CLASS I ECHINOIDEA 113: 
closed circle formed by the double column of paired ambulacral plates ; and 
this solitary interambulacral plate is followed by a double column of 
successively added plates, the last-formed being contiguous with the corre- 
sponding basal of the apical system. But in the Hndocyclica, according to 
Lovén, the primordial interambulacral plate is soon resorbed, so that the 
interambulacra enter the peristomial margin with two plates each. 
Ambulacra.—Each ambulacrum consists of an interporiferous area, placed 
between two poriferous zones ; only a few Palaeozoic genera have the whole 
ambulacral area poriferous. As a rule, the ambulacral pores are in pairs ; 
but in some cases they are unpaired. The pores may be either all similar, 
or the outer pores of a series of pairs may differ in shape and size from those 
of the inner series. There is usually a septum between the pores of a pair, 
and the pairs may be separated by costae. When the pores of each pair are 
united by transverse furrows, they are said to be conjugate. 
The arrangement of the pairs of pores may be in simple series, when one 
pair is placed over the other from peristome to apex; they are biserial 
(“bigeminal” auct.) when placed so that there are two vertical rows of pairs, 
one nearer the ambulacro-interradial suture than the other; and friserial 
(‘trigeminal ”) when there are three vertical rows of pairs. Simple series of 
pores are either absolutely straight, or in arcs of three or more pairs. 
Oblique series are those in which there is an outward slant of three or more 
consecutive pairs from above downwards. 
Ambulacra are simple or perfect when they are band-shaped and continuous 
from pole to pole, or when the direction of the pairs of pores is but slightly 
flexuous. Petaloid or circumscript ambulacra are those which enlarge between 
the apex and equatorial circumference (ambitus), and contract again more or 
less perfectly before reaching that region. Sub-petaloid ambulacra are more 
elongated than the petaloid, and the pairs of pores do not tend to close 
distally. The pores do not cease altogether at the end of the petaloid parts, 
but remain traceable for some distance beyond, often as far as the mouth. In 
such cases, however, the rows become uniserial, and the pores are greatly 
reduced in size, or present other marked differences from those of the petaloid 
parts. The poriferous zones are said to be discontinuous or interrupted when 
the pairs of pores cease at the ends of the petals, and reappear in the vicinity 
of the mouth. Sometimes the dorsal portions of the ambulacra are petaloid, 
and the rest simple, except often near the peristome. 
Dorso-central or Apical System.—This is abactinal or dorsal, and is ordinarily 
composed of ten plates arranged in two alternating zones or circles of five 
plates each. The uppermost circle is interradially situated, and consists of 
large pentagonal or hexagonal pieces, called the basal or genital plates. These 
are usually perforated by one or more ducts communicating with the genital 
glands ; but when less than the full number of glands are developed, the 
corresponding basal plates are impunctate. The posterior basal is very often 
imperfectly developed or absent. The lower circle of plates occupies the 
summit of the ambulacra, and consists of five smaller radial plates (formerly 
termed “ocular plates”), which are also perforated. In the Palwechinoidea 
the radial plates may be pierced by more than one canal; but all other 
Echinoids have but a single canal to each radial, terminating in a single or 
double orifice. These openings appear to relate to a primitive large tentacle, 
and not to an ocular organ. 

