/34 PHYL UM 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 Cliona 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 (Suberites 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 Algae live, like the " yellow 

 cells " of Radiolarians, in mutual partnership or symbiosis. 

 One of the cuttlefishes, Rossia 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. 



INCERTVE SEDIS. MESOZOA 



The title Mesozoa was applied by Van Beneden to some simple 

 organisms which appear to occupy a very humble position in the 



