54 MR. T. H. STEWART ON THE [Feb. 12, 



wise very low organization, possesses jaws (or, as I prefer to call the 

 apparatus, a splanchnic skeleton) of a more complicated arrange- 

 ment than any other animal in existence, from a simple sponge or 

 Amoeba up to man himself. This splanchnic skeleton, in Ciduris, 

 Echinus, and allied genera, is formed of forty separate calcareous 

 portions, arranged in a conical form, and, as we shall see by de- 

 scription and inspection of the specimens on the table, beautifully 

 and perfectly articulated together, and having forty separate muscles 

 to move the teeth in various ways. 



If we take up an Echinus, or Sea Urchin as it is commonly called, 

 and look at the flattened under-side, we see in the centre a circular 

 part which is membranous, and continued from the corona to the 

 points of the five protruded teeth. This peristomal membrane is 

 covered in most of the Echini (not in our he&\xt\?vi\E.Jlemingii, how- 

 ever) with minute, oval, and somewhat irregularly scattered calca- 

 reous plates, not (as in the rest of the corona) articulated together, 

 but with intervals between each other, leaving the membrane partly 

 bare. On these calcareous particles are placed organs called ^jedi- 

 cellaria, and also, on some, minute spines, the tubercles for which 

 may be seen with a lens ; these particles are of various sizes. Around 

 the teeth, on the peristomal membrane, are situated, ambulacrally, 

 five pairs of large oval plates, each with a pit excavated in the centre, 

 and having a minute perforation, over which is placed externally a 

 modified form of cirrus. Internally an exceedingly small vessel 

 comes from each perforation and joins the large longitudinal ambu- 

 lacral vessel : these plates are also covered with numerous tubercles 

 for minute pedicellaria. To this series of plates succeeds a soft 

 circular lip, containing excessively fine particles of lime in a radiating 

 linear arrangement, not bearing either pedicellaria or spines, and im- 

 mediately surrounding theprotiuded points of the sharp, hard, white, 

 Rodent-Uke teeth. 



Seeing these points of teeth in so humble an animal — and the first 

 appearance of such, makes one, like a child with a new toy, long 

 to see the interior. We set to work to open the shell, as erroneously 

 called, of our Echinu.s. We there find, besides the intestines and other 

 viscera, a complicated conical apparatus surrounding the first part 

 of the alimentary canal, and enclosing the rest of the teeth (previ- 

 ously unseen), and having attached to it all the numerous muscles 

 which act on it. Now it is to this that I principally wish to draw 

 your attention. 



In the first place, I will enumerate the parts which make up these 

 curious jaws. There are ten triangular pieces, called alveoli (PI. X. 

 fig. 1), which wheu articulated together form five prismatic-shaped 

 sockets for the five teeth, and all together constitute a conical mass, 

 with the apex external, formed by the points of the teeth. The apices 

 of the alveoli are firmly fixed to the peristomal membrane ; but the 

 lip is loose over the teeth. The bases of the alveolar pairs are united 

 by wedge-like pieces called ^/ir/^ces (PI. X. fig. 3), five in number, on 

 each of which is placed an arched portion, divided into two ; there 

 are ten pieces arching over the external surface of the alveoli at the 

 base, which maj' be called the epiphyses of the alveoli, — making in 



