Ill 



pi, radiate calcareous bodies, forming the foundation 'for the terminal calcareous plate of 

 the arm. 



Fig. 14. A somewhat further developed germ of an arm, viewed from below. This 

 preparation had been lying in maceration for a long time in a solution of potash; so that 

 all the organic parts are quite transparent, whereby the calcareous skeleton forming itself 

 is made to appear very distinctly: a, nascent ambulacral -plates; ad, adambulacral plates; 

 p, furrow- spines; p*, marginal spines with rudiments of pedicellaria?; p 2 , thin spines issuing 

 from the terminal calcareous plate; 6, the terminal organ of sense. 



Fig. 15. The extremity of the same germ of the arm, viewed from above, to shew 

 the formation of the cuticular skeleton: b, the terminal organ of sense; r, calcareous con- 

 cretions in the dorsal skin forming the foundations for the transversal calcareous ribs; 

 i, the growing terminal calcareous plate with its spines (p 1 ); p, marginal spines with 

 nascent pedicellaries. 



Fig. 16. The basal part of the skeleton of a recently formed arm, magnified, viewed 

 from above: a, ambulacral plates; ad, adambulacral plates; r r, the 2 interior dorsal mar- 

 ginal plates (here distinctly separated) ; r ', rudimentary marginal plates further out on the 

 arm; p, furrow-spines; p ', marginal spines. 



Fig. 17 23. Successive development of an ambulacral plate, strongly magnified. 



Fig. 24. A recently formed adambulacral plate. 



Fig. 25. One of the radially ramified calcareous concrements which form the foun- 

 dation of the terminal calcareous plate, strongly magnified. 



Fig. 26 31. Successive development of an arm-spine. 



Fig. 32. One of the thin spines attached to the terminal calcareous plate, isolated. 



Fig. 32 (bis). A recently formed water-foot. 



Fig. 33. A piece of the embryonal cuticular skeleton of the dorsal skin of the 

 disc, strongly magnified. 



Fig. 34. One of the echinulated spines attached to the same. 



Fig. 35. Schematic representation of the digestive system (the stomach with radial 

 caeca) of a 10-armed specimen, viewed from above, natural size. 



Fig. 36. 'Section of the disc, and base of an arm of Brisinga coronata (schematic 

 figure). The section goes to the left, through an interradial space; and to the right, through 

 a radial space. The dotted line refers to the next figure. 



Fig 37. A similar section of Solaster endeca, 



The following corresponding indications will serve for both figures: 

 ft, the furrow for the radial ambulacral vessel. 

 g, genital organ. 



h, the ,,heart" with the stone canal. 

 m, the madreporic body. 

 m (bis), the oral aperture. 



