DE VEL OEMENT 
3 1 5 
themselves can, in many cases, grow fresh bodies, and become complete individuals. 
A star-fish of the genus Linckia commonly avails itself of this faculty; and it is 
by no means rare to find big arms with a small body at one end, and four little 
arms growing out of it; these are known as comet-forms. This power of repro¬ 
duction is probably due to the extension of all the systems of the body into the 
arms: the arms of brittle-stars, which do not contain all the systems of the body, 
have not been known to reproduce individuals. In some cases echinoderms have 
been seen definitely to reproduce themselves by fission or splitting in half. Such 
division is well known to take place 
by some to take place even in brittle- 
stars ; the specimens of the six¬ 
armed green snake-star ( Ophiactis ), 
shown in the annexed illustration, 
being thought to represent the 
result of such a process. The 
specimen A consists of two almost 
similar halves; but the three arms 
towards the bottom of the illustra¬ 
tion, marked a, are smaller than the 
others marked a, and indicate that 
this half is the later grown. The 
specimen B, which is seen from the 
back, has only just separated itself 
from its other half. The separation 
appears to take place by a forcible 
though spontaneous rent, and the 
edges of the wound subsequently 
grow together, and not merely heal 
up but reproduce the lost parts of 
the animal. As a rule, however, 
echinoderms reproduce by the 
ordinary sexual methods; although 
this, too, presents peculiarities. Just 
as a butterfly does not develop 
directly from the egg, but passes 
through the intermediate larval stage of the caterpillar, out of whose chrysalis the 
butterfly springs, so the sea-urchin or the star-fish egg gives rise to a larval form, 
in whose body, as it were, the mature form is developed. The particular shape of 
the larva varies in the different classes of echinoderms; but the differences are not 
essential, and it is clear that all the larval forms are modifications from one primitive 
type. The changes passed through in the development of the common sea-urchin 
{Strongylocentrotus droebachiensis) are depicted in the illustration on p. 318, in 
which the drawings are very greatly magnified. 
The fertilised egg divides and subdivides until a round ball of cells is formed. 
This is then pushed in at one end, as one might push in a soft indiarubber ball, 
so that there is formed a little sac with a double wall to it (stages 1, 2). Stage 3 
among the sea-cucumbers; and it is believed 
green snake-star (enlarged 5 times). 
