iSO CHORDATA. 



and pulmonary artery to the lungs. Becoming arterial there, it llows l\v the 

 pulmonary \ ein into the left auricle and ventricle, and thence b)' the aorta into 

 the systemic circulation. I-'or details see fig. SjS- 



Besides blood-^'essels, Iviiiph vessels occur as complements of the \enoiis 

 svstem. The fluids which collect in the spaces of the connecti\e tissue are taken 

 bv them and carried (Ihoracic dnet) into the large venous trunks. Usually the 

 action of the heart and the movements of the bodv are sufficient to cause the 

 flow of this Ivmph, but special lyinpJi hearts may occur. The Ivmph vessels 

 distributed to the digesti\"e tract plav an important role, since thev ser\e in the 

 resorption of digested food. They are called eliyle duels because their contents, 

 the chvle, rencicred white bv oil globules at the time of digestion, distinguishes 

 them from other lymphatics. The most important features of Ivmph and blood 

 have already been noticed (p. jS). In special places small bodies, the lymph 

 i^hvids, are inserted in the course of the lymph vessels, in which lymph corpuscles 

 arise. Among these from its structure is to be enumerated the spleen, colored 

 bright red bv its rich blood supply. 



The sexual and cxerclory orgaits are so closely associated that they are 

 general!)' united as the iirogcnilal sysIcjji. The sexual products are formed 

 in the embryo in a special region of the peritoneal epithelium on either 

 side of tlte yertebral column. These primordial cells arise from the ento- 

 derm, migrate into the epithelium and then early leave this position, and 

 sink into tli.e underlyirtg connective tissue (hg. 34), formuig in the male 

 glandular tubes, ill the female cords which break up into numbers of 

 rouiad follicles, each contairiiiig a single larger cell, the ovum. In the 

 male the gonads thus formed are compact and frequently oval testes; in 

 tire female tliey are looser and follicular cn'ories. 



In many fishes the sexual cells pass out by way of the body cavitv and 

 the abdominal pores, and in this case a part of the ccvlom mav be cut off as 

 a special vas deferens or ovidiiet. In mo.-t vertebrates the ducts are formed 

 from a part of the nephridial system. The urogenital system thus formed 

 recalls that of the amtelids and in botlt there is the same origin of gonads 

 and nephridia from the ctxdomic epithelium. As the lower \ertel)rates 

 (Amphibia and ITasmobranchs) show, the first stage of the excretory 

 system is furnisheil by segmentally arrangeil cat\als [nephridial liiluiles) 

 which open into the body cavity by ciliated fuiuiels, the nephrostomies. 

 In many forms the nejihrosloines persist throughout life, in others they 

 degenerate. In the higher \-ertebrates they are usually itot formed, a 

 fact connected with the loss of the excretory function of the cu'lom and 

 its assumption by the blood system. T.his has entered into connexion 

 in a characteristic way with the i\ei.)hridial tubules, and especially by the 

 formation of glomeruli, networks of l)lood capillaries which, pushing the 

 wall of the tubules Ijefore them, produce the Malpighian bodies. Rarely 

 there is a rich L)Iood vascular development in the wall of the body cavity, 



