12 Mr Stevenson on the Building Materials 



coat of the cyst may be of this nature, but it is not so easy 

 to conceive the inner tunic to be due to the same cause. Pro- 

 fessor Owen, in his memoir on the Trichina spiralis — the en- 

 tozoon of the human muscles, — ^liolds that the cyst of that ani- 

 mal, although apparently consisting of two tunics, is the result 

 of irritation. Dr Knox, again, considers it to be a part of the 

 animal, although the latter lies free in the cavity. This latter 

 opinion is inadmissible, according to the usual conception of an 

 individual animal. Might we not conceive the cysts to be 

 essential parts of all such entozoa, inasmuch as they are never 

 absent ? and may we not suppose them to be parts of the origi- 

 nal ovum within which the animal was formed, and in which 

 it passes the term of its existence ? Without having any facts to 

 adduce in proof, I hazard this supposition as a hint for future 

 research ; and as it is not at variance with any of the known 

 conditions of animal existence, it is worth consideration in a 

 fresh investigation of the subject. 



EXPLANATION OF THE FIGURES (PLATE I). 



Fig. 4. Entozoon inclosed in botli cysts. 



Fig. 5. The internal transparent cyst, with the worm seen through it. 



Fig. 6. The worm removed from the cyst and fully expanded. 



Fig. 7. The cervical receptacle opened to shew the retracted head and 



neck. 

 Fig. 8. The four muscles of the proboscides. 



On the Building Materials of the United States of North Ame- 

 rica, By David Stevenson, Esq., Civil Engineer, Edin- 

 burgh. Communicated by the Society of Arts for Scotland. 



There is, perhaps, nothing connected with the useful arts, 

 which has a greater share in forming the characteristic appear- 

 ance of a country, than the materials which it produces, and of 

 which its public works are necessarily constructed. I use the 

 word materials, in the technical sense in which it is employed 

 by engineers and architects, to denote the several productions 

 of the mineral and vegetable kingdoms which are used in the 

 construction of engineering and architectural works ; and we 

 have only to look around us for a moment, to be at once con- 

 vinced how much these, in their almost endless variety, affect 



