222 THE BLOOD. 



happened even after twelve hours had elapsed from the 

 exsection. The heart, in another experiment, being ex- 

 posed and laid naked to the eye, was observed to beat spon- 

 taneously for a few seconds, then to stop for about the 

 space of a quarter of an hour, when it again began to beat 

 quickly and violently without any obvious cause, so that 

 this learned physician thinks it certain that, in these ani- 

 mals, the motion of the heart is not beyond its control, but 

 also voluntary or subject to the will,* an inference in 

 which you will not be disposed to coincide. In a small 

 Snail just hatched, Bradley counted about three seconds 

 between each pulsation ; and in an old Snail from six to 

 seven seconds elapsed.f Gaspard saw the heart of a Vine- 

 yard Snail (Helix pomatia) beating, in the summer, twenty- 

 five to twenty-eight times in a minute ; and that of a Pond- 

 mussel contracted, according to Pfeiffer, fifteen times in the 

 minute. J On examining a specimen of Carychium lineatum 

 our mutual friend, Mr. Alder, observed, through the trans- 

 parent shell, that its heart beat only eight times in a 

 minute, and he was about to conclude that the slowness of 

 locomotion, in this class of animals, resulted from the tardy 

 circulation of the blood, when, on examining a few speci- 

 mens of Vitrina pellucida, he was surprised to find that 

 their heart beat about a hundred and twenty times in a 

 minute ! The latter was in a state of action, and the former 

 of rest. He has since found, that the number of pulsations 

 in the same individual is very unequal at different times, a 

 variableness dependent on external influences, doubtless, 

 and principally, perhaps, on the temperature of the air or 

 water to which the animals are exposed. 



The blood itself is of a bluish-white colour, and glutin- 

 ous consistence. 1 1 Lister tells us, that when he kept the 



* Exer. Anat. de Coch. p. 38. Exer. Anat. tert. p. 13. 



f Phil. Ac. of the Works of Nature, p. 129. 



$ Tiedemann's Comp. Phys. i. 156. 



Mr. Garner states that in the Lamellibranchiata the pulsations of the 

 heart are generally from twenty to thirty in the minute. Charlesw. Mug. 

 N. Hist. iii. 168. Messrs. Alder & Hancock have given the number of pul- 

 sations in the following Nucjibranchiata : 



Polycera ocellata 72 88 Ancula cristata 72 75 



Doto coronata 60 Polycera lessonii 62 



Eolis coronata 65 ' Hermsea dendritica 96 



Eolis papillosa 50 



|| Sir E. Home says that the blood of the Teredines is red, Comp. Anat. 

 i. 32; and that of the Planorbis is almost purple. MILNE-EDWARDS, Elem. 

 de Zool. p. 18. Milne-Edwards has found, in the neighbourhood of Pa- 

 lermo, an Ascidia with red blood. Ann. and Mag. N. Hist. xv. 69. 



