52 Descriptions of Preparations. 



along the concavity of the visceral spire^ and joins the free thick- 

 ened rim or ^collar' of the mantle^, a little posteriorly and su- 

 periorly to the respiratory orifice. One of the afferent pulmonary 

 veins is figured by Milne Edwards^ Memoires de I'Institut, torn. 

 XX., 1 849, pi. V. d., as taking a similar course. The muscular floor 

 of the respiratory sac discharges a secondary function, in propelling 

 the animal downwards when it has been retracted into its shell, 

 as well as a more obvious and primary function in the act of 

 breathing. In the completion of the act of forcing the animaVs 

 body out of the shell, the '^ collar' takes a share. And it is to 

 this thickened muscular and glandular border of the mantle that 

 the secretion of the epidermis of the shell and its increase of capa- 

 city is due in all shelled Gasteropoda, whilst the formation of the 

 epiphragm is also to be ascribed to it in the air-breathing repre- 

 sentatives of the class. 



For the formation of the epiphragm, as observed in the Helix 

 /wrfensis, see Binney, Terrestrial Air-breathing Molluscs of 

 the United States, vol. ii. p. 114; Keferstein, Klassen und 

 Ordnungen des Thierreichs, iii. ii. p. 11 86. 



For figures of the pulmonary vessels in Helix Pomatia, see Milne 

 Edwards' Memoires de I'Institut, tom. xx. 1849, pi. iv., pi. v.; 

 C. G. Carus_, Tabulae Comp. Anat. iv. vii., 2, 5. 



17. Edible Snail {Helix Pomatia), 



Dissected so as to show its nervous system. 



The mantle has been separated from its attachment around the 

 base of the visceral mass, except for a short distance, corresponding 

 to the middle line of the foot posteriorly, and along from that 

 attachment, a short distance round on the animal's left side, where 

 a triangular lappet, the so-called ' columellar lobule,' is developed 

 upon it. It has been turned over to the left side, together with 

 the organs in connection with the roof of the cavity it developes, 

 viz. the heart and the pulmonary vessels, the rectum, and the 

 kidney with its duct. The integument covering the head and 

 neck has been divided in the middle line, and fastened out on 



