268 



ECHINODERMATA— ECHINOZOA 



PHYLUM IV 



each interamhulacral area. New coronal plates are formed at the ventral border of 

 the five ocular plates, ambulacral pores pass through amhulacral plates, rarely 

 (Clypeastroids) in part hetween plates. 



The peristome is on the under or actinal surface, and in all but the 

 Exocycloida bears from one to many rows of ambulacral plates with or with- 

 out non-ambulacral plates. There are five oculars (apparently in part or 

 wholly wanting in some of the Pourtalesiidae), and five genitals or fewer, the 

 whole being fused into a mass in certain types of Exocycloida. The genitals 

 typically have each one or more pores as exits of the interradiaÜy situated 

 reproductive glands. In addition, typically, madreporic pores exist in genital 

 2, but are not recognizable in most Paleozoic forms. The periproct is more 

 or less plated, situated within the oculogenital ring, or in irregulär types out- 

 side of that area in the posterior interambulacrum ; the anus is in the peri- 

 proct. The masticatory lantern is composed of forty pieces (or Clypeastroids 

 thirty pieces) ; it is wanting in adult Spatangoids. Respiratory organs consist 

 of Stewart's organs, peristomal, or ambulacral gills. Locomotion is eff'ected 

 by ambulacral feet, or by spines, or both. 



The Test. — The test or main skeleton of the Echini is composed of numerous 

 calcareous plates, firmly united by their edges so as to form a more or less 



rigid case or box and disposed in certain 

 regions or Systems. In some genera, 

 however, the plates overlap one another 

 in an imbricating manner so as to im- 

 part a certain degree of flexibility to 

 the test. When coronal plates are 

 imbricaie, the ambulacral plates overlap 

 adorally and the interamhulacral over- 

 lap aborally and from the centre out- 

 ward and oyer the ambulacrals on the 

 p^^, gg^ adradial suture (Fig. 365). When peri- 



" Lepidestiies coiiettiyf\vL. Synthetic ii^ure show- stomal plates are imbricate, all overlap 



ing method of iinbrication of coronal plates (after adorallv (^Fi^ 371 B^ 

 Jackson). rm • * i ' r i 



ihe main element oi the test is 

 termed the Corona, which is composed of five ambulacral and five interam- 

 hulacral areas. At the summit is situated the apical disc, or oculogenital 

 plates, which in regulär Echini surrounds the periproct and anal opening. 

 The periproct is usually plated, always carries the anal opening, and in 

 irregulär Echini lies outside of the apical disk in the posterior interam- 

 bulacrum. Yentrally is situated the peristome, a membrane which is usually 

 more or less completely plated, or may be naked, and extends from the 

 mouth opening to the base of the Corona. The peristome is either central in 

 Position or anterior to the centre in some of the Exocycloida. 



Echini are oriented by an antero-posterior axis drawn through an ambul- 

 acrum and opposite interambulacrum in such a plane that the madreporite lies 

 in the right anterior interambulacrum. This is the axis on which bilaterality 

 is attained in the Exocycloida, and the same axis is indicated in regulär Echini 

 by the order in which ocular plates reach the periproct when such occurs. 

 With known axes Lovön devised a nomenclature of areas which is of very 

 great value in brevity and clearness of expression. He numbered the ambul- 



