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 



