Introduction to Animal Morphology. 305 



little dorsal eminences (Nerea). 9. Tethyidse — tentacles as 

 last ; tongue and radula none. 



§ III. Pygobranchiata — dermis spiculigerous. 10. Onchi- 

 doridac — gills retractile, in separate cavities ; semilunar (Hep- 

 tabranchus), or in a circle (Hexabranchus), or in two lobes 

 (Villiersia) ; skin stiff, spicular (Onchidoris), or soft. 11. Tri- 

 opidae — gills retractile in one cavity; teeth 8. o. 8 (Triopa), 

 or 2. I. 2 (Idalia). 12. Dorididas — gills retractile (except in 

 Acanthodoris), teeth numerous, equal ; mantle weak (Casella), 

 none, or well developed, with the body depressed, rounded 

 above (Doris), or angular (Goniodoris). 



§ IV. Pleurobranchiata(Mono-, orDi-pleurobranchiata) — 

 gills covered by the mantle edge on the right or on both 

 sides, the first sub-section including the following fami- 

 lies : — 13. Runcinidse — shell-less, slug-like, marine; a 

 shelly gizzard. 14. Pleurobranchidae — shell internal, asym- 

 metrical (Pleurobranchus, &c.), or none (Pleurobranchaea, 

 Neda) ; genital openings close together ; tentacles free. 

 15. Umbrellidse — shell external, discoidal ; apex sub- 

 central ; eyes intertentacular ; head in a pre-pedal fissure 

 (Umbrella), or projecting (Tylodina). 16. Lophocercidse — 

 shell coiled, unsymmetrical ; side lobes of foot undivided 

 (Lophocercus), or lobed (Lobiger). 17. Aplysiidae — tentacles 

 ear-like ; stomach gizzard-like ; foot with lateral lobes ; 

 teeth 00 . o. 00 ; shell internal, calcareous (Dolabella), or 

 subcartilaginous (Aplysia)*, or none (Bursatella, Notarchus, 

 &c.). r8. Philinidae — shell subspiral or none (Postero- 

 branchaea) ; foot borders reflected round it ; gizzard with 

 calcareous plates ; teeth i. o. r or 2. i. 2 ; tentacles none, or 

 adherent, as in the succeeding four families ; foot rudimental, 

 with two lateral epipodia (Gastropteron), or large ; shell 

 forming a complete whorl (Philine). 19. Bullidae — frontal 

 lobe quadrangular; eyes sessile ; teeth 00 . o. 00 ; shell invo- 

 lute, making a half whorl (Smaragdinella), or less (Cryp- 

 tophthalmus), or one or more whorls (Bulla, Atys, &c.) 

 20. Cylichnidae — shell perfect, with an elevated spire in 

 Tornatina; teeth 6. o. 6; head disc with lateral lobes. 



* The purple secretion of Aplysia contains iodine. The secretion of 

 its depilatory glands sometimes urticates the hand. 



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