174 THE GASTROPODA 



Mediterranean. FAMILY 2. UMBRELLIDAE, Gray. Shell external, conical, 

 and much flattened ; anterior tentacles very small and situated together 

 with the mouth in a notch in the foot below the head ; ctenidium very 

 large, extending above the neck. Genus Umbrella, Lamarck (Fig. 158). 

 FAMILY 3. PLEUROBRANCHIDAE, Gray. Shell covered by the mantle or 

 absent ; the interior tentacles form a frontal veil ; spicules are formed in 

 the mantle ; foot flattened. Genera Pleurobranchus, Cuvier ; mantle 

 long and broad ; shell internal, with a short spire. Bertkella, Blainville. 

 Haliotinella, Souverbie. Osca?iius, Leach ; British. Oscaniella, Bergh. 

 Oscaniopsis, Bergh. Pleurobranchaea, Meckel ; mantle .short and narrow ; 

 no shell (Fig. 157). 



I'lO. 158. 



Umbrella metliterrmua, right side view, a, mouth; b, cephalic tentacle ; A, ctenidium. The 

 free edge of the mantle is seen just below the margin of the shell. (From Lankester, after 

 Owen.) 



SUB-ORDER 2. NODIBRANCHIA, Cuvier. 



Naked Opisthobranchs without a shell in the adult state ; without 

 ctenidium and osphradium. These animals are generally slug-like and 

 exhibit an external symmetry. The visceral mass, except in the Hedylidae, 

 is no longer a sac marked off from the foot, and the dorsal integuments 

 frequently give rise to appendages which are subservient to respiration. 

 The nervous system is much concentrated ; the ganglia are generally 

 united on the dorsal side of the oesophagus ; the supra-intestinal and 

 infra-intestinal ganglia are fused with the pleurals (Fig. 159, a) ; the fusion 

 of the centres is sometimes carried to a great extent (Tethys), but the 

 several infra-oesophageal commissures (pedal, visceral, and stomato-gastric) 

 always remain distinct. The visceral commissure is always reduced, and 

 is generally without a ganglion. Accessory stomato-gastric or "gastro- 

 oesophageal " ganglia are present. The gonad is subdivided into male 

 and female acini (Fig. 102, B) except in the Elysiomorpha. The Nudi- 

 branchia are marine, generally carnivorous, and brightly coloured, 

 affording many instances of mimicry. There is no osphradium, but its 

 absence is compensated by the increased development of the olfactory organ 

 or rhinophore. In ontogeny the free veliger stage of Nudibranchs (Fig. 61) 

 is followed by a planariform creeping stage, during which the shell is 

 rapidly lost (Fig. 116, B) ; and finally the dorsal appendages are acquired, 

 notably the dorsal papillae of the Eolids, of which the most anterior are 

 the first to be developed. Cenia is the only form that leaves the egg in 

 the adult condition ; it has no embryonic shell, and the embryonic velum 



