ee 
pee tee a ee NR TE NET rp NENT 
aa = Sere ees 
= A Se 
eae tre ree eee? 
Fic, 32.—Spicules 
originating in 
single cells. 
After Sollas. 
I. From Stelletta. 
adult Craniella cranium. 
METAZOA 41 
diaphragms in the exhalent canals of the flagel- 
late chambers are believed to be muscular, and 
a nervous nature has been attributed to others 
situated near the inhalent pores. 
The skeleton of Grantia consists of cal- 
careous spicules, most of which are of a 
triradiate shape (6, 7, and 8, Fig. 29). Each 
spicule commences to appear in one of the 
mesodermal cells, but as it increases in size it 
may protrude from this. The spicules are said 
to gradually work towards the exterior of the 
sponge, and to be discharged as waste matter. 
The character of the skeleton is made use 
of in classification. Only a very few sponges— 
Halisarca, Oscarella, and Chondrosia—are devoid 
of any kind of supporting structure. Those 
which possess calcareous spicules have been 
grouped together as the Calcarea, and opposed 
to all the other sponges, or Non-calcarea. The 
skeletons of this latter group may consist of 
siliceous spicules, or of a fibrous substance— 
spongin. The siliceous, lke the calcareous 
spicules, originate in a single mesodermic cell 
(Fig. 32); both assume a great variety of size and 
shape, and the former may be articulated or 
fused to one another. The organic skeleton, 
found in the Order Cornacuspongiae, consists of 
spongin, a substance chemically allied to silk. 
It is secreted by a number of mesodermic cells 
termed spongoblasts, which form a layer all 
round the fibre, and a multicellular cap covering 
the ends (Fig. 34). The fibrous skeleton of Hu- 
spongia, devoid of spicules, and characterised 
by the regular arrangement of the network and 
the smallness of the meshes, forms the bath 
sponge of commerce. 
In Grantia some of the amoeboid mesodermic 
Ill. The same from 
IV. A four-rayed spicule from Theonella Swinhoei. 
II, From embryo of Craniella cranium. 
