96 
kqom xi. the sheaths is narrow, and they arise from a 
creeping fibre. In the Mamellifera the sheaths 
are placed on a membranaceous expansion, and 
in the Corticifera the sheaths are covered with a 
sandy coat, and are united side by side, so as 
to form a kind of crust on the marine bodies 
to which they are attached. 
The major part of the animals of this Class are 
surrounded by a stony covering or coral, into 
which they can withdraw themselves from ex¬ 
ternal danger. This coral is always solid and 
calcareous, and the cells which are inhabited by 
the animals are furnished with more or less 
distinct longitudinal lamellae, placed in a radi¬ 
ating position round the central axis, so as to 
give the cavity a star like appearance. These 
corals are most of them attached to marine 
bodies during the whole of their growth. Some 
few kinds, as the Sea Mushroom ( Fungia ), and 
Sea Slug ( Polyphyllia ), are attached when they 
are young by a short stem, but the crown sepa¬ 
rates from the stem by a natural absorption as the 
coral enlarges, leaving it at length quite free. 
The reefs and islands, which are constantly 
and gradually rising above the sea, especially 
in the Pacific Ocean, are formed by animals 
inhabiting these kinds of coral. 
Corals, on account of the different forms 
which they present, have been divided into 
several 
