4 



ASTEROIDEA. 



In the genus TJrasfer (PI. I, fig. 2 a, b) the rays are long, and the ossicula on each side 

 of the ambulacral valley support many rows of spines ; the ossicles on the sides and upper 

 surface of the ray form a hollow cavity for lodging the viscera (fig. 1 c) ; and the numerous 

 small bones entering into this net-like structure support blunt or pointed spines (fig. 2 a), 

 the integument between the osseous pieces is naked, and perforated by pores which 

 communicate with the interior. Numerous pincers-like pedicellariae, supported on soft 

 stems, encircle the basis of the spines, or are distributed amongst them, whilst others are 

 disposed at the angles of the rays. All the Urasters possess an excentral anal opening. 



In Plate I, fig. 2 «, I have figured a portion of the upper surface of a ray of Vraster 

 tenuispinus, M. and T., which shows the recurved spines raised upon the inter-ambulacral 

 ossicles, and the naked integument between the spines perforated with respiratory pores. 

 PI. I, fig. 2 b, is the under surface of the same ray ; four rows of tubular sucking-feet 

 occupy the ambulacral valley, which is fringed with two rows of small spines arranged in 

 an oblique comb-like order ; external to these other rows of larger spines arm the lateral 

 parts of the ray. PI. 1, fig. 3, represents a portion of the ambulacral skeleton, and shows 

 the spaces for the passage of the tubular retractile feet. 



The Astropedens have a stellate body, flattened on both sides, and furnished with two 

 rows of tubular feet \ in the ambulacral valleys, the rays are narrow and elongated, and 

 their sides are bordered with two rows of large, regular, marginal plates (fig. 3). The 

 ventral marginal plates carry long moveable spines, and the dorsal series in general are 

 covered with granules more or less developed, which sometimes likewise carry spines. On 

 the upper surface of the ray the inter-marginal space is thickly set with bunches oipaxilla, 

 and the under surface is crowded with regular rows of short spines. 



All these parts are well seen in situ in the section of a ray of the common Butthorn {Astro- 

 pecten aurantiacus, Lin.) If the ray is laid open along the middle of the upper surface, 

 and the dorsal integument and marginal plates are folded over into a horizontal position, 



the homology of the different parts of the 

 ray, with the ambulacra and inter-ambu- 

 lacra of the Echinida becomes evident. 



The annexed figures of the upper 

 and under surfaces of a ray of Astro- 

 pecten poly acanthus, M. and T., shows the 

 general structure of this part in the genus 

 A. B. Astropecten. The ambulacral areas have 



Portion of a ray of Astropecten polijacanthus, M. and T. A, under j.^^^ rOWS of tubular retractile feet • the 

 surface ; B, the upper surface of the ray. _ ' 



valley is bounded by two rows of short, 

 stout spines, arranged in groups by the side of the suckers, each of the lower marginal 

 plates is armed with four long, recurved, tooth-like spines (fig. 3 A), and other shorter 

 spines cover the plates at their base ; the upper marginal ossicles likewise support long, stout. 



