CLASS I 



ECHINOIDEA 213 



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 Endocyclica, according to 

 Love"n, the primordial interambulacral plate is soon resorbed," so that the 

 intfianil.ulacra 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 liserial 

 ("bigeminal" auct.) when placed so that there are two vertical rows of pairs, 

 one nearer the ambulacro-interradial suture than the other ; and triserial 

 ("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 

 nexuous. 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 Palceechinoidea 

 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. 



