GENERAL STUDY OF TYPICAL CHORDATES 29 



4. Perineal region. — In mammals the region which includes both the anal 

 and urogenital openings is designated the perineum. The anus is situated in the 

 ventral median line just in front of the base of the tail. On each side of the anus 

 in the rabbit is a deep, hairless depression, the inguinal or perineal space, onto 

 which open the inguinal glands (not visible externally) whose secretion produces 

 the odor characteristic of the animal. In the female, the urogenital opening is 

 situated immediately anterior and ventral to the anus; it is similar in appearance 

 to the anus. It is inclosed by a fold of skin extending around the rim; this fold 

 is named the greater lips or labia majora. The labia together with the urogenital 

 aperture constitute the vulva. In the male rabbit there is a hillock in front of 

 the anus. In the center of this is usually visible a pointed projection, the end 

 of the penis or organ of copulation; the penis bears at its tip an opening, the 

 urogenital aperture. The hillock of skin which folds up about the penis is named 

 the prepuce or foreskin. To each side of the prepuce and extending forward is 

 an oval swelling, caused by the male gonad, or testis, which is inclosed within it. 

 The double pouch of the body wall which incloses the two testes is named the 

 scrotum or scrotal sac. In the male cat there is a pair of rounded eminences 

 anterior to the anus; each eminence contains a testis or male gonad, and the 

 double pouch of the body wall inclosing the testes is named the scrotum or scrotal 

 sac. Anterior to the scrotum is a hillock of skin, the prepuce or foreskin. In 

 the center of the prepuce is an opening, which is not, however, as might be sup- 

 posed, the urogenital opening. It is merely the depression left by the withdrawal 

 into the prepuce of the penis; the penis is generally so far withdrawn in male 

 cats as to be invisible externally. 



N. SUMMARY 



Our study of the external features of representative vertebrates may be utilized to direct 

 the student's attention to the following points. 



1. The vertebrate body is typically fusiform, that is, moderately thick through the trunk, 

 tapering toward each end. In every group of vertebrates forms may be found which deviate 

 from this typical shape, but such deviations bear no relation to the position of the animal in 

 the vertebrate scale, being rather adaptations to particular modes of life. 



2. The body is divided into head, trunk, and tail in the lowest vertebrates. A neck is 

 added in land vertebrates. 



3. The head tends to increase relatively in size and the tail to decrease as one ascends the 

 vertebrate series. The former change is associated with increase in the size of the brain; the 

 latter with greater speed and agility of movement. 



4. The skin of vertebrates is commonly clothed with protective structures, such as scales, 

 feathers, or hairs. These are more complex in structure in the higher vertebrates and better 

 fitted for keeping the body warm. 



5. The head throughout bears three pairs of sense organs. Of these the eyes undergo 

 little change throughout the series. The olfactory sacs are blind in fishes and open to 

 the exterior only, by means of the external nares. As soon, however, as vertebrates left the 

 aquatic environment, internal nares were developed, connecting the olfactory sacs with the 



