122 PORIFERA — SPONGES. 



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



trisenes, in which a main shaft bears at one end three 



branches diverging at equal angles, e.g. Geodia, Pachyma- 



tisma, Plakina. 



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



perhaps survivals of primitive types (Oscarella, Halisarca) or degraded 



forms (Chondrosia). 



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 

 domunculd) 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 AlgK live, 

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

 ship or symbiosis. Finally, sponges deserve mention as 

 factors in human civilisation. 



General zoological interest and position. — Sponges have 

 this great interest, that they form the first successful class of 

 Metazoa. They illustrate the beginnings of a " body," and 

 the beginnings of tissues. Along with the Ccelentera, from 

 which it is the almost unanimous opinion that they must 

 be held distinct, they differ markedly from the triploblastic, 

 Ccelomate Metazoa, which do not retain the radial symmetry 

 of the gastrula. 



Their origin is wrapped in obscurity, though there is 

 much to be said for the view that they are the non-pro- 

 gressive descendants of primitive gastrula-like ancestors of 



