XXVll 



tion than contra6ling and dilating themfelves alternate- 

 ly, by which means they ejeft the water taken in, with 

 confiderable force. 



The Afcidia is rather a numerous genus : is found to 

 inhabit Pholas, Solen, fome of the Mya, MaSlra, and 

 probably part of other bivalve teftacea : many fpecies 

 are mollufca. 



It is probable this, as well as moft other aquatic ani- 

 mals poffefiTed of little or no locomotion, are androgy- 

 nous ; the power of felf-propagation moft likely extends 

 to all the inhabitants of multivalve and bivalve teftacea, 

 but to few, if any, of the univalve, except Serpula, or 

 fuch as are feflile. 



4 TETHYS. 



Body detached, rather oblong, flelhy, without pe- 

 duncles: mouth with a terminal, cylindrical probofcis, 

 under an expanded membrane or lip : apertures two on 

 the left fide of the neck» 



The Tethys is not found in a mollufca flate in the 

 Britifli feas, but is fuppofed to inhabit the greater part 

 of bivalve fhells ; according to Linn^us, the Tcllina, 

 Cardium, Madra, Venus, OJlrca, and others ; this, how- 

 ever, is not without exceptions in many inllances. 



5 LIMAX. 



