366 



ZOOLOGY 



SECT. 



canal communicates with the exterior. The two ambulacral areas 

 between which the madreporite lies constitute the bivium, the 

 remaining three the trivium. 



On the inner surface of the shell, close to the edge of the peri- 

 stome, there project inwards five processes, the auricles (Fig. 297, 

 aur), one opposite each ambulacral area. Within the ring of auricles 

 lies a complex structure termed Aristotle's lantern (Fig. 296). 

 This consists of the five teeth (c) the apices of which are to be 

 seen projecting through the mouth, together with a system of 

 ossicles. The teeth are long, curved, and pointed : proximally each 

 is supported by and partly embedded in a pyramidal ossicle, the 

 alveolus (a), consisting of two halves united by a longitudinal suture. 

 Firmly united to the base of the alveolus is a stout bar, the 

 epiphysis (/;). Adjacent epiphyses are in close contact with one 



FIG. 200. Lantern of Aristotle of Echinus. A, Two of the five chief component parts apposed 

 and viewed laterally, li, Lateral and C internal view of a single part, a, alveolus ; n, suture 

 with its fellow ; //, epiphysis ; V, suture with alveolus ; c, rotula ; (', radius ; c, tooth. (From 

 Huxley'* /.<' /'../<, -atts, after Muller.) 



another, and running inwards from their points of union are five 

 radially-directed, stout bars, the rotula; (c), the inner ends of which 

 unite to bound a circular aperture through which the oesophagus 

 passes. With the. inner end of each rotula is movably articulated 

 a more slender bar, the radius (d\ which runs outwards, parallel 

 with, and closely applied to, the rotula, to end in a free, bifurcated 

 extremity, . Aristotle's lantern as a whole is in the shape of a five- 

 sided pyramid, at the apex of which project the five teeth : the 

 pyramid is hollow, containing a^ passage which is the beginning of 

 the oesophagus. The base has the appearance of a wheel, the tyre 

 of which is represented by the five epiphyses, the spokes by the 

 five rotuke, with the five radii in close contact with them, and the 

 hub by the rounded central aperture. Passing between the various 

 ossicles of the lantern, and from them to the auricles, are systems 

 of muscles by means of the contractions of some of which the 

 lantern as a whole can be protruded or retracted, while the action 



