118 



ZOOLOO T. 



B 



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

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

 by rows of dots, r, ambulacra]; ir, uiterambulacral 

 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 attachirtent of the spines, of which micro- 



scoiDic 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 is a flattened sphere, consisting of ambulacral plates, 

 perforated for the exit of the 

 feet, and a broader series of 

 five rows of interambulaei-al 

 j)lates to which the spines 

 are attached, and of sucb 

 form and ari-angement 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 

 (as the animal moves mouth 

 downward), being a modifica- 

 tion of one of the genital 

 plates (Pig. 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 



Fig. 81.— Aboral end of the shell of an 

 Echinus, with the upper end of the rows of 

 plates, a, ambulacral area; i, interambii- 

 lacni] area; gr, genital plates; ify, mterceni- 

 tal plates; m, one of the genital plates 

 f oi-ming a madrepoiic plate; a;, anal opeDiiiff 

 m the aboral area em-rounded by the geuitiD 

 plates. The tubercles to which the spmes 

 are atttiched are only drawn on one ambula- 

 cral and one interambulacral area ; ou the 

 former are also di-awn the pores through 

 which the suckers protrude.- After Gegen- 

 baur. 



