POLYGYRA ALBOLABRIS AND LIMAX MAXIMUS 267 



the heart by the aorta, and is distributed by the arteries to the 

 different parts of the body, and passes from the minute branches 

 of the arteries into a flexus of capillaries spreading over the 

 whole body ; passing from them into the veins, and from the veins 

 into the sinuses previously described; finally all the blood enter- 

 ing the pulmonary circulus; and thence the pulmonary veins, 

 where, circulating freely through efferent and afferent vessels, it 

 becomes thoroughly aerated. 



The renal organ or kidney is supplied with blood, which has 

 previously been aerated, but only a small part of the blood 

 passes each time through it. 



Though the veins are situated in the substance of the body, 

 and their walls are much thinner than those of the arteries, they 

 are not simply lacunae or wall-less passages in the body, as they 

 have frequently been described. The walls though thin are dis- 

 tinct. 



Blood. In animals of the simplest structure all the fluids seem 

 to be of the same nature and seem to be " only water charged 

 with organic particles, but in animals higher in the scale of 

 being the fluids cease to be of the same nature, and there is one, 

 distinct from all others, destined to nourish the body. This fluid 

 is the blood. It not only nourishes the body, but is the source 

 from whence is derived all the other secretions, such as saliva, 

 urine, bile, etc." 



In the higher animals the blood is of a red color; but in the 

 Invertebrata it is of different densities aud of various colors. 



The blood of the Helix and L i m a x consists of a nearly 

 transparent fluid in which float solid corpuscles. 



For the following facts in regard to the composition of the 

 blood I am indebted to Dr Griffith's Physiology of the Invertehrata. 



In the majority of the Invertebrata the carrier of oxygen to the 

 tissues is haemocyanin, contained in the blood, but in many of the 

 Annelida, as well as in nearly all of the vertebrates, the transport 

 of oxygen from the surrounding medium (air or water), to the 

 living tissues is made by the hemoglobin of the blood. This sub- 

 stance, as is well known, forms an oxygenized condition which is 

 very unstable, and which is carried by the blood across the tissues 

 of the animal, and is there dissolved, yielding its oxygen to those 

 tissues which require it. 



