346 GILMAN A. DREW 
pens, a genus whose general body structure indicates nearrela- 
tionship may have the similar arm of the other side modified. 
The position of the arm on the right or left side of the body is not 
generally considered very significant. ‘The somewhat frequent 
occurrence of genera showing hectocotylism of arms symmetri- 
cally placed on the two sides of the body may indicate a primitive 
paired condition that has been replaced among the majority of 
existing cephalopod genera, by specializing on one side and drop- 
ping out on the other. Whether this accounts for the condition 
or not, the shifting of a modification from one side of the body 
to the other, sometimes involving modifications of other body 
structures and sometimes apparently not, is not uncommon among 
animals, and even if not easily explained, evidently has no very 
great phylogenetic significance. Shifting in series is not so com- 
mon and when we find in the same family, genera with the fourth 
and others with the first arm hectocotylized it becomes difficult 
to imagine ancestral conditions that made this posisble. 
Wherever known, male cephalopods use one or more of the 
arms to transfer sperm to the female. Copulation has not been 
described for many of the species but the presence of more or less 
modified arms in more than half the recognized families may be 
taken as an indication that either these animals or their ancestors 
used their arms in copulation. 
If the spadix of Nautilus is used in copulation we have a pos- 
sible indication that a number of arms may have been employed 
in the transfer of sperm by primitive cephalopods. It is of course 
possible that all the arms were used for this purpose and that the 
present diversity can be accounted for by the specialization of 
one or the other of the arms involved in this primitive condition. 
This, however, does not seem reasonable when the diversity within 
the limits of a single family is considered. 
The arm that is used, and the way in which it is used, is asso- 
ciated with the character of the spermatophores and the position 
of their final discharge. The Octopoda show the greatest struc- 
tural modification in their hectocotylized arms. While two of the 
families of this group give no evidence of hectocotylism, none of 
the genera of the remaining families are known to be free from it, 
