Respiration in Invertebrate Animals. 317 



viscera are enclosed. The interior surface of the chamber of the 

 shell is so nacreous and polished, and the corresponding portion 

 of the animal is so serous and smooth, that every condition 

 which can affect the facile motion of the one surface upon the 

 other is thus secured. From this circumstance the mind is led 

 forcibly to the idea, that this arrangement has really nothing 

 more than this mechanical purpose for its object. It might be 

 supposed, from the intimate contact thus effected between the 

 external element and the visceral cavity, that the former might 

 incidentally enter m considerable quantities by endosmosis into 

 the latter, and thus replenish the diminishing volume of the 

 nutritive fluids. In order to determine this latter point, and 

 whether the water in the chamber of the shell (fig. 2 a, a) were 

 capable of fulfilling an accessory part in the office of respiration, 

 it became necessary to ascertain by actual observation two 

 doubtful points of structure, viz. whether the membrane cover- 

 ing that portion of the body of the animal which is enclosed 

 within the shell be ciliated, or otherwise favourably organized 

 with a view to such an office ; and secondly, whether the circu- 

 lating fluids beneath this membrane were disposed conformably 

 with such an intention. 



The great bulk of the soft parts, the abdomen proper 

 (fig. 6d,d; fig. 4: b, c), by which the coil of the shell is filled, con- 

 sists of the liver, a portion of the stomach and intestinal canal, 

 and the reproductive organs. They are invested by a mem- 

 brane which is the continuation of the mantle. The membrane 

 here becomes thinner and smoother, assuming the characters 

 of a serous striicture ; it is not adherent at any point to the 

 shell. On the inferior as])ect of the body it is drawn up into a 

 frenum, in the layers of which are enclosed muscular fascicles. 

 It is by means of this contrivance that the animal is enabled 

 to coil itself firmly round and to grasp the columella (fig. 2 c). 

 Although this coat has a serous aspect, it is the continuation of 

 the fibrous mantle which forms the vault of the respiratory 

 chamber (fig. 3 A-«, & fig. 3 B-«). If by a very careful dis- 

 section this covering be removed from the viscera underneath, 

 the nature of its connexions with the latter will be readily seen. 

 It nowhere leaves an open space between it and the solid organs 

 which it invests. It is, on the contrary, so intimately united to 

 them, that numerous fibrous threads and bands descend from its 

 internal surface, penetrating into the substance of the viscera 

 and becoming continuous with their stromatous fibrous struc- 

 ture. It is inseparably identified with the serous tunica propria 

 of each viscus. The latter cannot be said to exist as an inde- 

 pendent structure. In no single species of the Prosobranchiata 

 (M.-Edw.), or the shelled Pulmonifera, is it provided with ciliary 



