Chap. IX.] MOLLUSKS. 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 flight- 

 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 (Littorina 

 Uttorea) 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 Poccasion 

 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. 



