BIONOMICS. 125 



B. Poi-ifera non-Calcarea, skeleton of silica or of spongin, or of both, 

 (i) Hexactinellida, with sexradiate siliceous spicules, canal 

 system usually simple, with Sycon chambers. The 

 members live chiefly in deep water, e.s;., Venus' 

 Flower Basket {Etiplectella) and the Glass Rope 

 Sponge (Hyaloneiua). 



(2) Monaxonida, with siliceous spicules (which are not quadri- or 



sex-radiate), or with "horny" skeleton, or with 

 both. 

 Order I, — Monaxona, with spicules only, e.g.. Mermaid's 

 Gloves [Chalina ocidata). Crumb of Bread 

 Sponge (Halichondria or Amorphina panicea). 

 Freshwater Sponge (Spongilla). 

 Order II. — Ceratosa, " horny " sponges with or without 

 spicules, e.g., the Bath Sponge {Euspongia). 



(3) Tetractinellida, mostly with quadriradiate spicules, or with 



triienes, in which a main shaft bears at one end 

 three branches diverging at equal angles, e.g., 

 I'etilla, Geodia, Pachyniatisma, Plakina. 



There are also a few sponges (MyxospongiLx;) without any skeleton, 

 perhaps survivals of primitive types (Oscarelhi, Halisarcd) or degraded 

 forms {Choiidrosia). 



History. — Sponges, as one would expect, date back almost to the 

 beginning of the geological record. Thus the siliceous Protospotigia 

 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 

 domunadd) 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 AlgcB live, 

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



