34 SIXTEENTH REPORT. 



SUMMARY. 



a. Environmental Differences Between Lakes, Pools and Swamps. 



There seemed to be seven main factors that differentiated the various 

 stages of this form of the lake-pond-swamp series, and these were all largely 

 dependent on the size, depth and amount of protection of these bodies of 

 water. The seven factors were: (1) the amount of wave action, and the 

 "roughness" of the water, (2) the comparative extent of the shallow littoral 

 conditions, as compared to the remainder of the body of water, (3) the con- 

 stancy in amount of water, (4) the isolation, (5) the amount and character 

 of the vegetation, (6) the temperature, and (7) the proportion Ijetween the 

 surface and the entire volume of the body of water. Some of these factors 

 were reflected by marked adaptations in the molluscs; the effects of others 

 would have to be determined by closer observation and experiment. 



/. The Amount of Wave Action. The lakes naturally developed much 

 larger waves than did the ponds and swamps; Douglas Lake sometimes was 

 disturbed by waves six feet in height between trough and crest, while the 

 pools and swamps were mostly smooth or were affected by small ripples. 

 The lagoons, however, altho much less disturbed than the open lake or the 

 unprotected shores, were much more open to wave action than were the 

 enclosed pools. 



Ill this connection, two adaptations were noticeable among the molluscan 

 forms of the lake, in contrast to the pools and swamps: (1) the prominence 

 of burrowing forms in the lake, and (2) the adaptations of the lake molluscs 

 for breathing water. Group 1, of the lake, has alread}^ been described as 

 consisting entirely of rather large burrowing forms, and, while their relative 

 prominence fell off with increasing protection, they were present in Groups 2 

 and 3 at certain depths. The varieties of Planorhis bicarinatus and Planorbis 

 campanulatus smithii were also semi-burrowing in habit. This fauna be- 

 came inconspicuous in the lagoons and usually disappeared in the pools, 

 altho the Sedge Point Pool contained two clams; it was entirely absent in 

 the temporary swamps. 



The second of these factors was even more prominent. All of the shells of 

 Douglas Lake breathed water (with the possible exceptions already noted), 

 while all of the forms of the Sedge Point Pools, with the exception of the 

 Sphaeridae and the two, rare clams were air breathers, as were also the 

 forms of the temporary swamps with the exception of Sphaerium occiden- 

 tale. The pulmonates of the Pine Point and Hook Point Lagoons appeared 

 to breathe water like those of the lake, but those of the Swamp Point Lagoon 

 were air-breathers as far as could be examined. The comparative infre- 

 quency of the gill-breathing forms in the pools might have been partly due 

 to the next factor to be taken up, but it certainly seemed i)robable that the 

 water-breathing adaptations of the larger pulmonates in the lake Avas forced 

 on them by their inability to take air unless the surface of the water was 

 comparatively smooth. The six, deep littoral forms of group 4 were all 

 peculiar varieties, as has been mentioned in a previous paper (6), with their 

 basal whorls considerably enlarged, apparently to allow for the increased 

 capacity of their lungs which was correlated with this habit of filling them 

 with water. Two of them, Physa ancillaria parkcri and I.ymnaea stagnnlis 

 pera)npla, as has already' been mentioned, liatl changed in Sedge Point Pool 

 to the more slender, air-breathing forms, Physa ancillaria, var., and Lymnaea 

 stagnalis appressa, respectivel3^ 



