MOLLUSCA. 



125 



(a) STREPTONEURA (twisted nerves), in which the Gallery 

 loop formed by the visceral nerves is twisted. Since the VIII. 

 gill-cavity is in front of the heart, they are also known as 



the Order Prosobranchia. Example: the whelk, Bioccinum. 

 To this Order are now referred certain small gastropods 

 (e.g. Carmaria), with very thin shells and a muscular 

 expansion forming a vertical fin by which they swim near 

 the surface of the sea. They are sometimes distinguished 

 as Heteropoda ; some are found in Tertiary rocks. 



(b) EUTHYNEURA (straight nerves), in which the 

 nerve-loop has untwisted. These are as:ain divided into 

 the Orders : Opisthobranchia (hind- 

 gills), marine forms, e.g. Bulla ; and Pul- 

 monata (with a lung, i.e. air-breathers), 

 terrestrial or fresh-water forms, e.g. Helix. 

 With the Opisthobranchs 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), 

 to which the Conularida were also 

 referred. 



G-astropods already existed in early 

 Cambrian times, but all through the 

 Palaeozoic Era they remained of simple 

 type, being mostly marine Streptoneura 

 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 

 those now living, though the species are different. Daring 

 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 increasing 

 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 Bmtalinm 

 is an example, have a foot adapted for burrowing in mud or 

 sand. The shell is tubular, 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 



Fig. 67.— a Pfceropod 

 living in the Atlan- 

 tic, Gleodora pyra- 

 midata. Natural 

 size. 



The number of 



