48 TESTACEA. 



PLANORBIS SPIRORBIS. Plate XI., Figs. 1, 2. 



Diameter of the shell four lines ; thickness half a line ; volutions five 

 or six ; figure flat, resembling a very thick wafer. Animal a limax ; ten- 

 tacula two, long and taper, with a black eye at the root of each. Sole 

 short, with an obtuse posterior extremity. Colour of the animal reddish, 

 whereof the shell slightly partakes. 



In December, while the temperature of the water was 54, the pul- 

 sations of the heart were thirty-two in a minute. 



This species feeds on certain parts of the Veronica Beccabunga, equise- 

 fum, and other fresh- water plants. Some specimens have fed on oysters. 



Its motion is slow and unsteady, owing to the disproportion between 

 the sole and the size of the shell, which is always perpendicular to the 

 plane of position. When the side of the shell comes accidentally in con- 

 tact with the side of the vessel, it claps close, adhering immovably and 

 beyond the power of the animal to relieve it. 



The animal is incited to motion by the prevalence of a mild tempera- 

 ture and exposure to the light. It generally seeks the higher parts of 

 the water. When crawling above the surface, the shell becomes unma- 

 nageable, adheres to the side of the vessel, and its tenant perishes. 



In the beginning of August the shell is often invested by parasite 

 colonies of the Vorticetta, whose multiplication is apparently repressed by 

 repeated renewal of the water. 



This little creature is very tenacious of life, and it survives intervals 

 of desiccation which would be fatal to many other animals. Towards 

 the end of August, a specimen was discovered with the side applied flat 

 to its glass, wherein it had probably remained about two months. The 

 body had contracted far within the orifice, therefore I removed a whole 

 volute to reach its retreat. It was then immersed in water, while the 

 temperature of the atmosphere equalled 59. In two or three hours 

 some small air bubbles escaped, and in other two, the animal, though 

 very weak and languid, appeared crawling on the vessel. 



Here it had been in a state of known desiccation, deprived of water, 



