MOLLUSKS 87 
twelve to thirty years; and the average length of life of 
the snail appears to be from two to five years. Certain it 
is that mollusks have numerous enemies besides man which 
prevent multitudes from living lives of normal length. 
Birds, fishes, frogs, and starfishes beset them continually, 
and many fall a prey to the ravages of internal parasites or 
to other mollusks. Under ordinary circumstances the shell 
is sufficient protection, and the spines disposed on the sur- 
face in many species render the occupant still less liable to 
attack. Many snails carry on the foot a horny or calcare- 
ous plate known as the operculum, which closes the en- 
trance of the shell like a door against intruders. Certain 
noxious seeretions poured out from the skin also serve as a 
means of defense, and many Nudibranchs (Fig. 50) bear 
nettle-cells on the processes of the body, which probably 
render them distasteful to many animals. Finally, there 
are numerous clams, mussels, snails, and slugs whose colors 
harmonize so closely with their surroundings that they al- 
most completely baffle detection, and enable them to lead 
as successful a life as those provided with special organs of 
defense. 
90. Cephalopods.—The animals belonging to this class, 
such as the squids and cuttlefishes (Fig. 52), are by far 
the most highly developed mollusks. They are of great 
strength, capable of very rapid movements, and several spe- 
cies are many times the largest invertebrates. In almost 
every case there is a well-defined head bearing remarkably 
perfect eyes, and also a circle of powerful arms provided 
with numerous suckers which aid in the capture of food 
(Fig. 52). Posteriorly the body is developed into a pointed 
or rounded visceral mass which to a certain extent is free 
from the head, giving rise to a well-marked neck. Some 
forms, such as the squids (Fig. 52, upper figure), are pro- 
vided with fins which drive the animal forward, but in com- 
mon with other cephalopods they are capable of a very rapid 
backward motion. By muscular movements water is taken 
7 
