130 PHYLUM FOR I PER A SPONGES. 



spiculcs (triaxons). The members live chiefly in deep water, e.g. 

 Venus Flower-Basket (Enplectella'] and the Glass-Rope Sponge 

 (Hvalonema). 



Class III. Demospongiae. 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. Pachy- 



matisma^ Tetilla. 



Grade II. Monaxonida, with monaxon spicules, sometimes with 



spongin in addition, e.g. Mermaid's Gloves (Chalina oculata), 



Crumb-of-Bread Sponge (Halichondria or Amorphina panicea], 



Fresh-Water Sponge (Spongilla). 



Grade III. Ceratosa, "horny" sponges with or without spicules, 



e.g. the Bath -Sponge (Enspongia}. 

 Grade IV. Myxospongida, without any skeleton, e.g. Halisarca 



and Oscarella. 



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. 



Bionomics. Sponges are living thickets in which many 

 small animals play hide-and-seek. Many of the associations 

 are practically constant and 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 Clione 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 eats into the 

 shell-substance. Within several sponges minute Algae live, 

 like the " yellow cells " of Radiolarians, in mutual partner- 

 ship or symbiosis. 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, 



