220 SUPPLEMENT 
number of species, or even more or less longitudinally pe- 
dicled, as those of many buccina. 
Those envelopes which may be called adventitious or 
casual, usually viscous at first, to determine the adherence of 
the egg, pass subsequently to a corneous or concrete mucous. 
state, and sometimes even to a cretaceous state, so that they 
resemble tolerably well the calcareous envelope of the egg of 
a bird. This may be observed in many terrestrial mollusca. 
The proper envelopes are but little known, but it is pro- 
bable that they do not differ much from those of the eggs of 
animals of a higher rank. 
We are not much better acquainted with the form and dis- 
position of the germ, except when it may be sufficiently de- 
veloped to resemble the parents which gave it birth. We 
only see that this germ is contained at first in a lodge or 
superficial excavation of a true vitellus, which communicates 
as usual with the intestinal canal, perhaps even altogether 
near the mouth. This vitellus is evidently a mucous or gela- 
tinous matter concrescible by alcohol, translucid, and not very 
thick in the fresh state. 
The development of the germ in the interior of the egg of 
the mollusca is so complete that the little animal which comes 
from it almost entirely resembles its parents. Accordingly it 
often happens that this development takes place in some part 
of the mother, and that both in the cephala and the acephala. 
They, therefore, in such cases bring forth their young living, 
and these mollusca are termed viviparous. All the acephala 
appear to be in this predicament. 
The disposition of the eggs, laid by the malacozoaria, ex- 
ternally is also very variable. Thus sometimes they are 
placed and attached one by one on submarine bodies, as in a 
great number of cephalous mollusca, but at other times they 
are joined together so as to form masses more or less consi- 
