484 A Sketch of Comparative Embryology. [July, 
from their external edges hyaline amoeboid processes, which 
probably help the larva to hold on (Fig. 18, B). The central 
ends of the cells approach one another, meet and close the mouth. 
Fig. 19 presents a side view of a larva in this stage, and shows 
the inner cavity ¢, 
now closed; its lining 
of small cells 4, and 
the exterior layer of 
granular cells a, the 
arrangement of which 
is particularly obscure, 
but they ultimately 
make the ectoderm 
¿and mesoderm. 
The development 
19.—Vertical optical section of the attached 
a of Sycandra see i us. After F. E. Schulze, now proceeds by the 
magnified about 500 d : s 
: vertical elongation of 
the sponge to a cylindrical shape; the formation of a large 
secondary opening, the osculum at the upper end, and of small 
openings, pores, around the sides, leading into secondary tubes, 
which communicate with the large central cavity; finally the 
development of the skeletal spicules and of the mesodermic 
intercellular substance. The first spicules that appear are simple 
rods tapering towards both ends, and slightly curved. They lie 
nearly parallel to the external surface, scattered irregularly. 
Three and four rayed spicules also soon appear, and the whole 
skeleton grows rapidly. The sponge is now in the Olynthus 
stage. 
The above account, though necessarily brief, shows that our 
present knowledge does not render the morphology of sponges 
explicable, because, although we should certainly consider, if we 
knew the larve alone, the small flagellate cells to be strictly 
homologous in all the embryos, yet in one case these cells form 
the internal digestive cavity, in another the external skin. At 
present the meaning of this divergence is unknown. 
The systematic position of the sponges has been much dis- 
cussed, At one time they were considered protozoic colonies, 
which they certainly are not. German zodlogists usually connect 
them with the Ccelenterata, but inasmuch as the development is 
not in the least ccelenterate, and the structure of the adult sponge 
