228 TlMEHRI. 
yields excellent leather, while the flesh is good for food. 
An arftic form of the group has become extinft within the 
last century. 
Next follow cases of birds, but these will be referred 
to later. Above them is suspended a large alligator or 
cayman from the Essequebo. In the alligator the toes 
are incompletely webbed ; and the large, prominent, 
canine tooth in the under jaw is received into a hole in 
the upper jaw, and is not visible outside when the mouth 
is closed ; while in the crocodile the toes are completely 
webbed, and the canine tooth, when the mouth is closed, 
projects outside in a groove of the upper jaw (seen in 
the skull hung on the other side of the Museum.) 
Next follow cases of sponges and corals and polished 
shells. The sponges exhibited are of two kinds ; one in 
which, as in the ordinary washing-sponges, the skeleton 
consists of horny fibres, and the other in which it consists 
of siliceous or glassy matter. A third or calcareous form 
is not represented. During life, the framework is covered 
with a slimy, gelatinous matter continued all through 
the interlacings of the fibres, and this matter is made up 
of small units of flesh, or cells, provided with vibratile 
hairs, or cilia, which cause currents to traverse the body, 
and by means of which food is brought to the sponge. 
The sponges, though for a long time regarded as of a vege- 
table nature, are by no means the lowest forms of animals. 
They start life as a single cell or unit, and by repeated 
division form a bag-like strufture made up of two layers — 
the outer layer for sensation, the inner for digestive 
purposes. By repeated budding the sponge grows to a 
large size of very varied shape. A nervous system, 
developed from the outer layer, has lately been dis- 
