382 
REPRODUCTION — VELIGER. 
Fig. 719. — Early embr^’O of Limncea 
fcregra (Mull.), highly magnified (modi' 
fied after Dumortier), showing the 
general character of the embryonal 
rotation. 
The arrow indicates the direction of 
the motion. 
tlie Ti’ochosphere stage ; their action sets the albuminous contents of 
the egg in motion, and facilitates the inception through the egg 
membrane not only of oxygen, but of the nutritive matter from the 
enveloping albuminous mass within which some eggs are enveloped ; it 
also establishes a kind of vortex in which the embryo itself is gradually 
involved, so that it begins an oblique 
rotatory movement upon its own axis, 
with the anterior part of the animal in 
front, varying in raj)idity according to 
temperature and age, and eventually 
also revolving around the internal 
walls of the egg cavity; this complex 
motion has been happily likened in a general sense to that of the diurnal 
and annual rotation of the planetary bodies around the sun, and con- 
tinues up to the time the embryo can perform voluntary movements. 
The Veliger {velum, a sail ; gei-o, 1 bear) stage is distinguished 
from the Trochosphere by the more distinct development of molluscan 
attributes, by the torsion of the visceral sac, characteristic of adult 
Gastropods, and especially by the enlargement of the anterior region, 
to form the Velum, which may be i)r(donged into lobes or processes 
fringed with powerful cilia, and constitute the organs of locomotion 
persisting in the adult of Limnmi as the subtentacular lobes. 
In the terrestrial and tiuviatile genera the animal hatches out in 
the adult form, the various larval 
stages being very transient or even 
suppressed and passed through within 
the egg. The Velum is not therefore 
needed as a locomotory organ and is 
reduced to one or more rings of cilia, 
l)ut in most marine spepies and in 
Drelsseusia the embryo is hatched 
early as a free swimming Veligerous 
larva, the highly contractile and well- 
developed pre-oral ciliated velar lobes not oidy acting as organs of 
locomotion, but probably also assisting in respiration and circulation. 
Certain groui)s, probably owing to special dangers to their offspring, 
have acquired the habit of retaining the ova within the body until the 
hatching has actually taken place, as in Vivljxtra, Anndonta, and 
certain species of Papa, Cl(tnPdl<(, Tlelir, etc., the young being 
Fig. 720. — Veliger stage of Drehsensla 
polyifiorpha{ Palla.s), with velum extended, 
greatly enlarged (after Korschelt). 
7'. velum fully extended and showing 
pigmented median area ; sh. shell ; p. 
pigmentation beneath oral aperture. 
