ECHINODERMATA. 9 



which arch over the ambulacra, and are called 'auriculae/ 

 Through these apertures the radial nerve-cords and the radial 

 canals of the water- vascular system pass from the nerve-collars 

 and water-vascular ring. From the relation which these ' auri- 

 culae ' bear to the nervous and water-vascular systems it would 

 seem as though they represented the ambulacral ossicles of the 

 Starfishes. 



13. A Clypeaster bisected to shew the great development 

 of the 'auriculae/ which here are so large as to unite the oral 

 and apical walls of the ' corona/ 



14. A Spatangus (8. purpureus), bisected to shew the total 

 absence of ' auriculae/ 



15. A Sea-Urchin (Echinus sphcera), prepared to shew its 

 generative and digestive systems. 



The test has been horizontally bisected, and the apical half 

 of the shell suspended above the oral half. 



The oesophagus, as it emerges from the arrangement of 

 calcified jaws called Aristotle's lantern, to be described in the 

 next specimen, is of moderate calibre, but subsequently it 

 somewhat abruptly expands into a large and thin-walled in- 

 testine, which, after turning twice round the inside of the 

 test, passes along an inter-ambulacral space to the anus. At 

 the commencement of its second and superior convolution 

 round the shell it bends upon itself and runs in an opposite 

 direction to its first circuit, to which however it still remains 

 parallel. The two convolutions are attached by a much- 

 fenestrated mesentery to the inside of the test, and are disposed 

 in festoons. Two pseud-haemal vessels can be seen in relation 

 with the intestine, one running along its inner, and the other 

 along its outer or mesenteric border. 



The ovaries are five in number, and are arranged round the 

 apical area, one in each of the five inter-ambulacral spaces. A 

 short tubular oviduct conveys the ova to the apertures before 

 described as existing in the five genital plates. 



The ampullae are seen on the internal surface of the shell in 

 the ambulacral areas ; and mesially between each of the two 

 rows of ampullae in an ambulacral area one of the five radial 

 nerve-cords can be seen. 



Rolleston, p. cl. 



