MOLLUSCS. 259 



sea-slugs) is chiefly due to the biliary glands being seen through 

 the translucent Integuments — this beauty being probably of no 

 service to these animals. The tints of the decaying leaves in an 

 American forest are described by every one as gorgeous; yet no 

 one supposes that these tints are of the least advantage to the 

 trees. Bearing in mind how many substances closely analogous 

 to natural organic compounds have been recently formed by chem- 

 ists, and which exhibit the most splendid colors, it would have 

 been a strange fact if substances similarly colored had not often 

 originated, independently of any useful end thus gained, in the 

 complex laboratory of living organisms. 



The sub-kingdom of the Mollusca. — Throughout this great divis- 

 ion of the animal kingdom, as far as I can discover, secondary 

 sexual characters, such as we are here considering, never occur. 

 Nor could they be expected in the three lowest classes, namely in 

 the Ascidians, Polyzoa, and Brachiopods (constituting the MoUus- 

 coida of some authors), for most of these animals are permanently 

 affixed to a support or have their sexes united in the same individ- 

 ual. In the Lamellibranchiata, or bivalve shells, hermaphrodit- 

 ism 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 find- 

 ing, securing, or charming the females, or for fighting with other 

 males. As I am informed by Mr. Gwyn Jeffreys, the sole external 

 difference between the sexes consists in the shell sometimes dif- 

 fering a little in form; for instance, the shell of the male peri- 

 winkle (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 reproduc- 

 tion, 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 to- 

 gether 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 

 hermaphrodites, are compelled by their structure to pair together. 

 Agassiz remarks,^ "Quiconque a eu I'occasion d'observer les amours 

 "des limagons, ne saurait raettre en doute le seduction deployee 

 "dans les mouvements et les allures qui preparent et accomplis- 

 "sent le double embrassement de ces hermaphrodites." These 

 animals appear also susceptible of some degree of permanent 

 attachment: an accurate observer, Mr. Lonsdale, informs me 

 that he placed a pair of land-snails (Helix pomatia), one of which 



' 'De I'Espece et de la Class. &c., 1869, p. 106, 



