150 Dr. T. Williams on the Mechanism of Aquatic 



as only another illustration of tlie illimitable fertility of resource 

 by which every part of every living mechanism is distinguished. 

 It is only by such a contrivance that the necessary surface for the 

 aerating process can be realized ; for compare the area of this 

 lung-sac with the extraordinarily multiplied superficies secured 

 by the laminose arrangement of the gills in the branchiferous 

 orders, and how considerably the latter will be found to ex- 

 ceed the former ! In comparing a gill with a lung, even in 

 the same class of animals, and organized on the same type, it is 

 necessary to remember that in the former case the oxygen, dis- 

 solved in water, is brought up to the blood under certain opposing 

 circumstances, and that a compensation is ofi'ered in the divided 

 state of the blood, while in the latter a similar amount of work 

 may be done with large vessels and coarse streams, in conse- 

 quence of the unmixed and direct manner in which the active 

 element is applied to the vital fluid. 



The pulmonary chamber of the Helicidse is distinguished by 

 several anatomical particulars from that of the Limacidse. In 

 the Snail (fig. 3) the rectum [a) enters the precincts of the 

 chamber, and the heart [b], mucus-gland {g), and pulmonary 

 vessels {d) are seated on the roof [h) of the cavity. In the Slug, 

 as already detailedly explained, the rectum is external to the 

 chamber, and the heart, gland, and pulmonary rete are placed at 

 the bottom of the cavity. 



Helix aspersa (figs. 3, 4 & 5) is the best, because the most 

 familiar, example of the family of the Helicidse. In it, as com- 

 pared with LimaXj a marked deviation is observed in the posi- 

 tion of the mucus-gland. That of Helix presents the appearance 

 of a soft oval mass (fig. 3 g) situated to the right of the heart 

 when in situ ; it is considerably less developed than it is in Limax. 

 This proximity of place to the heart has probably some meaning 

 which is not yet understood. But in structure the gland is the 

 same in the two families. The auricle (c) and ventricle {b) of 

 the heart in Helix are placed on the same axis ; they are separated 

 only by a slight constriction. In Limax the divisions of the 

 centre of the circulation are placed on difi'erent axes, and more 

 individualized. In this latter genus it is planted in the centre 

 of the respiratory plexus, and also in that of the cavity. In 

 Helix it is situated at the posterior boundary of the chamber. 



The roof in this shelled genus is a flexible membrane (fig. 3 b, 

 and figs. 4 & 5 c). It is fixed posteriorly to a sort of diaphragm 

 (fig. 3 i) which imperfectly divides the thorax from the abdomen (J). 

 In the substance of the roof is lodged a stratum of muscles which 

 contract and relax synchronously with the expiratory collapsing 

 and inspiratory expanding of the cavity. These respiratory move- 

 ments are very markedly defined in the animal of Helix removed 



