764 
The An 
Naturalist. 
properly speaking, a new name for a larval form called Planula 
by older writers ; but the term is now generally employed to de- 
signate an ideal embryonic stage supposed to be common to all 
multicellular animals. 
The blastula changes into a gastrula by a process of invagina- 
tion. The entodermal area of the blastula flattens out, the ecto- 
derm meanwhile expanding by multiplication of its cells ; after 
flattening, the entoderm turns inward, forming at first a shallow 
cup, then a pit which has an opening or mouth, the rim of which 
is the ectental line. The larva is now a double sack, and has an 
external wall or ectoderm, and an internal wall or entoderm ; the 
entodermic cavity is entirely distinct from the segmentation cavity. 
The process of gastrulation is here described as it occurs among 
the lower invertebrates. 
Typical gastrulae are the free swimming larva; of many marine 
invertebrates. We may 
take as an example that 
of a sea-urchin, (Fig. 19)- 
The larva is round ; at 
one pole it has an open- 
ing, M, the gastrula 
mouth leading into an 
internal cavity; as this 
is a free swimming larva 
it is provided with long 
cilia for organs of loco- 
motion ; the cilia in many 
gastrulas are distributed 
over limited areas or they 
may be wanting alto- 
gether. The larva con- 
sists of a double sack,— a larger outer one of small epithelial cells, 
ec, the ectoderm; and a much smaller inner sack composed of 
larger entodermal epithelial cells, en ; at the mouth, M, of the inner 
sack the two layers are continuous with one another; in the space 
etween the two sacks, which corresponds to the segmentation 
