260 ECHINODERMATA— ECHINOZOA phylum iv 



The generative organs are superficially alike in both sexes, and are in the form 

 of glands (usually five, sometimes four, three, or even two), situated dorsally 

 and interradially on the inner surface of the test. The genital ducts terminate 

 in pores in the so-called genital plates, to be described presently. 



Coronal Plates. — The plates of the Corona are arranged in ten meridional 

 areas. Five of these, the ambulacral areas, are composed of perforated plates, 

 and correspond in position to the radiating ambulacral vessels ; the remaining 

 five, the interambulacral or interradial areas, alternate with the first, and are 

 imperforate. 



In all Recent and in the majority of fossil Echini the ambulacral areas are 

 each composed of two columns of alternately arranged plates, the inner 

 edges of which meet in a zigzag median suture, and the actinal and abactinal 

 edges in horizontal sutures. In some Paleozoic genera there are more than 

 two columns in an ambulacral area, and there may be as many as sixteen, or 

 even twenty, at the mid-zone (Fig. 367, o). The interambulacral areas are 

 each composed of from one to fourteen columns of plates, but nearly all 

 post-Paleozoic and all Recent types have two columns. Interambulacral plates 

 are usually larger than ambulacrals and meet the latter in vertical adradial 

 sutures. There are therefore from fifteen vertical columns of coronal plates, 

 Bothriocidaroida (Fig. 377, A), to twenty columns Cidaroida, Centrechinoida, 

 Exocycloida, or more than twenty, as in the Paleozoic Echinocystoida and 

 Perischoechinoida (Figs. 429, 432), and the Triassic Plesiocidaroida. One 

 additional case of more than twenty columns is known in the peculiar 

 Cretaceous Tetracidaris. The number of columns of plates is the same for an 

 individual in each of the ambulacral areas, and usually for each of the 

 interambulacral areas as well, but the two Systems are entirely independent 

 of one another as respects the size and number of plates in a vertical column, 

 also, especially in the Paleozoic, as regards the number of columns in an area. 

 In the Cidaridae, for example, the ambulacra are very narrow and are 

 composed of numerous, thirty to sixty low plates in a column ; the inter- 

 ambulacra are broad with few, five or six to fifteen high plates in a column. 

 On the other hand, in the Paleozoic Lepidesthes colletti, the ambulacra are 

 broad, with sixteen columns of plates in each area, and the interambulacra 

 are narrow with four columns in each area (Fig. 434). In the regulär or 

 endocyclic Echini, all of the ambulacra and all of the interambulacra are 

 essentially similar in the individual ; but in irregulär or exocydic Echini, the 

 anterior ambulacrum and the posterior interambulacrum often differ consider- 

 ably from the corresponding areas. 



Interambulacral (interradial) plates are always simple ; ambulacral plates 

 may be either simple or Compound. In the latter case, they are formed of 

 two or of several component elements, all of which are joined by sutures and 

 form a more or less geometrical plate. Most simple plates, and some of the 

 components of Compound plates are primaries — that is, they extend from the 

 outer edge of an ambulacrum to the median suture of the area. Demi-plates 

 is a name applied to those component elements which reach the inter- 

 ambulacrum but do not extend to the median suture (Fig. 396). Isolated 

 plates are component elements which do not reach either to the interam- 

 bulacral or median suture. Occluded plates are component elements which 

 reach the median suture, but do not reach the interambulacrum. These 

 terms, based on Compound plate elements, can be also applied to the 



