174 THE GASTROPODA 
Mediterranean. Famity 2. UMBRELLIDAB, 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). 
Famity 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— Plewrobranchus, Cuvier; mantle 
long and broad ; shell internal, with a short spire. Berthella, Blainville. 
Haliotinellu, Souverbie. Oscanius, Leach ; British. Oscaniella, Bergh. 
Oscaniopsis, Bergh. Plewrobranchaea, Meckel ; mantle short and narrow ; 
no shell (Fig. 157). 
Fic. 158. 
Umbrella mediterranea, right side view. a,mouth; b, cephalic tentacle ; h, ctenidium. The 
free edge of the mantle is seen just below the margin of the shell. (From Lankester, after 
Owen.) 
Susp-OrDER 2. NUDIBRANCHIA, 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 
orrhinophore. 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 veluin 
