142 



ECHINID.E. 



jointed together with wondrous completeness, is especially 

 durable ; consequently we find the hard parts of the ex- 

 tinct species frequently as perfect as those of the recent 

 examples preserved in our cabinets. 



The Sea-Urchins are distinguished from all the other 

 Echinoderms by their form, which is more or less rounded, 

 without arms of any kind, and by their integument, in 

 which calcareous matter is deposited so as to form series 

 of regular plates, which plates are studded with tubercles, 

 bearing jointed on them spines of various forms and sizes 

 according to the genus or family. Ambulacra, variously 

 arranged, perforated with pores for the exsertion of suckers 

 or feet similar to those of the true Starfishes, are seen on 

 the surface of this hard integument. On the apex the 

 ovarian holes are seen, and there are two openings to the 

 digestive canal, a mouth always below, and sometimes 

 armed with an internal dental apparatus, and a vent which 

 is very variable in its position. The intestine is winding, 

 and is attached to the inner surface of the shell by a 

 mesentery, the surface of which, as well as of the mem- 

 brane lining the shell, is covered with vibratile cilia, and 

 serves doubtless for respiratory purposes. There are 

 ovaries, in number four or five, forming large masses placed 

 in the spaces between the ambulacra. There is a more or 

 less complicated vascular system, having a portion with 

 muscular parietes, and exhibiting pulsations, therefore 

 regarded as a heart, and other branches considered to per- 

 form the offices of arteries and veins. M. Van Beneden 

 has asserted that a nervous system exists in the Urchins 

 similar to that described by Tiedemann as existing in 

 Holothuria, but no one has as yet confirmed the observation. 

 It is probable, however, that if what is regarded as a 

 nervous system in the Starfishes and Holothuria be really 



