486 Ajnerican Seashelh 



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 



