5. And these passages, both in fish and other animals, are joined to the loins below the stomach and between the entrails and the great vein, from which passages proceed to each of the kidneys; and, as in fish, the semen may be seen entering them at the period of coition, when these passages become very conspicuous, but when this season is passed the passages again become invisible. So also the testicles of birds are either small or entirely invisible when not excited, but when urged by desire they become very large; this is so remarkable in pigeons and partridges, that some persons have supposed that they had no testicles during winter. 6. In some of those animals in which the testicles are placed forwards, they are internal and upon the abdomen, as in the dolphin; in others they are externally conspicuous upon the extremity of the abdomen. These animals are similar in other respects, but differ in this, for in some the testicles are uncovered, and others that have external testes they are placed in a scrotum. 7. This is the nature of the testicles of all viviparous animals with feet: from the aorta, passages like veins proceed to the head of each testicle, and two others from the kidneys, these last are full of blood, but those from the aorta contain no blood. From the head of each testicle to the testicle itself, there proceeds a thicker and more muscular passage, which is in each testicle reflected back to the head of the testicle, and from this point they again unite upon the penis towards the fore-part of it. 8. And both these passages which are reflected back upon themselves, and those which are seated upon the testicles, are covered with the same membrane as the testes themselves, so that unless this membrane is taken away, they all appear to be one passage. These last passages, which are seated upon the testicle, contain sanguineous fluid, but less than those above from the aorta; but in the reflected passages of the duct which is upon the penis, the fluid is white. A passage also leads from the bladder, and is united to the upper part of this duct, which is enclosed in the part called the penis as in a husk. The accompanying diagram will illustrate the position of these parts. 9. The origin of the passage from the trachea, _a_; the head of the testes and the descending passages, _b b_; the passages which proceed from these, and are seated upon the testicle, _c c_; the reflexed passages which contain the white fluid, _d d_; the penis, _e_; the bladder _f_; the testicles, _g g_. But when the testicles are cut out or otherwise destroyed, the upper passages are retracted; in young animals castration is performed by bruising the testicles, in older animals by excision. And it has happened that a bull has begotten young if admitted to the female immediately after castration. This is the nature of the testicles of animals.