154 On the Mechanism of Aquatic Respiration. 



layer of ciliated epithelium. It is detectible only on certain 

 parts which chiefly correspond with the lines of the large trunks. 

 The Limneids differ from the Helicidse in being furnished with 

 a richly ciliated epithelium on the exterior of the respiratory 

 cavity. In young specimens vibratile cilia may be detected over 

 the entire abdominal portion of the body. In the old, how- 

 ever, it disappears from this region, and is replaced by a non- 

 ciliated variety. 



In Planorhis the distribution of cilia is more limited. It is 

 detectible on the siphon and margins of the thoracic cavity. 

 With care it is possible to lay open or to remove the roof of this 

 cavity in a large specimen of Lymneus, fixed by pins whilst 

 floating in water. The roof delicately cut away is placed between 

 two slips of glass and examined as an opake object. The larger 

 vessels present a general arrangement analogous to that for- 

 merly described in Helix. A large central trunk runs obliquely 

 from the left anterior to the right posterior angle of the cavity 

 (fig. 7 d). It is the main venous channel which terminates in 

 the auricle (e). The lateral or secondary trunks converge upon 

 the line of this vessel from all sides. It is scarcely possible, in 

 consequence of the absence of lime from the coats of the vessels, 

 to trace the distribution of the ultimate channels. From glimpses 

 obtained here and there, it is however most probable that they 

 observe a parallel mode of division such as that already repre- 

 sented in the Helicidse. The heart differs from that of Helix. 

 The ventricle stands at an angle on the auricle. The axes of 

 these two divisions meeting at the auriculo-ventricular orifice 

 would form nearly a right angle. The gland (/) occupies a 

 position to the left of the heart ; it resembles that of Helix ; 

 it is an oval flocculent mass. To the right of the heart, the 

 rectal intestine {ff) enters within the precincts of the respiratory 

 cavity ; it traverses the chamber along its floor, and terminates 

 in the siphon {a). 



The breathing-chamber in the Lymneadse exceeds that of all 

 other air-breathing Gasteropods in size relatively to that of the 

 body. The surface for the outlaying of the pulmonary plexus 

 exhibits a greater relative area. The pulmonary sac of Planorbis 

 is probably organized after the model of that of Lymneus. It is 

 formed however on too small a scale to admit of a direct and 

 satisfactory examination. The siphon is extensile like that of 

 Lymneus, and like that of the latter genus, the cavity communi- 

 cates externally by means of the siphon alone. 



[To be continued.] 



