MOLLUSCS. 763 
molluscs the arterial system also is imperfect, especially in the 
nudibranchiates’. 
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. 
Tn the organs of propagation in molluscs great variety prevails. 
Some are bisexual, others have distinct sexes. 
In those molluses 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 Ohiton, Patella, and Haliotis (the cyclo- 
branchiata and aspidobranchiata), which were formerly supposed 
to be all bisexual or rather female”. 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 Bucc’nwm is of 
unusual size, in the Cephalopods short only. The ¢esé’s or the 
ovary is single and lies in the G‘asteropoda 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 Mine Epwarps found an arterial cavity in the head, in 
which the large artery ends freely forwards. 
? Compare what we said above on a similar opinion respecting bivalves, p. 713. 
