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302 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
If an Echinus be placed mouth downwards, with the 
interainbulacrum in which the madreporite is placed 
forming the right anterior (Pl. I., fig. 1), it will be found 
that one ambulacrum is anterior and one interambulacrum 
posterior. The anterior ambulacrum, with its fellows on 
the right and left, form the trivium; the remaining two, 
bounding the posterior ambulacrum, constitute the bivium. 
According to Loveén’s formula, the ambulacra forming the 
trivium are numbered, counting from right to left, I., I1., 
and III.; the left bivial ambulacrum is IV. and the right 
one V. 
The membranous peristome has embedded in it a 
number of detached plates, of which ten, much larger 
than the rest, are arranged in pairs, coincident in position 
with the ambulacra, and close to the margin of the mouth. 
These are the buccal plates (Pl. I., fig. 2). Hach one is 
traversed by a double pore, and bears a corresponding 
tube-foot of relatively large size (Pl. L., fig. 7, ¢.f’.), in 
addition to minute clavate spines and pedicellariz (fig. 7). 
The peristomial edge of the test bears five arches, which 
project upwards into its interior, and are connected by 
intermediate ridges. Hach arch (Pl. IIL., fig. 23; Pl. V., 
fig. 87, aw.) is an auricula, and is formed by a process or 
apophysis from each of the marginal plates of an ambu- 
lacrum, which bend towards each other and unite in the 
middle line, the point of union coinciding exactly with the 
median suture of the ambulacrum. The intermediate 
ridges are formed by apophyses arising from the marginal 
plates of the interambulacra, which unite with each other 
and with the auricule to form the perignathic girdle. 
The external surface of the body is covered by an ecto- 
derm, which is everywhere ciliated except upon the sucker 
discs of the tube-feet. The filamentous ends of many of 
the cells become continuous with the fibres of a nerve plexus 
