164 PHYLUM PORIFERA — SPONGES 



Class II. — Hexactinellida, or Triaxonida, with sexradiate siliceous 

 spicules (triaxons). The members live chiefly in deep water, 

 e.g. Venus Flower-Basket (Euplectella) and the Glass-Rope 

 Sponge (Hyalonema). 

 Class III. — Demospongi^. 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, 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 {Euspongia). 

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

 and Oscarella. 

 A very remarkable form called Merita 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 Triaxonida and Tetraxonida. 



Ecology. — Sponges are living thickets in which many 

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

 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-sub- 

 stance. 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 carefully into pockets in the substance of a siliceous 



