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534 
THE SCIENTIFIC MONTHLY. 
The embryo bat resembles the ordinary small animal (mammal), the long Un¬ 
gers and persistence of the web between them, and its continuation from the border 
of the body and tail, are features of later appearance. 
In conclusion we will say that bats bring forth their young alive and nurse 
them at the breast by milk; have red blood corpuscles containing no nucelous ; 
are clothed with hair, and possess a corpus collosum, that is, a band of fibers con-^ 
necting the two cerebral hemispheres. The ovum or egg is very small. 
Bats are all included in the sub-order cheiroptera (handwinged). Bats are . 
generally nocturnal animals, spending the day in caves and hollow trees. They ■ 
are found in all portions of the world except the very coldest, but are most.j 
numerous and of the greatest size in the tropics. Those of the temperate regions 
of the globe generally spend the winter in a state of torpidity, in which, although 
circulation continues very languidly, respiration does not ordinarily take place. 
The bats of the temperate climates are all small, the largest British bat being only 
fifteen inches in stretch of wings, and its body not so large as a mouse. The 
United States bats are all of small *size, the largest being only fourteen inches 
stretch of wing. But in the tropics bats are of great size, the kalong being five 
feet stretch of wing. Many bats have a peculiar leaf-like appendage attached to 
the nose, resembling a horseshoe. In others the ears are wonderfully large. 
Some of the species are gregarious; others are found only singly or in pairs. 
Twilight is that light that remains after the sun has set, or which is seen just 
before sunrise. The duration of twilight is, in many respects, an interesting and 
useful element to be acquainted with. It depends in a great measure upon the 
angle to which the sun has descended below the horizon, but it is modified by 
several circumstances, the chief among which is the degree of cleanness of the 
atmosphere. The direct light of the sun at the time of sunset reaches to the 
West; as the sun sinks its boundry line rises, and some little time afterwards 
crosses the Zenith, when civil twilight ends; the planets and large stars then be¬ 
come visible to the naked eye. The eastern half of the sky being thus first de¬ 
prived of direct solar light, night begins there. Afterward, the boundry line (the 
crepuscular curve) itself disappears in the west; then the astronomical twilight 
ceases and night has fully set in. Twilight begins or ends.when the sun is at a 
certain distance below the horizon; this distance is variable, depending upon the 
state of the atmosphere. It may be taken that civil twilight ends when the sun is 
about 8° below the horizon, and that astronomical twilight ends when the sun is 
18° below the horizon. The phenomena of twilight are hardly known in tropical 
climates; as soon as the sun has descended below the horizon darkness sets in 
suddenly. This was remarked by Bruce at Seng'al, where, however, the air is 
BOTAN ICAI 
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