146 Dr. T. Williams on the Mechanism of Aquatic 



like its pulmonary rete, is located in the roof. These are features 

 of striking dissimilarity. 



The pulmonary vessels («) of Limax are not, as in Helix, 

 gathered into one large trunk, debouching in the auricle. They 

 consist of several trunks, the cylinders of which are so super- 

 ficially situated as to stand in prominent relief above the main 

 plane of the cavity. This peculiar appearance results from the 

 rigid calcareous walls of the vessels. It seems, according to the 

 author's dissections, as if two or three veins in Limax opened 

 into the auricle, but it is not easy to determine their exact num- 

 ber. The peculiar tenacious mucus secreted by the gland [d, c/) 

 which surrounds the heart {d) is poured out into the respiratory 

 cavity. But although this is the case, it does not interfere with 

 the function of the surface over which it is diffused. It does 

 not become adherent to the latter. This mucus is capable of 

 enclosing globules of air, and of becoming frothy. In this state 

 it is frequently extruded from the pulmonary orifice. The study 

 of these glands and their structure is for the present postponed, 

 since they share in no way in the process of respiration. 



Cuvier and the older anatomists supposed, when they observed 

 the white colour of the vessels in the Limacidse, that the phseno- 

 menon was due to the milbj character of the blood by which 

 they were filled. This is an error. The white colour results 

 from the mixture of fat and chalk which abounds in the sub- 

 stance of the walls of the vessels, imparting to them a peculiar 

 character and extraordinary thickness ; such thickness as renders 

 it extremely difficult to understand how they are not thereby 

 disqualified for the office which they are designed to discharge. 

 The larger vessels (fig. 1 ff) stand upon a more superficial plane 

 than the smaller ones. This disposition gives a cellulated ap- 

 pearance to the surface (fig. 2), like that of the inside of the 

 frog's lung. It is an arrangement which, more completely than 

 a smooth surface, detains the air in contact with the blood. 

 But though a tendency to the cellular form is displayed by the 

 pulmonary membrane, it must be understood that it is not 

 organized after that fashion, as is the case with the lung of the 

 frog. 



The pulmonary vessels in the Pulmonated Gasteropods form 

 but one sheet, that is, the blood traverses the area of aerating 

 surface only once. The blood-currents converge upon the respi- 

 ratory chamber from all parts of the body. Large trunks (fig. 2) 

 may be seen at the sides of the cavity before, behind, and at the 

 sides. These trunks are individually walled vessels ; they are 

 true pulmonary arteries. They subdivide into smaller vessels, 

 and these break again into a network constituting the real lung 

 of the animal. Near the position of the heart, they begin to 



