50 TYPES OF LARVAE. 
by which it rotates freely about the centre. It represents 
; the typical centro-symmetric and 
ae aB monoblastic organism. The layer 
SECTION OF BLASTULA. oF cells is called the archib/ast and 
the internal cavity the archicele (or 
blastocele). As the organism is com- 
pletely centro-symmetric there can 
be no division of labour between the 
cells ; hence the blastula represents 
the phyletic stage of a colonial protozoan rather than a true 
metazoan. 
2. MoruLta —The morula differs 
from the blastula in having the in- 
ternal cavity filled up with cells, 
thus forming a solid ball or mul- 
berry-mass, It is difficult to imagine 
a living adult organism like a morula, 
and it is probably a ccenogenetic 
larva. 
Fig. 17. 
SECTION OF MoRULA. 
2. Diploblastic Larve. 
3. GASTRULA.—This is possibly the most widespread 
and important larval form. It is typically of a “bell” shape, 
varying from a ‘‘cup” to nearly a sphere or cylinder. Like 
Fic. 18 the blastula it is usually a free-swim- 
: ae ming marine larva. It has two layers 
SECTION OF GASTRULA. OF calls—the outer, termed the ¢fv- 
: blast, and the inner, the Aypod/ast. 
The internal cavity is termed the 
archenteron, and its opening to 
the exterior is called the 4lastopore. 
The epiblast cells are usually cili- 
ca ated, and the larva is free-swimming, 
astopore- with motion in a spiral direction 
along the long axis through the blastopore. The gastrula 
is the typical diploblastic axo-symmetric larva, with physio- 
logical division of labour between the epiblast and hypoblast, 
the latter being specially concerned with the function of ali- 
mentation, the former with those of locomotion, sensation, 
and excretion. Its body-plan is much the same as that of 
living Calenterata. The gastrula is produced from the 
OWNED, 
ELLA 
te 
oun 
08 
Sa 
S 
D> 
> 
a, 
® 
tl 
& 
EX 
