INTRODUCTION 21 
functions. In higher forms, however, the body is composed of an 
organized system of cells wherein we see a division of labor, some 
of the cells being digestive, others muscular, others serving to con- 
duct nervous impulses, etc. 
In the great majority of marine animals the eggs or young 
larvee are cast out into the water,.and it is interesting to ob- 
serve that even such sedentary forms as clams, oysters, starfishes, 
sea anemones, corals, etc., are nearly all free-swimming in their 
early life. 
It has long been known to naturalists that, in their develop- 
ment, animals pass through stages which recall the adult states of 
their more simply organized ancestors of the remote past, and when 
it was discovered that the vast majority of marine larve are free- 
swimming, it seemed probable that the most ancient marine ani- 
mals were so, and that the sedentary habits of life seen in sponges, 
corals, mollusks, etc., were of comparatively recent origin. We must 
not forget, however, that the free-swimming habits of the young are 
of immense advantage in leading to a wide distribution of animals, 
and that it may therefore have been maintained through the agency 
of natural selection at all times, or have been brought about as a 
result of adaptation. 
The stages passed through in the development of all animals 
are so complex, and yet show such a striking similarity of plan 
that there can be no doubt of the blood relationship of all forms one 
to another. 
As this is a most interesting and important matter, we will 
devote some space to its consideration. 
We will first speak of a typical case of development, such as 
is seen in the common starfish, and will then consider the more 
complex conditions exhibited in the worms, crustaceans and 
mollusks. 
The immature eggs of the starfish resemble a minute sphere, in 
the centre of which we find a small round body called the nucleus. 
When the egg is cast out into the water this nucleus divides 
into two nuclei, both exactly like the original nucleus. One of 
these new nuclei is, however, soon detached from the egg as a minute 
ball, and is thrown out into the water. The nucleus which still 
remains in the egg then divides into two half-nuclei, and one of 
