Dr. T. Williams (tn the Mechanism of Aquatic 



l^irailis^ reunite, assume a more rapid course, passing over the 

 "surface and between the folds of the " mucous glands/' under 

 the character of effete and deoxygcnated 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 X)f this 

 most beautiful aiTangement. .e'loilqoJ 



' The thoracic cavity of the aquatic and terrestrial Pulm6tiifera 

 (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- 

 ^Diical 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 the left side, in all others on the right. 

 Under the former circumstances the respiratory movements of 

 the parietes of the cavity are reversed. The pulmonary cavity 

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

 body (fig. 4 «, h), is larger in dimensions than the branchial 

 ; chamber of the Pectinibranchs. This difierence 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 which has been made in the instance of 

 the Pulmonaia 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 



jinfundibulum (figs. 4 & 6 «, «). 'Si^iiioqo 



* The acts of inspiration and respiration are remarkably slowly 

 performed. So spacious is this chamber in the Planorbidae 



, (fig. 6 c), Limneadse (fig. 4 a, b), and Helicidte, that a supply of 



'air capable of sustaining life for a considerable tinjie under water, 



or in an irrespirablc medium, can be stored. ^'^^^^ liimaizo an m 



- The normal muscular movements of re^!i^t;iBri*'W^*fei6fet 



<^^sfactorily studied in Helix aspersUy previously carefully, and 



^'Hvithout injury to the 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 



expiration the walls of the whole of the dorsal and lateral regions 



of the body simultaneously contract, and the breathing-chamber 



