102 
Hogg, on the Water-Snail. 
finds an appropriate nidus for their evolutions in the body of 
the lacustrine snails ; and the curious transmutations of form 
they undergo in the interior of the animals, and the circum- 
fluent water, afford one of the most striking illustrations of 
Steenstrup's theory of alternating generations." * 
Upon observing the Limneus in my glass rather closely, I 
noticed that its body was covered Avith the " little white line- 
looMmj leeches" described by Dr. Gould and M. Baer ; upon 
carefully detaching one or two, and viewing them with a half- 
inch object-glass, it had the formidable appearance represented 
in the drawing at fig. 8. It has an anterior mouth, surrounded 
with minute teeth or spines, over which it possesses great 
power. Suddenly it may be seen to dart out its body, at the 
same time projecting its mouth to some distance apparently for 
the purpose of seizing its prey, when it as quickly retracts 
itself within the shell of the animal, where it securely attaches 
itself to its body by a posterior sucker. It is possessed of a 
great number of booklets or feet, by these it creeps from one 
part of the body to another, but is always found adhering to 
those parts affording security in times of danger. Eventually 
they become so numerous that the animal's life falls a sacrifice 
to its troublesome tormentors, having apparently no power to 
rid itself of them. 
In conclusion, I would offer a word or two on the cell ; the 
primordial wall of which does not enter into the formative 
process of the embryo. The cell contents only are required 
for the purpose of affording nourishment to the vital blastema 
of the nucleus, in which a cycle of progressive development 
once set up, goes on until the animal is sufficiently matured to 
break through the cell- wall, and escape from the ova-sac. At the 
same time it may be inferred, that this is in some way assisted 
by the process of endosmose, and in this way certain gases 
or fluids become drawn into the cell-interior, arid thus mate- 
rially aid in the supply of nourishment for the growth of the 
animal. 
The cell-wall bears the same relation to the future perfect 
animal that the egg-shell of the chick does to it ; it is but an 
external covering to a certain amount of gaseous and fluid 
matter, and for the purpose of placing the germ of life in a 
more favourable state for development, assisted as it is by 
an increase of temperature usually the result of a chemical 
action set up, or once begun, in an organism and a medium. 
The ovum, destined to become a new creature, originates from 
a cell enclosing a gemmule, from which its tissues are formed, 
and nutriment is assimilated, and which eventually enables 
* Agassiz and Gould's ' Principles of Zoology.' 
