MOLLUSKS 315 
In bivalves the eggs are retained within the parent shell until 
hatched, and the young, which then escape into the water, are very 
different-looking creatures from the parent. Generally the young 
bivalve is free-swimming and exceedingly lively in its move- 
ments; sometimes by means of cilia it is enabled to attach itself 
to any passing host, and thus the species is widely disseminated. 
The fry soon lose their embryonic form and take on the charac- 
teristics of the adult. 
The longevity of mollusks varies greatly with the species. Some 
attain an age of from ten to fifteen years, while some are only 
annuals. Oysters continue to grow for four or five years, after 
which they may survive many years more. In truth, little is 
known concerning the duration of life even of our better-known 
shore species. 
In point of intelligence, mollusks must be relegated to a very 
low position. They give evidence of possessing no more than the 
most primary instincts, those of self-preservation and of repro- 
duction. There are some forms that exercise a high degree of 
protective mimicry. The highest class of mollusks, the Cephalop- 
oda (cuttlefishes, octopi, etc.), is widely separated from the other 
classes in the development of faculties that appear to correspond 
with intelligence. They are exceedingly crafty in the pursuit of 
their prey as well as in eluding capture. 
STRUCTURE OF MOLLUSKS 
Among the invertebrate animals, especially in the lower orders, 
wide physiological departures from central or typical forms 
are frequent. It often happens that a whole group or even sub- 
order will differ so materially in its general characteristies from 
another group or suborder belonging to the same phylum that the 
student will be puzzled at first to understand how the two can be 
nearly related. In many such cases it is only when the essen- 
tial features of the phylum are thoroughly understood that the 
relationships of its suborders can be appreciated. An example 
of widely divergent groups within the same phylum has already 
been shown in the Echinodermata. Who would at first suppose 
that the graceful Astrophyton could bear kinship to the inert holo- 
