(^ MAMMALS. PHYLUM CHORDATA 



remain separate. There are generally five digits on each limb and 

 the animal walks with the palms and soles on the ground, and 

 so is called plantigrade. The brain is of very reptilian type, with 

 large olfactory lobes, and the specially mammalian features of 

 corpus callosum and neopalhum poorly developed. Other 

 primitive features are found in the reproductive system ; there 

 is often a cloaca, the uterus is primitive and often duplex, that 

 is. the right and left tubes are separate, and join only in a median 

 vagina, and the testes, although they shift backwards from their 

 original position, do not descend into a proper scrotum. Another 

 link with the reptiles is found in the fact that the constant body 

 temperature is not permanently established, since many insecti- 

 vores hibernate and in so doing become cold-blooded. 



The insectivores, however, are not mere primitive mammals, 

 for nearly all of them show specialisations of one sort or another. 

 Although they are all small and retiring in habits, and mostly 

 nocturnal, they have exploited their environment in various ways. 

 The hedgehog [Erinacetis europcBus), with its spines and habit 

 of rolling into a ball when touched, has achieved a high degree 

 of freedom from enemies ; it has also pecuhar teeth, for the first 

 incisors are long and ' caniniform ', that is, they resemble the 

 canine of a dog. The mole (Talpa europcBo) is highly specialised 

 for a digging or fossorial Ufe. Its fore -limbs are greatly 

 strengthened and the palms are turned backwards, so that they 

 have become paddles for digging ; the eyes are small and almost 

 or perhaps quite functionless, the pinnae are vestigial, and the fur 

 is short and lies equally well in all directions. The remaining 

 British insectivores, the shrews (Soricidse), are less obviously 

 specialised, but they have an elongated snout, associated with 

 hook-hke first upper incisors and long and horizontal first lower 

 incisors, and they have lost the zygomatic arch. They are rather 

 more than insectivorous, since they kill and eat small rodents, 

 even those larger than themselves. They include the smallest 

 known mammals, the pigmy shrew, Sorex minuhis, of Britain 

 being one of the least. Its length, apart from the tail, does not 

 exceed sixty millimetres nor its weight six grammes. Because of 

 their large surface area in proportion to their volume their rate of 

 loss of heat is high relative to their ability to produce it, and they 

 must be continually feeding to supply food to be burnt. A fast 

 of a few hours is fatal, except when they are hibernating and 

 they have ceased to be warm-blooded. 



