10 



The Urogenital System 



Because of their close morphological relationship, the ex- 

 cretory and reproductive systems are described together. The 

 relationship is particularly close in the male where tubules 

 of the kidney are utilized for the transport of the sexual 

 products. This area of comparative anatomy is generally 

 given close attention and its contribution to our knowledge 

 of vertebrate phylogeny is often stressed. Homer Smith be- 

 lieved that the fine structure of the vertebrate kidney in- 

 dicated a specialization for the removal of water and that, 

 therefore, the vertebrate group arose in fresh water. It may, 

 however, have been a preadaptation making possible the 

 invasion of fresh water. The urochordates and hemichor- 

 dates lack any kidney and have apparently always been 

 marine in habitat. Amphioxus has a large number of ne- 

 phric vesicles, associated with clumps of solenocytes, in the 

 pharyngeal region. The vertebrate system shows no agree- 

 ment with that of the cephalochordate, so it can be assumed 

 that this type of excretory system arose in the vertebrate 

 group as an adaptation to some special condition, metabolic 

 or environmental. The vertebrate reproductive system also 

 probably arose independently since it is not indicated in the 

 protochordates. 



THE EXCRETORY SYSTEM 



The function of the excretory system is to separate excess 

 or waste materials from the blood. It consists of the kidney, 

 the tubes draining this structure, and an exit from the body. 

 Frequently there are storage structures associated with this 

 system. Since the process of excretion is generally defined as 

 removal of metabolic wastes from the body, this term would 

 also include the removal of carbon dioxide by way of the 

 respiratory system. The excretory system also is involved in 

 water and salt balance, which are only in part related to 

 metabolism; therefore, it seems best to use the rather indef- 

 inite statement "removal of excess or waste materials" in de- 

 scribing its function. 



Mammals 



The kidneys in the mammal (or man) are fairly large 

 structures lying retroperitoneally against the dorsal wall of 

 the body cavity, to either side of the midline (Figure 10-3). 

 Conspicuous renal arteries and veins enter the medial con- 

 cavity (hilus) of the kidney. From the hilus exits the ureter. 

 If the kidney is sectioned frontally, much of its interior 

 structure can be observed (Figure 10-1). In its simplest form, 

 as in the monotremes, small marsupials, edentates, bats, 

 some rodents, insectivores, and some carnivores (cat), the 

 ureter opens into a simple cavity or pelvis, into which pro- 

 jects a papilla. This is the unilobular (or unipyramidal) 

 type of kidney. From the tip of the papilla numerous dark 

 lines indicate the collecting tubules and collectively these 

 form a pyramid. Capping the pyramid is cortical tissue in 

 which dark granules, the knots of arterial capillaries called 

 glomeruli, can be observed. The major blood vessels, which 

 enter at the hilus, branch to either side between the outer 

 cortical material and the inner medullary (tubular) sub- 

 stance. 



In a more complex type of kidney, the tubules of a pyra- 

 mid open along a crest or ridge instead of on a papilla. This 

 crest projects into the cavity of the pelvis. Such a type oc- 

 curs in the dog or sheep, as well as in many carnivores and 

 cloven-hoofed mammals, the artiodactyls. The next step in 

 increasing complexity of structure involves subdivision of 

 the pelvis into a number of outpocketings, the major and 

 minor calyces. Into each of these subdivisions a papilla pro- 

 jects. Each papilla has a pyramid with its cap of cortical 

 tissue, and the whole forms a lobule. This type of kidney is 

 described as polylobular and compact. The several lobules 

 are enclosed and separated by connective tissue, the columns 

 of Bertin. A compact polylobular kidney is observed in the 

 pig, kangaroos, some carnivores, and some primates (man). 



The variations of kidney structure are interesting when 

 viewed in taxonomic terms. For example, among small 

 carnivores, the weasel has a papilla, while the marten has a 

 ridge projecting into the pelvis. The lion, cat, and lynx have 

 a single papilla, while the tiger has a ridge. A specimen of 

 Hyrax was observed in which there was a single papilla in 



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