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. 

 Gen.: 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, 

 ivhich 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, Tiiecosomata. 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. : 

 Scyllasa, 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 



