Chap. IX.] MOLLUSRS. 315 



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 Brachiopods (con- 

 stituting 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 Lamellibran- 

 chiata, or bivalve shells, hermaphroditism is not rare. In 

 the next higher class of the Gasteropoda, or marine uni- 

 valve 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 fight- 

 ing with other males. The sole external difference be- 

 tween 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 {Littorlna 

 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 repro- 

 duction 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. Nevertheless with 

 the pulmoniferous gasteropods, or land-snails, the pairing 

 is preceded by courtship ; for these animals, though her- 

 maphrodites, are compelled by their structure to pair to- 

 gether. Agassiz remarks, 1 " Quiconque a eu l'occasion 

 d'observer les amours des limacons, ne saurait mettre en 

 doute la seduction deployee dans les mouvements et les 

 allures qui preparent et accomplissent le double embrasse- 

 ment de ces hermaphrodites." These animals appear also 

 susceptible of some degree of permanent attachment : an 

 accurate observer, Mr. Lonsdale, informs me that he placed 



1 'De l'Espece et de la Class.' etc., 1869, p. 106. 



