348 MANUAL OF NATURAL HISTORY. 



simple, contractile ; polyps with a double 

 sheath. 



II. ORDER. FRESH-WATER ASCIDIAN-POLYPS 

 (Hyppocrepia). 



Lacustrine, or natives of fresh water ; polyps com- 

 pound ; mouth surrounded with ciliated, retractile 

 tentacles, interrupted or depressed on one side so as 

 to assume a crescentic or horse-shoe form ; ova not 

 ciliated. 



1. FAMILY. Crested Ascidian- Polyps (Cristatel- 



lidse). Polyzoary free, floating, contractile, 

 locomotive ; polyps issuing from apertures 

 arranged upon the upper surface ; tentacular 

 disk crescentic. 



2. FAMILY. Plumose Ascidian- Polyps (Plumatel- 



lidae). Polyzoary fixed, incrusting, confer- 

 void or sponge-like, inarticulate, composed 

 of aggregated membranous tubes opening on 

 the surface. 



3. FAMILY. Marsh Ascidian- Polyps (Paludicel- 



lidse.) Polyzoary fixed, jointed, coriaceous ; 

 composed of a single series of claviform cells 

 with a concatenated arrangement ; apertures 

 uni-lateral, tubular, near the wide end of the 

 cell. 



CAVITARY-PARASITES. 



The Ccelelminiha of Owen, or the " Vers cavi- 

 taires " of Cuvier, assume a much higher standard 



