METHODS OF PROTECTION AMONG ANIMALS. Toe 
but most spiders have soft, flexible surfaces on the under and 
a harder chitinous covering on the upper part. Scorpions 
have a chitinous shell all over the body. In spiders the 
segments which represent the antenne of insects are very 
efficient pincers, used for prehension. Scorpions have still 
more formidable nipping-claws, and have the power of 
stinging their prey by means of the tail, which is hooked 
and has two poison-glands with minute canals opening into 
the tips. The spider’s web must ever be borne in mind as a 
wonderful and beautiful method of protection devised for 
the double purpose of protection and supply of the animal’s 
needs. Its formation and origin need not be here described. 
Centipedes and millipedes also possess a chitinous 
covering and glands in the integuments which secrete an 
acrid fluid for protection. 
The remaining group of Invertebrates or non-chordate 
animals is the large sub-kingdom of Mollusca or soft-bodied 
animals. For us the interest centres on the shells, which 
almost all possess. They are aquatic, inhabiting sea and 
fresh water, and terrestrial. It is computed that 50,000 
species of the former and less than half that number of the 
latter have been identified. The vast majority of Mollusca 
have shells consisting either of one piece shaped after 
diverse patterns, or of two valves, thus constituting the two 
main divisions of Mollusca, univalve or gastropod molluscs, 
and bivalve or lamellibranch molluscs—e.g., snails on the one 
hand and oysters on the other. The shell is in nearly all 
composed of calcareous matter mixed with a small amount 
of animal matter, and is formed by the outer layer of the 
“mantle,” so called. This shell is essential to the life of the 
animal, and it cannot, in the convenient manner mentioned 
among Crustacea, shed its coat and form a fresh one. 
Injuries to the shell can be repaired, but no new one has 
ever been known to be produced. Shells are described as 
porcellanous from their dense white structure, horny, fibrous, 
or nacreous, such as those of the-mother-of-pearl. In addition 
to the ordinary protection of the hard shell, there is in many 
a further protection of the shell itself. This “overcoat” of 
the molluscan shell is called the ‘‘periostracum,” and is a 
tough, smooth coating laid over the calcareous surface, 
efficiently protecting fresh water shells, in particular, from 
the eroding chemical action of the water, in which carbonic 
acid gas is dissolved. A past generation of men thought the 
discover y of copper coatings for the bottoms of ships a great 
