TUN1CARJES. 



organized, provided with two apertures, the one 

 oral, for respiration and nutrition, the other anal; 

 the interior envelope, analogous to their mantle, 

 provided also with two apertures adhering to 

 those of the outer one. Body oblong, irregular, 

 divided interiorly into many cavities, without 

 a head ; gills occupying, entirely or in part, the 

 surface of a cavity within the mantle ; mouth 

 placed towards the bottom of the respiratory 

 cavity between the gills ; alimentary tube, open 

 at both ends ; a ganglion, sending nerves to the 

 mouth and anus. 



These animals are either simple or aggregate ; 

 fixed or floating : the simple ones are sometimes 

 sessile, 1 and sometimes sit upon a footstalk. 2 The 

 aggregate ones possess many characters in com- 

 mon with the polypes, inhabiting, as it were, a 

 common body, somewhat analogous to the poly- 

 pary, except that it is more intimately connected 

 with the animal that inhabits it : the mouth of all 

 is surrounded with rays or tentacles, as is also, 

 in many, the anal orifice; but in their organi- 

 zation they differ very widely, exhibiting traces 

 of a nervous system, and even, in some, of one 

 of circulation. The fixed ones are commonly 

 attached to rocks or other inorganized substances, 

 but sometimes they are parasitic ; thus a species 

 of botrylle 3 envelopes, like a cloak, certain asci- 



1 Cynthia, ~ Clavelina. 3 Botryllus polycyclus. 



