Respiration in Invertebrate Animals, 153 



should be enabled adequately to discharge a function so im- 

 perious ? 



Can there be any peculiarity in the structure of the vessels ? 

 As the calcareous layer is less developed here than in Limax, it 

 is obvious that the presence of this layer is not an indispensable 

 constituent of the organ. The pulmonary vessels in the Snail 

 exhibit a structure which cannot well be proved to exist in the 

 Slug. Their coats bulge out in a cellular-like manner. This 

 occurs all round the circumference of each vessel,, most di- 

 stinctly in the smallest. It is impossible to determine whether 

 the convexities on the exterior correspond with concavities on 

 the interior of the vessels. But, by inference from the character 

 of the outside, it seems almost certain that such an arrange- 

 ment does really exist. If it does, it is not difficult to under- 

 stand how considerably it is calculated to augment the surface 

 of contact between the blood and the air. Though such cellu- 

 lated parietes might mechanically slacken the speed of the cur- 

 rent, the function of the part cannot fail to be considerably raised 

 in amount. 



Lymneadce. — "These freshwater pulmoniferous snails consti- 

 tute a very natural family, the animals of all bearing a great simi- 

 larity to each other, and being similarly organized. All have 

 short, broad snouts, and two tentacula of considerable size, either 

 triangular or subulate in shape, with eyes placed at their inner 

 bases.^' (Forbes and Hanley.) Little has been done by anato- 

 mists to elucidate the structure of this interesting family. In 

 several features they approximate closely to the Helicidse. The 

 most striking character is the siphon-like tube into which the 

 respiratory orifice is prolonged. This siphon (fig. 7 a) is capable 

 of being considerably extended beyond the edge of the mantle (Z>) . 

 When the animal is floating reversely near the surface of the 

 water, it may be seen from time to time to be slowly pushed above 

 the surface into the air. At this moment, and through it, the eff'ete 

 contents of the respiratory cavity are emitted, and a supply of 

 oxygen is drawn in. So wonderfully sensitive and discriminative 

 is this little organ, that it opens only when it peeps into the 

 atmosphere. Water never enters into the pulmonary chamber 

 of Lymneus. The same observation applies to Planorbis, 



But in these water-snails the breathing-chamber (fig. 7 c) 

 is more actively ciliated than in the terrestrial families. Not 

 because they breathe at any time on the aquatic principle, but 

 probably because the whole tissues in these water -inhabiting 

 genera are less calcified, and therefore more favourable to the 

 development of cilia. It must, however, be supposed from these 

 statements that the breathing-chamber is lined with a continuous 



