134 PHYLUM PORIFERA—SPONGES. 
tions are harmless, but some burrowing worms do the 
sponges much damage. The spicules and a frequently 
strong taste or odour doubtless save sponges from being 
more molested than they are; the numerous phagocytes 
wage successful war with intruding micro-organisms. Some 
sponges, such as C/iona on oyster-shells, are borers, and 
others smother forms of life as passive as themselves. 
Several crabs, such as Dromia, are masked by growths of 
sponge on their shells, and the free transport is doubtless 
advantageous to the sponge till the crab casts its shell. 
A compact orange-coloured sponge (Suderites domuncula) 
of peculiar odour often grows round a whelk-shell tenanted 
by a hermit-crab, and gradually dissolves the shell-substance. 
Within several sponges minute Algz live, like the “yellow 
cells” of Radiolarians, in mutual partnership or symbiosis. 
One of the cuttlefishes, Ross¢a glaucopis, puts its eggs care- 
fully into pockets in the substance of a siliceous sponge. 
Finally, sponges deserve mention as factors in human 
civilisation. 
General zoological position. — Sponges form the first 
successful class of Metazoa. They illustrate the beginnings 
of a “body,” and the beginnings of tissues. Along with 
the Ccelentera, they differ markedly from the triploblastic, 
Ccelomate Metazoa, which do not retain the radial 
symmetry of the gastrula. In their germinal layers and 
in their internal cavity they differ so much from Ccelentera 
and all other Metazoa, that they must be regarded as on 
a by-road of evolution. This has been emphasised by 
Professor Ray Lankester in the term ‘‘ Parazoa” ; he speaks 
of them as a sterile stock. 
Their origin is wrapped in obscurity; it may be that 
they are the non-progressive descendants of primitive 
gastrula-like ancestors with a sluggish constitution. The 
presence of choanocytes suggests a relationship with certain 
of the flagellate Protozoa (Choanoflagellata), and Protero- 
spongia (Fig. 55) may possibly be regarded as a connecting 
link. 
InceRT& SEpDis. MEsozoA 
The title Mesozoa was applied by Van Beneden to some simple 
orginisms which appear to occupy a very humble position in the 
