Page:American Seashells (1954).djvu/584

486 belong to closely related genera. The characters most relied upon in dis- tinguishing species are relative length of the arms, the skin surface, the nature and relative length of the small ligula (the tiny pad-like extension on the end of the third right arm in the males, i.e., the hectocotylized arm). The number of gill plates and color pattern are used to a less extent. The eight arms have each been given a number, in order that com- parisons may be made. This is done by setting the octopus down with the body up, and the arms spread-eagle out in all directions. Turn the octopus so that the two eyes are on the side away from you. By going from the eyes out to the mantle edge away from you, and choosing the first arm to the right, you have located the first arm. Further clockwise are the second, third and fourth right arms. Instead of counting further (fifth arm, etc.), return to the center again, and count to the left — hence, the first, second, third and fourth left arms. When giving an arm formula, only the right ones are gen- erally given, and they are set down in order of large to smaller size. Hence, 4.3.1.2 means the fourth arm is the largest, the second one the smallest in length. It may be pointed out, that on rare occasions an octopus may acci- dentally lose an arm. There are two simple sets of measurements (all in millimeters) which are important in distinguishing the species of Octopus. The first is the mantle-arm index which simply means the comparison of the length of the mantle (measure from the round, bulbous "head" end to a point just between the eyes) with the length of the longest arm (turn the octopus over, measure from the mouth to the tip of the longest stretched-out arm). An index is obtained by multiplying the mantle-length by 100 and then dividing the result by the arm-length. The ligula index is obtained only from males and from the third right arm which is a modified sex organ. The ligula is measured from tip to the last sucker. The arm length is obtained as explained in the preceding para- graph. The index is: length of ligula, multiplied by 100, the result divided by the total arm-length. The number of gill plates and the size of eggs are determined by cutting a deep slit in the body. Octopus vulgaris Lamarck Common Atlantic Octopus Figure looa Connecticut to Florida and the West Indies. Europe. Length, including the longest arm, i to 3 feet (the latter would give a radial spread of about 7 feet). Mantle-arm index in Florida and North Carolina is about 25 (that is, the arms are 4 times as long as the mantle). Ligula-index below 2.5. Cill plates 7 to 9 (in Bermuda, usually 10 or 11). In life, skin smoothish; preserved, it is rugose with variously shaped warts. Eggs 3 mm. or less in length. A common harmless species found hidden away