8 Colourless Blood in Animals. 



corpuscles. The lymph hearts occur in the tail region and are much 

 more than mere ill-defined spaces in the tissues. They are distinct 

 chambers with special walls, in which striated muscle fibres may be 

 made out. A long lymphatic vessel passes midway along the lateral 

 muscle masses receiving successive side branches, while two trunks run 

 alongside the lateral neives, one on each side, and two pass along inside 

 the spinal canal. Perhaps the amphibians, frogs especially, have this 

 colourless blood-circulation best developed. Two definite lymph 

 hearts occur, in the frog, between the short rib-like transverse processes of 

 the 3rd and 4th vertebrae, and a second pair behind the hip-joint, on 

 each side of the urostyle. These pulsating organs show striped muscle 

 fibres. Other large lymph spaces, which do not however pulsate, occur 

 on each side of the head, and a chain of irregular spaces, filled with 

 fluid, run down each side of the back, with corresponding ventral 

 vessels, and ramifications along the limbs. Lymph spaces and vessels 

 have not been noticed so prominently in reptiles, except in tortoises and 

 crocodiles. In the latter there are large abdominal spaces, and smaller 

 chambers near the root of the tail. In the tail region in birds, during 

 the early stages especially, there exist well-marked lymph spaces. The 

 existence of a lymph or colourless blood circulation in so many groups 

 of animals, including the highest vertebrates, must have some weighty 

 significance. Its primitive character is demonstrated by the fact that 

 the suspended corpuscles are nucleated cells, and quite unlike the red 

 corpuscles of warm-blooded mammals. When we thus find in the 

 lowest vertebrate (Amphioxus) and in the early stages of higher forms, 

 such as larval fishes, that the red blocd circulation is absent there is 

 every evidence that a colourless blood system is the original system, and 

 that red-blood is a modified and secondary arrangement. 



The blood circulation in the invertebrates is then a primitive system, 

 which persists in Amphioxus as the only system ; while "in fishesandthe 

 lower vertebrates it maintains an importance almost equal to that of the 

 red-blood circulation, but in the higher vertebrates, although itstill supplies 

 co!< ' and serum to the red bio id, the latter circulation 



has largely supplanted it and deprived it of its original importam 



