Introduction to Animal Morphology. 305 



little dorsal eminences (Nerea). 9. Tethyidce tentacles as 

 last ; tongue and radula none. 



III. Pygobranchiata dermis spiculigerous. 10. Onchi- 

 doridcc gills retractile, in separate cavities ; semihmar (Hep- 

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

 (Villiersia) ; skin stiff, spicular (Onchidoris), or soft. 1 1. Tri- 

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

 or 2. i. 2 (Idalia). 12. Dorididae gills retractile (except in 

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

 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. Runcinidae 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. Umbrellidae shell external, discoidal ; apex sub- 

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

 (Umbrella), or projecting (Tylodina). 16. Lophocercidae 

 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 oc . o. oo ; shell internal, calcareous (Dolabella), or 

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

 &c.). 1 8. Philinidae shell subspiral or none (Postero- 

 branchaea) ; foot borders reflected round it ; gizzard with 

 calcareous plates ; teeth i. o. i 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. Bullidai frontal 

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



. half whorl (Smangdi&ella), or less (( 

 tophthalmus), or one or more whorls (Bulb, Atys, 

 20. Cylichnida; shell perfeet, with an elevated spire in 

 Tornatina; teeth 6. o. (> ; head disc with lateral ! 



The pur. ' '.: line. 1'lu- ^-action of 



:latory jjlai. the hand. 



