168 HOUSE, GARDEN, AND FIELD 



port itself far from the place of its birth. In the pond- 

 snail the girdle soon loses its cilia, and ceases to resemble 

 an organ of aquatic locomotion ; Lankester has shown 

 that part of it is represented in the adult by the tentacles. 

 The ciliated girdle of the embryo pond-snail is therefore 

 the last vestige of a migratory larval stage, which is still 

 effective in the sea-snails. Remote progenitors of the 

 pond-snails, it would seem, inhabited the sea. When they 

 betook themselves to inland waters, so restricted that a 

 crawling snail could traverse them in almost any direction, 

 the original purpose of the ciliated girdle ceased to exist. 

 What had been a free larval stage came to be included in 

 the period of embryonic development. The cilia, if useful 

 at all under the new conditions, are useful in a totally 

 different way. It may be that they still serve to prevent 

 adhesion to the embryonic membranes, a danger incident 

 to all animals which undergo a protracted development 

 within an egg. One part of the disused organ has under- 

 gone a strange transformation, being converted into a pair 

 of tentacles. The gelatinous egg-chain of a pond-snail, 

 familiar to every naturalist and so easily examined that 

 it may be put into the hands of a beginner as a first piece 

 of independent study, is at the same time worthy to occupy 

 the thoughts of the most advanced speculator. It leads 

 us far from the conditions of life of the existing pond- 

 snails, forcing us to consider the larval migrations of their 

 remote ancestors, their subsequent adaptation to terres- 

 trial, and finally to fluviatile existence, and the persistence 

 through long ages of faint but unmistakeable vestiges of 

 ancient history. 



THE MUD-FLATS IN AUGUST. 



It is now August, and the summer heats have dried up 

 the ditch until only one deep pool and one thin runnel of 

 water are left. The black ooze stiffens and cracks. Some 

 of the mud-flats, which have been long exposed to the air, 

 are covered with a mat of green threads, and in places 



