326 Dr. T. Williams on the Mechanism of Aquatic 



columns reunite, assume a more rapid course, passing over the 

 surface and between the folds of the "mucous glands," under 

 the character of effete and deoxygenated water, and sweeping 

 the termination of the intestine, and finally escape at the ex- 

 treme right cleft {b) of the respiratory chamber, mechanically 

 bearing on its current all the refuse products of the cavity. It 

 will be anticipating, what will afterwards be studied more mi- 

 nutely, to enter further at this place into the details of this 

 most beautiful arrangement. 



The thoracic cavity of the aquatic and terrestrial Pulmonifera 

 (figs. 4, 5 & 6) is mechanically arranged on the plan of that of 

 the Pectinibranchiata. Though in some genera, as in Parma- 

 cella, Testacella and Onchidium, it may assume a posterior posi- 

 tion on the back, it is not changed in mechanism or in anato- 

 mical structure. In all other families of this order it occurs on 

 the back near the head. In those species whose shell is sinistral 

 the orifice is situated on (he left side, in all others on the right. 

 Under the former circumstances the resjiiratory movements of 

 the parietes of the cavity ai-e reversed. The pulmonary cavity 

 of the air-breathing Gasteropods, relatively to the bulk of the 

 body (tig. 4 a, h), is larger in dimensions than the branchial 

 chamber of the Pectinibranchs. This difference is explained 

 by the difference between air and water, but, as will hereafter 

 be shown, it is due in reality to the comparatively rude and 

 imperfect provision v*'hich has been made in the instance of 

 the Pulmonata for the necessities of breathing. Here, the an- 

 terior fissure of the branchial cavity, so characteristic of this 

 part in water-breathing Gasteropods, is accurately closed. The 

 cavity communicates with the exterior by means only of a single 

 orifice, which, for the most part, is situated on the right side. 

 This orifice, in families of aquatic habits, is prolonged into an 

 infundibulum (ligs. 4 & G a, a). 



The acts of inspiration and respiration are remarkably slowly 

 performed. So spacious is this chamber in the Plauorbida> 

 (fig. 6 c), LimneadEe (fig. 4 a, b), and Ilelicidie, that a supply of 

 air capable of sustaining life for a considerable time under water, 

 or in an irrespii'able medium, can be stored. 



The normal muscular movements of respiration arc most 

 satisfactorily studied in Helia; aspersu, previously carefully, and 

 without injury to tiie soft parts, removed from its shell. It will 

 be seen that the volume of air drawn in by the inspiratory act is 

 driven by the slow vermicular movement of the parietes (arrows 

 in fig. 4) from right to left, chiefly along the roof. If the air 

 is long retained, it repeats the same orbit. During the act of 

 ex[)iration the walls of the whole of the dorsal and lateral regions 

 of the body simultaneously contract, and the breathing-chamber 



