544 HISTORY OF SYSTEMS. 



spiral, with a very wide and entire aperture : tentacula 

 or none, or rudimentary, or earlike. 



Family 1. SUBAPLYSIACEA. Two or four tentacular 

 appendages : orifices of the generative organs 

 approximate and without an intermediate furrow, 

 den. : Barthella, Pleurobranchus, Pleurobranchi- 

 dium. 



Family 2. APLYSIACEA. Four tentacular earlike ap- 

 pendages : sexual orifices distant and united by 

 a furrow. Gen. : Aplysia, Dolabella, Bursatella, 

 Notarchus, Elysia. 



,, 3. PATELLOIDEA. Body depressed and covered 

 by a large external non-symmetrical patelliform 

 shell. Gen. : Ombrella, Siphonaria, Tylodina. 

 4. AKERA. Body more or less globular, 

 gasteropod, divided into two parts, the anterior 

 often furnished with lateral lobes : head indis- 

 tinct, without tentacula or with rudimentary 

 tentacula. Ex. Bulla, Bullea. 



SECTION II. Respiratory organs and shell (when it exists, 

 which is rarely) symmetrical. 



Order I. APOROBRANCHIATA. Body variable in form but 

 always furnished with natatory appendages in pairs and 

 lateral ; no foot ; organs of respiration often inconspi- 

 cuous. 



Family 1, THECOSOMATA. Genera: Hyalaea, Cleo- 



dora, Cymbulia, Pyrgo. 

 2. GYMNOSOMATA. Genera : Clio, Pneumo- 



derma. 



3. PSILOSOMATA. Genus : Phylliroe. 

 Order II. POLYBRANCHIATA. Organs of respiration bran- 

 chial, in numerous filaments or tufts disposed symme- 

 trically and on the exterior of each side of the body, 

 which is naked. 



Family 1. TETRACERATA. Branchiae simple, filamen- 

 tous. Gen. : Glaucus, Laniogerus, Tergipes, 

 Cavolina, Eolida. 

 ,, 2. DICERATA. Branchiae arbuscular. Gen. : 



Scyllsea, Tritonia, Thethys. 



Order III. CYCLOBRANCHIATA. Organs of respiration 

 branchial, more or less arbuscular, collected symmetri- 

 cally near the vent which is on the median line of the 

 posterior part of the body. Gen. : Doris, Onchidoris 

 Peronia. 



Order IV. INFEROBRANCHIATA. Organs of respiration 

 branchial, in the form of lamellae under the projecting 



