292 SEXUAL DIMORPHISM 



secretion of a number of suckers. Careful investigation 

 might result in showing that the secretion of the shell is 

 directly caused by the position in which the arms are held. 



The peculiar modification of a pair of arms in the female 

 occurs only in Argonauta, but the modification of an arm in 

 the male, for the purpose of copulation, is common to all 

 Cephalopods. The degrees of the modification and the 

 particular arm affected are different in different genera. In 

 Nautilus, which possesses a large number of arms or tentacles, 

 the modification affects the four interior ventral tentacles on 

 the left side, which coalesce to form a single organ known 

 as the spadix. In Spirida the two arms of the fourth pair 

 are hectocotylised. In Rossia also the same two arms are 

 modified, but unequally. In other cases only one arm is 

 affected, the fourth on the left side in the Oigopsidse, a Sub- 

 order of the squids or ten-armed cuttle-fishes ; the third on 

 the right side in Octopus, Eledone, and other eight-armed 

 forms ; the second on the right in Cirroteuthis. 



In Philonexis and Tremoctopus the modified arm is separated 

 from the male in copulation as in the Argonauta, and probably 

 is succeeded by a new arm of the same kind. In other cases 

 no separation takes place. In Octopus, Eledone, and others, 

 the modification of the arm is very slight, the extremity 

 merely taking the shape of a spoon. In Sepia the suckers 

 are absent at the base of the hectocotylus, while in Idiosepion 

 and Loliolus they are absent altogether on the whole arm. If 

 each of these different modifications is thoroughly well adapted 

 for its purpose, it follows that the mode or habit of copulation 

 is different in the different genera, and if this is the case the 

 stimulations to which the arm is subjected in the different 

 cases must be very different. The facts in this instance 

 therefore, so far as they are known, are in harmony with the 

 general theory of the influence of external conditions in pro- 

 ducing sexual dimorphism. The degree to which the special 



