; on the 



the 



^f ^nn ^f +1-,^ ,,'f1 frmer are also drawn the pores through 



Of One Ol the genital which the suckers protrude. -After Gegeu- 



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

 five large plates, one at each end of the interambulacral 

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

 the ovarian openings through which the eggs escape ; these 



