RESPIRATION IN PULMONATA. 
cteiiidium or primitive gill has become atrophied and lost, and the 
anterior margin of the cavity has become fused with the dorsal 
integument, except at one point which constitutes the pulmonaiy 
aperture, where air is admitted and expelled for respiratory purposes, 
or, as in the aijuatic Pulmonata, for hydrostatic purposes also. Upon 
the walls and more especially upon the roof of the respiratory cavity, 
a complex network of delicate blood vessels has been developed, a 
change necessitated by the adop- 
tion of terrestrial life and aerial 
respiration by the ancestors of 
the Pulmonata ; although the Pul- 
monates are not a homogeneous 
group, but of polyphyletic origin, 
several distinct families having 
branched off in this direction. The 
pulmonary chamber has been aptl}' 
compared to a single air cell of 
the mammalian lung, as in both there is a cavity lined by a delicate 
vascular membrane supplied with impure venous blood, with air in 
contact with the free surface. 
In terrestrial and Huviatile Pulmonates, although the breathing or 
opening and closing of the single pulmonary orifice is more actively 
rhythmical than in the branchiferous species, yet there is not the same 
methodical and regular isochronous succession of respiratory move- 
ments as previously described in the movements of the heart. 
Si^/are of water ^^S ec ^^ Sec 
ScAi-E : — 30 mill. = one hour. 
Fig. 503. — Diagram illustrating the respiratory action at 66 F. of a specimen of Planorbis 
corneiis from Wansford, near Bev’erley. 
The time.s mentioned on the diagram repre.sent the number of seconds the respiratory orifice 
remained open to the air at each inspiration, their duration being too brief to allow of indication 
according to scale. The horizontal line below the level of the surface of the water is to scale and 
represents the time the animal remained beneath the surface between the respiratory acts. 
The renewal of the air within the pulmonary cavity takes place when 
fhe respiratory aperture is opened, and is chiefly accomplished by 
diffusion ; exactly as when a door is opened to ventilate a room, the 
incoming purer air being stated to follow a certain well-defined course 
within the lung-cavity, and to gradually replace that which has become 
effete by respiration ; the process being, however, assisted by the con- 
tractions and relaxations of the muscular walls of the cavity. 
21 10/9D. 
Fig. 597. — Blood sinus of lung wall of 
Helix pojiiatia L., highly magnified (after 
Vogt and Vung). 
b.s. blood sinus; ep. epithelium ; m. mus- 
cular layer beneath a stratum of connective 
tissue bearing pigment cells ; c.t. connective 
tissue cells. 
U 
