MOLLUSCA. 
125 
(a) STREPTONEURA (twisted iierves), in which the 
loop formed by the visceral nerves is twisted. Since the 
gill-cavity is in front of the heart, they are also kno\yn as 
the Order Prosobranchia. Example : the whelk, Bitcciniirn. 
To this Order are now I'eferred certain small gastropods 
(e.g. Carinaria), with very thin shells and a muscular 
expansion forming a vertical fin hy which they swim near 
the surface of the sea. They are sometimes distinguished 
as Heteropoda ; some are found in Tertiary rocks. 
(h) EUTHYNEURA (straight nerves), in which the 
nerve-loop has untwisted. These are again divided into 
the Orders : Opisthobranchia (hind- 
gills), marine forms, e.g. Bulla ; and Pul- 
monata (with a lung, i.e. air-hreathers), 
terrestrial or fresh- water forms, e.g. Helix. 
With the Opisthohranchs are now placed 
some small thin-shelled gastropods in 
which the foot is changed into two wing- 
like fins, one on each side (Fig. 67). 
They were formerly considered as a 
separate Class, Pteropoda (wing-foot), 
CO which the Conularida were also 
refeiTed. 
Gastropods already existed in early 
Cambrian times, hut all through the 
Palaeozoic lira they remained of simple 
type, being mostly marine Streptonenra. The number of 
genera and species increased greatly during Mesozoic time, 
but it was not till the later Jurassic and earlier Cretaceous 
rocks that non-marine forms were preserved in any abundance. 
In the older Tertiaries most of the genera are the same as 
Fig. 67. — A “ Ptero- 
pod ” living in the 
Atlantic, Cleodora 
pijramidata. Natural 
size. 
those now living, though the species are different. During 
the Miocene Epoch a few modern species made their 
appearance, and of the Pliocene species about 85 per cent, have 
persisted to the present day. On this gradually increa.sing 
proportion of recent species of Mollusca Lyell based his divi- 
sion of Tertiary time into Eocene, Miocene, and Pliocene. 
III. SCAPHOPODA (digger-foot), of which Dentalium 
is an example, have a foot adapted for burrowing in mud or 
sand. The shell is tubulai', since the folds of the mantle 
have grown together on the under side of the animal ; its 
broader front end, from which the foot can stretch out, is 
sunk in the sand, while the narrower hind end projects 
above it and discharges the waste products, Scaphopods 
Gallery 
VIII. 
