324 
SEXUAL SELECTION. 
Part II. 
The sub-Tringdom of the Mollusca . — -Throughout this 
great division (taken in its largest acceptation) of the 
animal kingdom, secondary sexual characters, such as 
we are here considering, never, as far as I can discover, 
occur. Nor could they be expected in the three lowest 
classes, namely in the Ascidians, Polyzoa, and Bracliio- 
pods (constituting the Molluscoida of Huxley), for most 
of these animals are permanently affixed to a support 
or have their sexes united in the same individual. In 
the Lamellibranchiata, or bivalve shells, hermaphro- 
ditism is not rare. In the next higher class of the 
Gasteropoda, or marine univalve shells, the sexes are 
either united or separate. But in this latter case the 
males never possess special organs for finding, securing, 
or charming the females, or for fighting with other 
males. The sole external difference between the sexes 
consists, as I am informed by Mr. Gwyn Jeffreys, in 
the shell sometimes differing a little in form ; for 
instance, the shell of the male periwinkle (Littorina 
littorea ) is narrower and has a more elongated spire than 
that of the female. But differences of this nature, it 
may be presumed, are directly connected with the act 
of reproduction or with the development of the ova. 
The Gasteropoda, though capable of locomotion and 
furnished with imperfect eyes, do not appear to be en- 
dowed with sufficient mental powers for the members 
of the same sex to struggle together in rivalry, and 
thus to acquire secondary sexual characters. Never- 
theless with the pulmoniferous gasteropods, or land- 
shells, the pairing is preceded by courtship ; for these 
animals, though hermaphrodites, are compelled by their 
structure to pair together. Agassiz remarks , 1 “Qui- 
<s conque a eu l’occasion d’observer les amours des lima- 
1 4 De l’Espece et de la Class.’ &c., 1869, p. 106. 
