CLASSIFICATION. 133 
the anterior pole ; (2) rounded, more or less amoeboid elements, rarely 
flagellated, forming the inner mass or aggregated at the posterior 
pole ; and (3) the archzeocytes, usually scattered in the inner mass, 
and often represented by undifferentiated blastomeres... . 
“TI, The larva fixes and undergoes a metamorphosis whereby the 
flagellated cells become placed in the interior, while the cells of the 
inner mass come to surround them completely. 
“JIT. (1) The flagellated cells of the larva become the choanocytes 
of the adult (gastral layer), acquiring a collar; . . . (2) the inner mass 
_gives rise to the dermal layer in its entirety; . . . (3) the archzeocytes 
become the wandering cells of the adult, from which the reproductive 
cells arise.” 
It is interesting to note that the primitive germ-cells are early set 
apart. 
Classification. __ 
Class I.—Catcarea. With skeleton of calcareous spicules :— 
Grade I.—Homoccela. — Continuous’ internal layer of collared 
flagellate cells, e.g. Ascetta, Leucosolenia. 
Grade II.—Heteroccela.—Collared flagellate cells restricted to 
radial tubes or chambers, ¢.g. Sycou (Grantia), 
Class II.—HEXACTINELLIDA, or Triaxonia, with sexradiate siliceous 
spicules (triaxons). The members live chiefly in deep water, 
e.g. Wenus Flower-Basket (Zzp/ectella) and the Glass-Rope 
Sponge (Ayalonema). 
Class III. —Demospongiz. Skeleton of siliceous spicules, but never 
triaxons, or of spongin fibres, or of spongin fibres and siliceous 
spicules, or absent. 
Grade I.—Tetraxonida, typically with tetraxon spicules, e.g. 
Pachymatisma, Tetzlla. 
Grade II.—Monaxonida, with monaxon spicules, sometimes with 
spongin in addition, ¢.g. Mermaid’s Gloves (Cha/ina oculata), 
Crumb-of-Bread Sponge (Halzchondria or Amorphina pantcea), 
Fresh-Water Sponge (Spongzl/a). 
Grade III.—Ceratosa, ‘‘ horny” sponges with or without spicules, 
e.g. the Bath-Sponge (Zuspongia). 
Grade IV.—Myxospongida, without any skeleton, eg. Halisarca 
and Oscarella. 
A very remarkable form called /er/za seems to have both a siliceous 
and a calcareous skeleton. 
History.—Sponges, as one would expect, date back almost to the 
beginning of the geological record. Thus the siliceous Protospongia 
occurs in Cambrian rocks, and in the next series—the Silurian—the 
main groups are already represented. From that time till now they 
have continued to abound and vary. 
The division between calcareous and siliceous sponges goes deep 
down to the very roots of the phylum, and the siliceous branch must 
have divided very early into Triaxonia and Tetraxonia. 
Ccology.— Sponges are living thickets in which many 
small animals play hide-and-seek. Many of the associa- 
