168 



LETTER XL 



THEIR SYSTEM OF AQUEDUCTS. 



I HAVE often been struck, wlien examining the testaceous 

 mollusca, with the great difference between the size of the 

 animal when fully extruded and when contracted within the 

 chambers of its shell. It may not be compared to that 

 power of expansion and contraction which Milton assigns 

 to the fallen spirits when they thronged the council-hall of 

 Pandemonium, — who 



" To smallest forms 

 " Reduced their shapes immense ;" 



but it is so remarkable that, when once observed, it can 

 scarcely fail to raise a question of its cause and end. You 

 may observe the difference in ahuost every marine mollusk, 

 though we have no species on our shores that exhibits it in 

 that excess of which we have an example in the " Yet " of 

 Adanson, (Cymba nejDtuni, Sow.,) where the protruded foot 

 far exceeds the entire bulk of the shell (Fig. 29). The 

 Cowries and the Tun-shells (Dolium) are examples of the 

 same excess, and in the former the breadth of the foot, and 

 the extent of the mantle-lobes, contrast strongly with the 

 narrowness of the shell's aperture. * Nor is the fact less 

 visible in the land tribes, for just recall to memory the size 

 and figure of the common snail as it crawls along, and you 

 will then admit it to be curious how such a broad elongated 

 foot, and all the tentacula, can be so nicely compacted toge- 

 ther as to be contained in the shell with ease, and with room 

 enough to spare. I have already called your attention to 



* Of Buccinum Isevigatmn, and B. achatenum, Mr. Svvainson says, — 

 " Both these have the foot of an immense size, so that it spreads over a cir- 

 cumference near three times as large as the shell, and is sufficient to envelope 

 it entirely." — Malacology, 74. The genera of Gastcropods, which have the 

 foot disproportionahly large, are Dolium, Oliva, Ancillaria, Bulliana, Harpa, 

 Voluta, Cymbium, Ovula, Cyprsea, and Natica and Bulla. An orifice for 

 the introduction of water within the foot is seen also in Conns and Nassa, 

 where the foot is comparatively small. See " Figures of Molluscous Ani- 

 mals," etched by Maria-Emma Gray, vol. i. Lond. 1842 : one of the most 

 valuable works which the conchologist can place in his library. 



