640 MOLLUSCA— CLASS IV.—GASTROPODA. 



In the genus Tormdhm, the shell is cylindrical, the apex of the short spire 

 being sinistral ; in Volvidella the spire is no longer raised, the shell being 

 convolute, i.e., wound on itself like a measuring tape; in Bulla and 

 Bullinella [ = Cylichna] it is involute, i.e., the spire is sunk, leaving an 

 aperture, whilst the columella is less pronounced ; in Scaphander the latter 

 has disappeared and the shell is without any central axis, the whole interior 

 being visible from the front ; in Philine this is carried still further, the 

 shell, which is internal, being widely open — unrolled, as it were. 



To the Bulloidea is now referred that section of the old class Pteropoda, 

 or sea-butterflies, in which the animal retains its shell in the adult state. 



The chief peculiarity of the Pteropoda lies in the conversion of the foot 

 into a pair of wing-like fins. Their shells vary greatly in shape, whilst their 

 general appearance will best be understood from the foregoing figures (Pig. 19). 



The following families are referred to the Bulloidea : — Actfeonidso, Torna- 

 tinidfe, Scaphandridre, Bullidre, Aplustridie, Ringiculidas, Gastropteridse, 

 Philinidse, Doridiidae, Runcinidse, Oxynoidaa, Limacinidoe, Cymbulidae, 

 Cavolinidfe. 



B. — Aplysioidea. 



In the Aplysioidea the shell is either very greatly reduced or wanting 

 altogether ; the head carries two pairs of tentacles. 



The Ai'LYSiiD.E, or sea-hares, have large bodies of greenish or oiive-greeii 

 hue, with a pair of lappet-like extensions of the foot folded over the back ; 

 the shell, concealed beneath the mantle-folds, is thin, covered with a horny 

 periostracum, triangular in shape, and flexible when first removed from 

 the animal (Fig. 18 D). 



The shell-less sea-butterflies are now placed with this group, which 

 includes the following families : — Aplysiidse, Pneumonodermatidas. Clionop- 

 sida3, Clionidfe, Notobranchseidse, and Eurybiidse. 



C. — Pleukobranchoidea. 



The Pleurobranehoidea have two pairs of tentacles, no lappet-like 

 extensions of the foot, and no gill cavity, the gill occupying a narrow 

 space between the mantle and the foot. 



In the PLEQKOBKANCHiDyE, the thin flexible shell which is covered by the 

 mantle is large, oblong, and nearly flat, with the nucleus at the hinder 

 margin. In Umbrella, the sole genus in the family Umbrellid.b, the 

 external shell is a flat, circular disc, with the nucleus nearly in the centre. 



Sub-order 2. — Nudibeanchiata. The sea- 

 slugs are shell-less in the adult state, but possess 

 when still in the egg both shell and operculum. 

 There are no true gills ; their place is taken by 

 certain tentacle-like organs that sometimes are 

 placed in a cluster near the end of the body 

 (Fig. 20 A), and sometimes ranged in rows 

 down the back (Fig. 20 B). These "branchial 

 leaflets," "dorsal appendages," or "cerata," 

 as they are variously styled, frequently bear 

 jPi(7. 20.— NnniniiANciis. * striking resemblance both in form and 



A, Doria. B, Dendronotus, colour to some other marine object, such as a 

 cluster of zoophytes or small sea-weeds, etc. 

 The radula is very variable, sometimes there are many teeth in each 



