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, i; ambulacra!; ir, interambulacral 

 areas; o, mouth; a, vent. After Gegeubaur. 



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 Avhich micro- 

 scopic sections 

 should be made. 

 The solid mouth- 

 parts, the oral 

 m e m b r a n e 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 Avell as their re- 

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



shell is a flattened sphere, consisting of ambulacral plates, 



perforated for the exit of the 



feet, and a broader series of 



five 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 



the shell, while the madre- 



poric plate is situated upon 



the top or end of the shell 



, HJ . _ ill i i i i < I I " / 1 i ( i i i I ' f * .'( I I ' i in n (i V I IM (ill iJilt.ll 



(as the animal moves mouth plates. The tubercles to which the spun 



i -, N i ,.,-, are attached are only drawn on one ambula- 



CiOAVllWarCtJ, being a motlinca- cral and one interambulacral area ; 011 the 



.- i -i , i former are also drawn the pores through 



tlOll OI One OI tlie genital which the suckers protrude. -After Gegeu- 



plates (Fig. 81, m). There are baur " 



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 



Fig. 81. Aboral end of the shell of an 

 Echinus, with the upper end of the rows of 

 plates, a, ambulacral area; i, interambu- 

 lacral area; g, genital plates; iff, intergeni- 

 tal plates; m, one of the genital plates 

 forming a madreporic plate ; x, anal opening 

 in the aboral area surrounded by the genital 



