CHAPTER XXXIX 

 ANIMAL RESPIRATION AMPHIBIOUS INVERTEBRATES 



As will already have been gathered from what has been said 

 in chapter xxxvii, numerous interesting illustrations of the amphi- 

 bious habit are furnished by certain members of the important 

 animal groups of the Mollusca and Arthropoda. 



MOLLUSCS (MOLLUSCA) 



ORIGIN OF LAND-SNAILS AND SLUGS. We have seen in the 

 last chapter how the study of amphibious fishes throws light upon 

 the evolution of land vertebrates, and amphibious species are 

 included in typical groups of both marine and freshwater molluscs, 

 which enable us to understand how land-snails proper have origin- 

 ated from aquatic ancestors. It is extremely probable that land 

 molluscs have been derived from two sources, some being de- 

 scendants of marine forms living between tide -marks, and others 

 offshoots from estuarine or freshwater groups. In the latter case 

 we are obliged to fall back in the end upon the sea as the original 

 home of molluscs, for from it have been populated the estuaries 

 and rivers of the globe. And, as in the case of land vertebrates, 

 aquatic molluscs have given rise to land molluscs as the result of a 

 keen struggle for existence, which has driven certain forms of life 

 from sea to land, either directly or after a more or less prolonged 

 sojourn in brackish or fresh water. 



Among the most interesting marine snails which afford a hint 

 as to one set of conditions under which amphibious habits may be 

 acquired are the species included in the Periwinkle Family (Lit- 

 torinida), plant-eating forms characteristic of the region between 

 tide -marks. The gill -cavity is here more or less adapted for 

 breathing damp air when the tide is down, and the contained 

 gill is reduced in size, as its chief work is done when the animal 



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