50 TESTACEA. 



diameter ; neither does the animal make any use of the fractured original 

 parts for obtaining a perfect dwelling. 



An albuminous mass of spawn, with four embryos, each in its own 

 capsule, has been produced in the beginning of March. The earlier form 

 of the young is an irregular spiral ; but the shape improves ; the eyes 

 become visible ; motion is perceptible, and in twenty-one days the nascent 

 animal quits its prison. But for a fortnight longer, the creature remains 

 a mere white speck, which is transparent under the microscope. The 

 tentacula then seem truncate or mutilated, and the eyes at their root, 

 are black and globular. Now the orifice of the shell is wide, and the 

 whole is void of symmetry. 



Many examples prove the progressive advances of most of the Tes- 

 tacea to symmetry. Sometimes the young so resemble the adult that it 

 might be mistaken for the offspring of a different parent. But all this 

 may be compared to lower degrees of metamorphosis, such as we have 

 described in higher stages. 



This species had been the subject of observation and experiment in 

 the year 1802, when I had favourable opportunities, continued for seve- 

 ral years subsequently. Specimens were then readily obtained from a 

 spot where the water afterwards failed, during an extraordinary drought 

 in 1813. Not a single animal could be obtained either there or in the 

 vicinity, from repeated researches for several years. At length on resort- 

 ing to the same place in 1820, I found a new or a resuscitated colony 

 dwelling in the water. Another drought followed in 1821, inducing me 

 ultimately to regret that I had been prevented, by circumstances, from 

 ascertaining whether the old animals had retreated into the earth, or 

 whether only their ova had been preserved. 



Specimens have survived eighteen months in my possession. 



PLATE XI. 



Fio. 1 . Planorbls spirorbis. 



2. Under surface, enlarged. 



PLANOREIS RHOMBEUS. Plate XI. Figs. 3, 4, 5. 

 The multitude of synonyms and the vague description annexed to 



