Research study 1

This view looking down on the Aristotle's lantern shows parts of the pyramids, large muscles joining the pyramids, rotulas, dental sacs, and the severed end of the esophagus. Note the groove in the pyramid that accommodates the top end of the tooth. Expansion and contraction of the lantern not only facilitates biting and feeding, but may also cause special gills around the mouth, the peristomial gills, to deflate and inflate. There is some thought that the combined metabolic activities of the lantern, including continuous production of teeth, and practically non-stop activity of the complex musculature during feeding, may require the auxiliary gas-exchanging surfaces of the peristomial gills.

The pyramids are sometimes referred to as "jaws", in the sense that they support the teeth that do the actual biting and scraping.

An interesting feature of the Aristotle lantern is that in times of food shortage the pyramids and other components grow larger.  The response is thought to increase the efficiency of food-gathering, as bigger lanterns relative to body size are able to scrape and gather in more food.  The calcium carbonate required for jaw formation is taken up from the surrounding seawater. As an example, measurements of pyramid sizes in populations of purple urchins Strongylocentrotus purpuratus in Oregon collected from areas with known low and high food availability show a 20% greater jaw dimension in the former.  Ebert 1980 Bull Mar Sci 30: 467.

NOTE the measurement is the linear extent of a half-pyramid from where it attaches to the epiphysis (this is the part at the top that links the 2 halves of the pyramid) to the oral tip

Within a sea-urchin species, it is generally assumed that feeding capacity is directly related to size of Aristotle’s lantern.  A comparison at the Bamfield Marine Sciences Centre, British Columbia of lengths of demi-pyramids of the Aristotle’s lantern against diameter of test in the sea-urchin species Strongylocentrotus franciscanus, S. droebachiensis, and S. purpuratus reveals significant differences in scaling.  Thus, whereas demi-pyramid length and test diameters scale linearly in S. purpuratus and S. droebachiensis (0.96 and 0.93, respectively), in S. franciscanus they scale allometrically (1.56).  This means that as an individual grows in size, its pyramids grow disproportionately larger. The authors note that this should translate to a greater relative capacity for feeding in S. franciscanus, leading to greater relative growth and gonad production, which seems to be the case.  Lawrence et al. 1995 J Nat Hist 29: 23.

NOTE  equivalent to a half-pyramid. The same relationships hold true for rotule lengths plotted against test diameters in the same species