200 



ZOOLOG Y. 



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 



Fig. 143. Schematic figures of a Sea-urchin. A, from 

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

 by rows of dots, r, ambulacral; ir, interarubulacral 

 areas; o, mouth; a, veut. After Gegenbaur. 



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 ambulacral 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. 143-144). 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 



Fig.144. Aboral end of the ehell of an 

 Echinus, with the upper end of the rows of 

 plates, a, ambulacra! area; i, interambu- 



the top Or end of the Shell forming a maurepoiicplate; x, anal opening 



. in the aboral area surrounded by the genit nl 



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



, . . , . ~ are attached are only drawn on one ambula- 



. , . ~ - 



being a modlhca- cral and one interambiUacral area ; ou the 



, . /> j_i "i i former are also drawn the pores through 



tlOn OI One OI the genital which the suckers protrude. -After Gegeu- 



plates(Fig.l44,w). 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- 



