XII THE CHICAGO ACADEMY OF SCIENCES. 



TABLE OF CONTENTS. 



Page. 



Preface V 



I. General Morphology of the Lymnaeas 1 



1. The Shell 1 



2. External Appearance of the Animal 5 



3. General Internal Anatomy 6 



a. The Digestive Tract 7 



b. The Jaw 8 



c. The Radula 9 



d. The Reproductive System 10 



e. The Nervous System 13 



f. The Respiratory System 16 



g. The Renal System 16 



h. The Circulatory System 17 



i. The Muscular System 17 



4. Bibliography of Works Consulted 18 



II. Ecology of the Lymnseas 19 



a. General Environmental Conditions 19 



b. Variation of the Lymnseid Shell in Different Environments. . 29 



c. Locomotion 32 



Gliding 33 



Hunching 33 



Thread Spinning 33 



Mechanism of Locomotion 34 



Rate of Locomotion 35 



d. Respiration 35 



e. .^Estivation 39 



f. Abnormalities 40 



g. Parasitism 41 



h. Food 42 



i. Food for Other Animals 44 



j . Reproduction and Nidification 45 



k. Experiments 47 



1. Length of Life 51 



m. Bibliography of Works Consulted. 51 



III. Distribution in Space 52 



A. The Present Distribution 52 



Eighteen Zoological Regions 56 



Table of Distribution of American Lymnseas 61 



B. The Glacial Period 67 



Post-Glacial Dispersal 69 



Tertiary Lakes of the Great Basin 73 



