304 Introduction to Animal Morphology. 



ming ; footless ; anus lateral, right ; tail thread-like (Acura)^ 

 or flattened (Phyllirhoe).* 3. Elysiidae — placobranchiate ; 

 jaws none ; anus dorso-lateral ; tentacles ear-shaped (Elysia), 

 or club-shaped (Placobranchus) : probably the aberrant gela- 

 tinous Pterosoma, with no proboscis, foot, gills, radula(?) nor 

 tentacles, sessile eyes, and lateral, fin-like folds, should form 

 the type of a family. 



Sub-order 2. Opisthobranchiata {Milne Edwards) — gills 

 exposed or slightly covered, behind the heart ;f hermaphro- 

 dite ; larva shell-bearing ; mostly carnivorous ; gelatinous, full 

 of sea water, which they discharge rapidly on being caught. 

 When a shell exists in the adult it is inclosed in a duplica- 

 ture of the mantle. This includes the following sections : — 



§1. Cerabranchiata (p. 294): — Family i. Hermseidse — 

 genital ducts separate, rachiglossate ; tentacles none (Alderia), 

 or two contractile thread-like (Stiliger), or longitudinally 

 folded (Hermsea). 2. ^olididse — rachiglossal ; tentacles not 

 retractile ; foot rudimental (FilurusJ) ; small (Chioroera), or 

 large ; gills on foot-stalks (Calma), or on sub-pallial expan- 

 sions (Fiona), or on the back ; head tentacles none (Cal- 

 liopaea), or four (^olis).§ 3. Glaucidae — gills on lateral 

 pedicles ; foot none. 4. Dotonidae — tentacles retractile into 

 a sheath ; gills muricate (Doto), or smooth (Gellina). 5. Proc- 

 tonotidae — ptenoglossal ; liver diffuse; tentacles simple 

 (Proctonotus), or pectinate (Janus). 



§ II. Cladobranchiata — gill papillae branched or leaf-like. 

 6. Heroid^ — tentacles not retractile; teeth 2. i. 2, and a 

 labial veil between the tentacles. 7. Tritoniadae — tentacles 

 retractile ; liver simple (Tritonia).|i 8. Melibaeidae — teeth 

 00. I. 00 . ;^ liver diffused ; tentacles retractile, linear (Meli- 

 baea), or lamellate ; gills tree-like (Dendronotus), on three 



* Sometimes infested by a parasitic gymnophthalmatous medusa,, 

 Mnestra. 



t The heart beats in some 50-96 times per i". 



X This genus has a ringed abdomen. 



§ -lEolis (Flabellina) punctata and Dendronotus arborescens emit sounds 

 apparently by ejecting water, but tliis does not disturb the surface of the 

 water in a basin. 



II T. Hombergi grows to i-i\ feet long. 1 In Scyllaea 24. i. 24^ 



