CLASS I 



ECHINOIDEA 



261 



characters seen in Paleozoic types which have many columns of simple plates 

 (Fig. 367, l-o). 



The growth of the corona of all Echini is effected by new plates being 

 successively added at the dorsal termination of the ambulacra and inter- 

 ambulacra, and by their increasing in size. In the young, and in adults 

 where the ventral border of the corona has not been resorbed in the advance 

 of the peristome, there is a single plate, which is the primordial interambulacral, 

 in the basicoronal row bordering the peristome in each area (Clypeastroids, 

 Spatangoids, many Paleozoic genera) (Fig. 366, a, h, f-h, etc.). Excepting 

 the Bothriocidaroida, we find passing dorsally from the primordial inter- 

 ambulacral plate that new columns are progressively added until the füll 

 number characteristic of the order, genus or species is attained. The new 

 columns come in in a perfectly definite order and System, although, where a 

 large number of columns is attained, there is some local Variation as regards 

 the point of introduction and also the number of columns. As progressive 

 development is marked by the addition of columns, senescent or regressive 

 development is marked dorsally in some types by the dropping out of columns 

 (Figs. 429, B ; 432). The primordial interambulacral plate ventrally represents 



. Fio. 366. 



Characters of the base of the interambulacrinn in lepresentative Echini : a, Bothriocidaris arcMica Jackson. 

 Ordovician. b, Goniocidaris canaliculata A. Agassiz. Young. c, Euddaris tribuloides (Lamarck). Bahamas. 

 d, Melonechinus multiporus (Norwood and Owen). Lower Carboniferous. e, Archaeocidaris woHheni Hall. 

 Lower Carboniferous. /, Echinocyamus pusillus (Müller). Recent. g, Rotula dentata (Lamarck). Recent. 

 h, Perlschodomus Usericdis M'Coy. Lower Carboniferous. In figures a, b, f-h, the primordial interambulacral 

 plate is in the basicoronal row ; in c-e, it, with or without additional plates, has been resorbed (after Jackson). 



a Single column, and may be compared with the adult of the Ordovician 

 Bothriocidaris which retains a single column in each area throughout life. 

 The ventral border may in the adult be retained intact, or it may have been 

 more or less extensively resorbed in the advance of the peristome. When 

 this occurs, the primordial interambulacral, with or without additional rows 

 of plates, are cut away. Such occurs in the Cidaroida, most of the Centre- 

 chinoida, and a number of Paleozoic genera (Fig. 366, c-e). 



Ambulacra. — Each ambulacrum has two columns of simple or Compound 

 plates, or in some Paleozoic genera, more than two columns of simple plates. 

 The ambulacrum is always composed of two halves which are equal on either 

 side of the median suture. Ambulacral pores are typically in pairs, rarely 

 (some Spatangoids) unpaired. Pore-pairs usually lie nearer to the inter- 

 ambulacral suture than to the middle of the plate in which they occur, 

 therefore as a result, where there are two columns of ambulacral plates, 

 there is a median interporiferous area between two marginal poriferous areas. 

 The pores of a pair may be vertically superposed, or usually the upper pore 

 of a pair revolves outward, through an angle of 90" or less, and toward the 

 interambulacrum, so that the axis of the pair is inclined or horizontal, the 

 inner pore being the lower of the two. A pore-pair is typically surrounded 

 by an elevated rim or peripodium, and the pores of a pair may be united by 

 transverse furrows, when they are said to be conjugate. 



