84 EVERS'S COMPARATIVE ANATOMY. 



mora distinct from the sanguiferous than in any of the preceding 

 classes, as manifested in the sanguineous characters of the chyle, 

 the elaborate structure of the vessels, the perfect condition of their 

 valves, the increased number of conglobate glands, and the unity 

 and distinctness of the thoracic duct. Sometimes this duct is 

 double, as in the dog, and sometimes its branches open into the vena 

 azygos, as in the hog. Occasionally its trunk divides, and having 

 enclosed a narrow elongated space, called insula Halleri, the 

 branches again unite. The mesenteric glands are of great magni- 

 tude in the cetacea, more detached in the pachydermata, and grouped 

 into a mass, named pancreas Assellii, in the carnivora. 



RECAPITULATION. 



1. Lymphatics have been described, but not saticfactorily demon- 

 strated, in the invertebrata. 



2. No conglobate glands have been found in fishes ; their absorb- 

 ents are thin, convoluted, and furnished with rudimentary valves, 

 and occasionally two thoracic ducts exist. 



3. This system is the same in amphibia as in fishes, but in the 

 reptilia the valves are more perfect. 



4. In birds the absorbents are very numerous, the valves still 

 more perfect, and glands for the first time met with. 



5. In mammalia the entire system is marked by a higher type of 

 formation : the thoracic duct is sometimes double as in the dog, 

 and occasionally opens into the vena azygos as in the hog. 



CHAPTER XIII. 



SANGUIFEROUS SYSTEM IN THE INVERTEBRATA. 



The systemic, or red-blood circulation was discovered in the 

 higher animals by Harvey, in 1619; and by the researches of 

 modern comparative anatomists, it has been found to have a much 

 more extended existence ; there is a considerable number, however, 

 of the cycloneurose classes of animals, in which no distinct vascular 

 system has as yet been detected. The first appearance of it which 

 we observe in the lower animals, as in the earliest condition of the 

 human embryo, consists of vessels alone, through which the fluids 

 move in a circle, like the colourless blood in the cells of a plant. 

 In the asterias, echinus, and holothuria among the echinodermata, 

 a large vessel, in the form of a ring, surrounds the commencement 

 of the alimentary canal, from which the systemic arteries are 

 derived ; the systemic veins send branches to the gills, from which 

 the blood is returned by one large vessel to the heart. 



