132 PHYLUM PORIFERA—SPONGES. 
(a) In the small calcareous sponge Sycandra raphanus (Fig. 61), as 
described by F. E. Schulze, the segmentation results in a hollow ball 
of cells—the J/astula. A few cells at the lower pole remain large, 
and are filled with nutritive granules; the other cells divide rapidly 
and become small, clear, columnar, and flagellate. The large granular 
cells become invaginated, forming what is called a “‘ pseado-gastrula.” 
This leaves the parent, and the cells forming the lower hemisphere 
of the embryo become rounded and 
non-flagellate. The embryo swims for 
a time actively, but the flagellate cells 
of the upper hemisphere are invaginated 
into or overgrown by the large granular 
cells, and thus what is generally called 
the gastruda stage results. This soon 
settles down, on rock or seaweed, with 
the blastopore or gastrula mouth down- 
wards, and is moored by amceboid 
processes from the granular cells, which 
likewise obliterate the blastopore. The 
granular cells lose their granules, for 
the larva is not yet feeding; the now 
internal flagella disappear in the absence 
of the stimulating water; a mesogloea 
with spicules begins to be formed 
between the inner and outer layer, 
probably by migrants from the latter. 
But this disadvantageous state of affairs 
cannot last. Pores open through the 
walls, the entrance of water enables the 
inner cells to récover their flagella, and 
an exhalant aperture is ruptured at the 
upper pole. 
Fic. 62.—Diagrammatic re- 
presentation of development 
of Oscarella lobularis. — 
After Heider. 
Bl, Free-swimming blastula with 
flagella; G., gastrula settled 
down. 
Next figure shows folding of inner 
layer (Fx.); £c., outer layer. 
Lowest figure shows radial cham- 
bers (2.C.); Mesoglea (AZg.); 
inhalant pore (/.); exhalant 
osculum (0.). 
hemispherical gastrula, which settles mouth downwards. 
The young sponge is now 
in an Ascon stage, from which, by 
the outgrowth of the inner layer into 
radial chambers, it passes into the 
permanent Sycon form, grows into a 
cylinder, and becomes differentiated in 
detail (Fig. 61). 
(6) In Oscarella (Halisarca) lobularis 
(Fig. 62), a sponge without any skeleton, 
the ovum segments equally into a 
blastula, which is flagellate all over. 
This free-swimming stage may’ be in- 
vaginated from either pole to form a 
Pores, an 
osculum, and the mesogloea are formed as before, and the inner layer 
becomes folded into flagellate chambers. 
The main features of sponge embryology are thus summarised by 
Minchin :— 
“I, The larva is composed of three classes of cell-elements: (1) 
Columnar flagellated cells, forming the outer covering or localised at 
