MOLLUSCS. 763 



molluscs the arterial system also is imperfect, especially in the 

 nudibranchiates 1 . 



Some molluscs breathe atmospheric air by a cavity rich in 

 vessels ; others, and indeed the greatest number breathe, as fishes 

 do, the air diffused through water by means of gills. The form of 

 the gills is very various, as will be explained in the view of the 

 different families in the systematic arrangement. In those that have 

 a shell, this is a special protection for the respiratory apparatus, 

 and is usually situated, whenever it does not cover the entire animal, 

 upon these organs. 



In the organs of propagation in molluscs great variety prevails. 

 Some are bisexual, others have distinct sexes. 



In those molluscs in which the sexes are distinct, no external 

 difference is on the whole to be observed ; an extremely remarkable 

 exception to this is however formed by the genus Argonauta 

 amongst the Cephalopods, in which the male individuals have 

 hitherto been entirely unrecognised, as will be explained in the 

 sequel when we treat of this genus. In some there are no external 

 sexual or copulating organs present, and here, as in the bivalves, the 

 sexual difference can alone be recognised by microscopic investiga- 

 tion of the content of the secreting organs, which has been done 

 within the last ten years in Chiton, Patella, and Haliotis (the vydo- 

 brancliiata and aspidobranchiata) , which were formerly supposed 

 to be all bisexual or rather female 2 . In others, on the contrary, 

 a copulation occurs, at least an approximation as in the Cephalo- 

 pods. Here there is a penis also, which in Buccinum is of 

 unusual size, in the Cephalopods short only. The testis or the 

 ovary is single and lies in the Gasteropoda close to the liver. In 

 Nautilus the oviduct has a free opening on each side, and thus the 

 eggs do not come, as is elsewhere the general rule with invertebrate 

 animals, immediately by a closed canal from the ovary. 



Amongst those molluscs which are hermaphrodite, to which, 

 besides many Gasteropoda, the Pteropoda belong, the arrangement 

 of the sexual apparatus has been best investigated in the genera 

 Helix and Limax. In these animals, backwards close to the liver 



1 In Haliotis and Patella MILNE EDWABDS found an arterial cavity in the head, in 

 which the large artery ends freely forwards. 



2 Compare what we said above on a similar opinion respecting bivalves, p. 713. 



