ORGANIC DEPENDENCE AND DISEASE 



57 



are silicious sponges allied to the living Euplectella or 

 Venus 's Flower-basket, and though we find no parallel ex- 

 pression of comniensalism in the living glass-sponges, yet 

 Euplectella carries a parasite in the form of a crustacean 



which in youth enters its 

 open cloacal cavity and re- 

 mains there so that when the 

 sponge has in adult growth 

 built the terminal or sieve- 

 plate over its aperture the 

 crustacean is wholly and 

 permanently caged. 



This ancient association 

 continued long after the 

 Paleozoic, for I have else- 

 where illustrated its occur- 

 rence in the sponges of the 

 Cretaceous from very strik- 

 ing specimens sent to me by 

 Dr. F. A. Bather of the Brit- 

 ish Museum. They are here 

 reprinted. All show a spiral 

 worm tube encircling the 

 long median cloaca of the 

 sponge, in one the spiral be- 

 ing dextral and the other 



Fig. 43. A silicified spoDge from the 

 Englisli Chalk exposing a spiral 

 worm tube encircling the wall of 

 the cloaca of the sponge. (Courtesy 

 of Dr. F. A. Bather. ) 



sinistral. The flat section 

 here shown is a direct photo- 

 graphic print made from a 

 thin slide and shows the 

 actual distance of the annelid tube from the cloaca, and 

 suggests also the presence of other commensal worms in 

 the upper left-hand part of the sponge-body (prepared by 

 Doctor Bather). All are from the Chalk series at Beck- 

 hampton. 



