200 ZOOLOGY, 
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 
= a 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 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 pig 144.—Aboral end of the shell of an 
the shell, while the madre- newt te upper cud wurows of 
poric plate is situated upon [air area: 9, genital plates; gee 
the top or end of the shell i 
(as the animal moves mouth 
Fig.143.—Schematic figures of a Sea-urchin. A, from 
the oral end; 3B, from one side. Ambulacra indicated 
by rows of dots. 7, ambulacral; ir, interambulacral 
areas; 0, mouth; a, veut.—After Gegenbaur. 
forming empten pete plate; x, anal opening. 
in the aboral area surrounded by the geni 
downward), being a modifica- 
tion of one of the genital 
plates (Fig. 144,m). Thereare 
former are also drawn the pores through 
ee the suckers protrude.—After Gegen- 
aur. 
plates. The tubercles to which the spines 
are attached are only drawn on one ambula- 
cral and one interambulacral area; on the 
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 
