I found the recent article “Evolutionary enigmas” (SN: 5/18/13, p. 20) fascinating because I know of another example of an invertebrate animal possessing a “strictly vertebrate” quality. As a high school human anatomy and physiology teacher, I sometimes have my students test the effects of the constituents in cigarette smoke on live Daphnia heart rates. These arthropods are known to have myogenic hearts, whereas most arthropods have neurogenic hearts. Myogenic hearts contain muscle cells with an innate ability to contract without neural input, just as vertebrate heart muscle does. This makes me wonder what other invertebrate phyla might contain oddball members with vertebrate characteristics, thus helping “fill in the dots” and establishing sound evidence for a parallel line of evolution that never quite fully blossomed.
John Gallo, Plano, Texas
The illustration of the magnetic poles in “Spinning the core” (SN: 5/18/13, p. 26) correctly shows that the field lines run from geographic south in Antarctica to geographic north in the Arctic. However, the convention is that magnetic field lines run from north magnetic poles to south magnetic poles, so the magnetic pole in Canada is actually a south pole, not a magnetic north pole as illustrated.
Francis X. Hart, Sewanee, Tenn.
Technically, yes. By geophysical convention, though, the magnetic pole located in the Arctic region is known as the north magnetic pole. — Alexandra Witze
The cricket shown in “Dying crickets less choosy” (SN: 5/4/13, p. 8) is male, not female. Thank you to astute reader David Shen of Reno, Nev., who noticed its lack of an ovipositor.