Variation Among Vertebrates
In cartilaginous and ray-finned fish it consists primarily of red pulp and is normally a somewhat elongated organ as it actually lies inside the serosal lining of the intestine. In many amphibians, especially frogs, it takes on the more rounded form and there is often a greater quantity of white pulp.
In reptiles, birds, and mammals, white pulp is always relatively plentiful, and in the latter two groups, the spleen is typically rounded, although it adjusts its shape somewhat to the arrangement of the surrounding organs. In the great majority of vertebrates, the spleen continues to produce red blood cells throughout life; it is only in mammals that this function is lost in the adult. Many mammals possess tiny spleen-like structures known as haemal nodes throughout the body, which, it is presumed, have the same function as the spleen proper. The spleens of aquatic mammals are in some ways dissimilar to those of fully land-dwelling mammals. In general, the spleens of aquatic mammals are bluish in colour. In cetaceans and manatees it tends to be quite small, but in deep diving pinnipeds it can be quite massive, owing to its function of storing red blood cells.
The only vertebrates lacking a spleen are the lampreys and hagfishes. Even in these animals, there is a diffuse layer of haematopoeitic tissue within the gut wall, which has a similar structure to red pulp, and is presumed to be homologous with the spleen of higher vertebrates.
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