E CHINODERMS. 
3iS 
illustration on p. 317, in which the letters have the same meaning. This larval 
form is called a Pluteus, on account of its frequent changes of shape, as it swims 
about with its arms constantly moving. It will be noticed that through the whole 
of its development it retains a two-sided symmetry, such that if cut down the 
middle it would be divided into two precisely similar halves. This is very different 
from the five-rayed symmetry of the 
sea-urchin, and the difficulties arise 
both in this class and in the others 
when we try to discover how the 
five-rayed form was produced from 
the two-sided one. 
From this larva only the stomach 
and the water-vascular system are 
continued into the sea-urchin, whose 
prickly body is now being formed 
around the stomach of the larva; 
and it is in just those two systems, 
especially in the madreporite and in 
the intestine, that we note in the 
adult the traces of the primitive 
bilateral symmetry. When the little 
body of the sea-urchin, which at first 
is like a flat box, has become pro¬ 
vided with a mouth of its own, and 
with a circlet of comparatively large 
spines, then the parts not necessary 
to the new structure disappear. The 
calcareous skeleton of the larva is 
absorbed, and the lime salts thus set 
free help to build up the test of the 
sea-urchin. The arms sink in, and 
at last the outer larva remains as 
nothing more than a skin over the 
test of the urchin. The mode of life 
of the little sea-urchin, about one 
millimetre in diameter, is now com¬ 
pletely altered. It is no longer 
carried about through the water, but crawls by means of its tube-feet and its spines, 
as shown in the above illustration. AVe cannot here follow the further changes 
that it undergoes; but a study of those later stages is of great importance. For 
by means of such study Agassiz has shown that many supposed genera are nothing 
more than undeveloped forms of well-known species, and he has thus been able 
to work out the relations of species and genera to one another. It is not, however, 
all eehinoderms that pass through these curious larval stages, for in many species 
the young are developed in the shelter of the mother. AVe have already seen this 
to be the case with many brittle-stars, which are protected in the so-called genital 
young sea-urchin [Strongylocentrotus). a, From below ; 
6. From above (enlarged 20 times). 
