80 ANIMAL FORMS 
ment proceeds at first much as in the sponge, but soon the 
shell, foot, gills, and various other molluscan structures 
put in an appearance, and the few surviving young which 
have been free-swimming now settle down in some favor- 
able spot, and attach themselves or burrow according to 
their habit. 
80. Life history of fresh-water clams.—The life history of 
our common fresh-water clams is perhaps one of the most 
remarkable known among mollusks. The parent stores the 
eggs, as soon as they are laid, in the outer gill plate, and 
there, well protected, they undergo the first stages of their 
development, which results in the formation of minute 
young enclosed in a bivalve shell beset with teeth. These 
are often readily obtained, sometimes as they are escaping 
from the parent, and when examined under the microscope 
are seen to rapidly open and close their shells in a snapping 
fashion when in the least disturbed. In a state of nature 
this latter movement may result in attaching the young to 
the fins or gills of some passing fish, which is necessary to 
its further development. Within a short time it becomes 
completely embedded in the flesh of its host, from which, 
as a parasite, it draws its nourishment, and during the 
next few weeks undergoes a wonderful series of transforma- 
tions resulting in a small mussel, which breaks its way 
through the thin skin of the fish and drops to the bottom. 
81. The gasteropods.—The gasteropods, including snails, 
slugs, limpets, and a host of related forms, fully twenty 
thousand different species in all, are found in most of our 
fresh-water streams and lakes and in moist situations on 
land, while great numbers live along the seashore and at 
various depths in the ocean, even down as far as three 
miles. Examining any of them carefully we find many of 
the same organs as in the clams, but curiously changed and 
adapted for a very different and usually active life. In our 
common land snails (Fig. 48), which we may well examine 
before passing on to a general survey of the group, the first 
