SEXUAL SELECTION 347 



lowest classes, namely, in tlie Ascidians, Polyzoa, and 

 BracWopods (constituting the Molluscoida of some authors), 

 for most of these animals are permanently affixed to a sup- 

 port or have their sexes united in the same individual. In 

 the Lamellibranchiata, or bivalve shells, hermaphroditism 

 is not rare. In the next higher class of the Gasteropoda, 

 or univalve shells, the sexes are either united or separate. 

 But in the latter case the males never possess special organs 

 for finding, securing, or charming the females, or for fight- 

 ing with other males. As I am informed by Mr. Grwyn 

 Jeffreys, the sole external difference between the sexes con- 

 sists in the shell sometimes differing a little in form ; for in- 

 stance, the shell of the male periwinkle {Liitorina 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 endowed 

 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 pul- 

 moniferous gasteropods, or land-snails, the pairing is pre- 

 ceded by courtship; for these animals, though hermaphro- 

 dites, are compelled by their structure to pair together. 

 Agassiz remarks," "Quiconque a eu 1' occasion d' observer 

 les amours des limagons, ne saurait mettre en doute la 

 seduction d^ployde dans les mouvements et les allures qui 

 pr^parent et accomplissent le double embrassement de ces 

 hermaphrodites." These animals appear also susceptible 

 of some degree of permanent attachment: an accurate ob- 

 server, Mr. Lonsdale, informs me that he placed a pair 

 of land-snails {Helix pomaiia), one of which was weakly, 

 in a small and ill-provided garden. After a short time 

 the strong and healthy individual disappeared, and was 



' "De VEspece et de la Claas.," etc., 1869, p. 106. 



