262 



ECHINODERMATA— ECHINOZOA 



PHYLUM IV 



The arrangement of pore-pairs is uniserial when one pair is placed over 

 the other in a continuous line from the peristome to the apex ; hiserial when 

 so placed that there are two vertical rows of pore-pairs in each half-area, 

 and poll/serial when there are three to many vertical rows of j^ore-pairs in a 

 half-area. When ambulacral plates are compound, the pore-pairs of each 

 component element may be arranged in an arc, when there results a biserial 

 or polyserial arrangement. In Paleozoic genera, where the structure of the 

 ambulacrum is complex with many columns of plates (Melonechinus), the 

 ventral portion is simpler, presenting stages of development through which 

 the animal has passed. In the same types the dorsal area of young last 

 added plates also shows simplicity as a localised stage in development. In 

 those types that have compound ambulacral plates, the young plates dorsally 

 are simple for a short distance, as seen well in Centrechinus. 



Fig. 367. 



Character of the aralmlacrum in representative Echini ; left half represented. The horizontal dotted line is 

 on the plane of tlie mid-zone (after Jackson), ft, Bothrwcidarh urchaku Jackson. Ordovician. /*, Gnnincidaris 

 omaVicAilata A. Ag. Young, c, Eucidaris tnhuloides (Lamarck). Bahamas. d,Strongylocentrotns drohachiensis 

 (O. F. Miiller). Young, e, The same ; Adult. York Harbor, Maine. /, Micraster cor-anfjuineiim (Lamarck). 

 Cretaceous, England, g, KchinaracJmiiis 2Xirina (huTnarck). Eastport, Maine, h, Metalia pectoralls (hanitiTck). 

 Bahamas ; showing plates of two areas, i, Palaeechinus deijans M'Coy. Lower Carboniferous, fc, Maccoya 

 hurlinntonensis (Meek and Worthen). Lower Carboniferous. I, Lovetierhinus missouriennh (Jackson). Lower 

 Carboniferous, m, (lligojiorus danae (Meek and Worthen). Lower Carboniferous, n, Mdonechinus muUiporus 

 (Norwood and Owen). Lower Carboniferous, o, Lepidcsthes colletti White. Lower Carboniferous. 



Ambulacra are usually band-shaped and continuous from the peristome 

 to the apical disc. Petaloid ambulacra are those which enlarge between 

 the apex and the circumference (ambitus), and contract again more or 

 less perfectly before reaching that region. Subpefaloid ambulacra are more 

 elongated than the petaloid, and the series of 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 

 peristome. In such cases, however, the pores are greatly reduced in size, or 

 present other marked differences from those of the petaloid parts. The 

 poriferous areas 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 

 peristome. 



Oculor/enital or Apical System. — This is abactinal or dorsal, and is ordinarily 

 composed of ten plates, five oculars and five genitals, forming ixsually (ex- 

 cepting some Echinothuriidae, always in regular Echini) a continuous ring. 

 The genitals are typically large angular plates interradially situated and 

 perforated hy one or more pores communicating with the genital glands. One 

 of the genitals, the right anterior, is also perforated by madrejDoric pores 



