504 HISTORY OF SYSTEMS. 



(b.) Plates sunk in the back of the mantle. — Crytoplax. 

 Order XL Dipleurobranchia. Branchia laminar, round 

 the under surface of the mantle, just above the 

 foot : mantle coriaceous, scattered with retractile 

 tubercles : anus dorsal, subterminal : tentacula 4 : 

 organs of generation on the right side : half herma- 

 phrodite : protectors none. Marine, breathing 

 water. Gen. Phyllidia. 



Subclass III. Gymnobranchia. Branchia on the out- 

 side of the mantle. Marine, breathing water. 



Order XII. Pygobranchia. Branchia arborescent, exter- 

 nal, on the back of the mantle round the anus : 

 mantle coriaceous, scattered with retractile tuber- 

 cles : anus dorsal, subterminal : tentacula 4 : organs 

 of generation on the right side : half hermaphro- 

 dite : protectors none. Marine, breathing water. 

 Gen. Doris. 



Order XIII. Polybranchia. Branchia arborescent or 

 vermicular, external, on the sides or on the back 

 of the mantle : mantle coriaceous : anus and organs 

 of generation on the right side : half hermaphro- 

 dite : protectors none. Marine, breathing water. 

 Gen. Tritonia, Scyllea, Eolis, Tergipes, Tethys, 

 Glaucus. 



Class III. Gasteropterophora. 



Head distinct ; organs of vision 2 : body elongate, free 

 (floating horizontally belly upwards) : foot on the ventral 

 disk: arms or fins round the head none: fin 1, towards 

 the hinder part of the ventral disk : branchia pectinated, 

 inside the conical mantle : anus lateral : shell symmetrical, 

 conical, involute, very thin ; aperture rather angular : oppo- 

 site to the ventral fin. Marine, breathing water. Gen. 

 Pterotrachea. (Carinaria, Argonauta.) 



Class IV. STOMATOPTEROPHORA=Pteropoda, Cuv. 



Head more or less distinct : body elongate, free (floating) : 

 foot none distinct : mantle mostly double, extended in front 

 into the form of two membranaceous fins : hermaphrodite : 

 protectors none, or very thin. Marine, breathing water. 

 Order I. Pterabranchia. Head distinct: branchia ex- 

 ternal on the fins : mantle sack-like : shell none 

 or univalve. 

 (a.) Shell spiral. Limacina. 

 (b.) Protector or shell straight. Cleodora, Cymbula. 



