350 
REPRODUCTIVE SYSTEM. 
upon which they feed. In other cases the foot may be sjn’ead out 
laterally and form a disc. 
iMessrs. Bo 3 ’cott and Bowell have, however, recorded that the Unto 
m((rg((rit'ifer progresses 15 feet i)er day, or about 3 millimetres per 
minute, and a mile in 352 daj'S. 
The most sluggish of onr species is the adult Dreissensia, which 
becomes tixed bv its byssns to some suitable spot for considerable 
periods, this inactive adult life is compensated for by the activity of 
the young, which are free swimmers in their larval stage. 
Within certain limits, snails can carry considerable weights without 
l)erceptibly imi)eding their rate of progress, showing that ordinarily 
they do not make full use of the strength they possess, certain Helices 
having been demonstrated to be able to travel on a horizontal surface 
with a weight tifty times their own, or ascend vertically, burthened 
with a weight nine times that of the weight of animal and shell com- 
hined. iNlr. 11. Crowther has also experimentally demonstrated that 
Heliv aspersa can, even when loaded with a weight of 50 grammes 
(gi’amme= 15 '432 grains) ascend a vertical glass plate at a speed of 
seven millimetres per minute, but when the weight was increased 
to G8'9 grammes, the animal though able to raise the load, could 
not progress ; while Jleli.v (irhusturum, whose foot area is barely 
eight inches, has, under the same conditions, ascended at a similar 
speed with a weight of <sj- grammes, hut a weight of 8| grammes was 
heyond its strength, as the animal was (pdte unable to raise the 
hnrthen and slijjped off the glass. 
The Reproductive or Sexual System in the mollusca is very varied 
in the character and complexity of its component jiarts, the simpler 
the organizatimi the fewer the sexual characters exhibited, while 
sexual differentiation is naturally most iwoiiounced in the most highly 
organized types, and although all organisms tend to vary in an 
intinite variety of ways, the different organs are not e(iually affected 
nor do they undergo modifications in a similar way nor in an e(|ual 
degree. 
The sexual organs at ordinary times are only normally of moderate 
bulk, but at the approach of the pairing season they become turgid 
and increase vastly in size, often occnjiying the greater jiart of the 
body cavity. This iiairing season is not the same for all .species, but 
si)ring is perhaps the most favoured season, the reproductive organs 
at that period being pre-eminently noteworthy for the great abun- 
