New Theory of the Ascent of the Sap, 54 1 



the same time analogous, habits. But I am more inclined to 

 the opinion that he has, in this instance, confounded the pul- 

 monary foramen with the anus, and been thereby deceived as 

 to the exact retreat of the parasite. Those who are acquainted 

 with the anatomy of these Molliisca are well aware that the 

 two apertures in question are situate close to each other, and 

 that, in the case of the snail, the anus may almost be said to 

 terminate in the pulmonary cavity ; the two together having 

 but one opening externally, which is common to both. This 

 common aperture, which is often very conspicuous from its 

 size (the animal having the power of enlarging and contract- 

 ing its dimensions at pleasure), is seated in that part of the 

 cloak which is called the collar ; and an ordinary observer, 

 who saw the excrement passing out of it, would naturally 

 suppose that it led directly to the intestines. 



Reaumur also states another circumstance with respect to 

 these insects, which deserves to be mentioned ; and that is, 

 that he could never meet with them in wet weather ; but that 

 if, at such times, he confined a number of snails in a close 

 vessel, however free they might then be from these parasites, 

 these last were sure to show themselves in plenty in a few 

 days. I have not myself observed any particular connection 

 between the appearance of these insects and the state of the 

 weather ; but the above circumstance will, perhaps, throw 

 some light upon the fact which I mentioned in the former 

 part of this article. 



Dr. Shaw appears to have met with this parasite upon the 

 great spotted slug, the Z/imax antiquorum of Ferussac; a 

 species on which I have not hitherto observed it. 



Swaffham BulbecJc^ Cambridgeshire, L. Jenyns. 



Sept. 13. 1831. 



Art. XIII. Nexjo Theory of the Ascent of the Sap. By H. 



Of all the various attempts to explain the laws of vege- 

 table physiology, which have at different times engaged the 

 attention of talented and scientific investigators, none have 

 been so unsuccessful as those which attempt to elucidate the 

 cause of the ascent of the sap. As obscure now as when 

 Grew first published his theory to the world, the lapse of a 

 century and a half has thrown no light upon the subject, 

 though during that time many patient labourers were en- 

 gaged in that arduous field of discovery. The results of 

 their labours were so unsatisfactory when given to the world, 



