250 



CLASS MAMMALIA (MAMMALS). 



State briefly the General Features of Mammals. 



Hairs always present. A projecting external ear-flap, 

 the pinna. Skin has sebaceous and sweat glands. The 

 mammary (milk-producing] glands are specialised sebaceous 

 glands. Chest and abdominal cavities separated by a 

 complete muscular septum, the diaphragm. Vertebral 

 centra and long bones generally have epiphyses (later 

 terminal ossifications). Typically seven cervical vertebrae. 

 Sutures between skull bones usually persistent. Two 

 (exoccipital) condyles. A tympanic bone around outer ear. 

 Three ear ossicles (malleus, incus, stapes}. Otic bones 

 (auditory capsule) fuse to form a periotic, which is fused with 

 squamosal. Internasal septum produced in front of nasal 

 bones, forming the cartilaginous support of a flexible nose 

 or muzzle. Lower jaw consists (each ramus) of a single bone, 

 and articulates on squamosal (quadrate absent). Teeth (in 

 sockets or alveoli) on premaxillae, maxillae, and mandible ; 

 and typically heterodont (i.e., different in form and function) 

 and diphyodont (i.e., two functional sets, a milk or deciduous 

 set replaced later by a permanent set). Sternum has 

 distinct segments. Coracoid rudimentary, a small hook-like 

 process of scapula.* Pubes, and also ischia, meet ventrally ; 

 acetabulum completely ossified (except in Echidna). Large 

 and often convoluted cerebral hemispheres connected by a 

 broad commissure (corpus callosum)f, and covering the 

 thalamencephalon and the four optic lobes (corpora 

 quadrigemina ; in higher forms also covering cerebellum, 

 which has one median and two lateral lobes. Twelve 

 pairs of cranial nerves. Lungs, within pleural sacs, largely 

 free. A complex larynx with vocal cords. Heart has two 

 auricles and two ventricles. Only one, the left aortic arch 

 is continued into the dorsal aorta. Without a ronal portal 

 system. Warm-blooded. Ureters open into bladder,* and 

 its outlet forms a urinogenital sinus (urethra of male, 

 vestibule of female), into which the genital ducts open, 

 and which is separated from the anus by a space, the 

 perinaeum.* Testes desoend into scrotal sacs situated 

 (except in Marsupials) behind penis, which is traversed by 

 urethra. Ovaries small. The uterine portions of the 

 oviducts are frequently united, or partly united (bicornuate 

 uterus) ; the vaginal portions are completely fused, f 



Ovum, very small,* contains little yolk* (segmentation 

 holoblastic), and develops within uterus (womb).* Embryo 

 has two foetal membranes (amnion and allantois) ; those, 



