GASTEOPODA. 9 



fourth that number has been found fossil from the Ordovician age 

 upwards. 



The second order of Amphineura, namely, the Aplacophora, Case 3. 

 comprises a few somewhat worm-like Molluscs which are devoid of a 

 shell, but have instead the dorsal surface more or less studded with 

 numerous minute calcareous spines or spicules. Neomenia, Pro- 

 neomenia, GhcBtocUrma are genera belonging to this order. 



Class II.— GASTROPODA. 



In contradistinction to the preceding class these Molluscs are Cases 

 asymmetrical, especially in respect of the gills and the spiral coiling *~1^5- 

 of the viscera and most of the shells. They may be divided into 

 two sections, Streptoneura and Eiithyneura, distinguished by differ- 

 ences in the arrangement of the visceral nerve-loop. 



Section Steeptoneuea. 



The Molluscs of this section are bisexual and furnished with a Cases 

 shell, and generally with an operculum. The gills are in front of *~^*- 

 the heart and the visceral nerve-loop is twisted into a figure of 8. 

 The section contains two groups or orders, Scutibranchia and 

 Pectinibranchia. 



Order 1. — Scutibeanchia. 



The Scutibranchia have a free bipectinate gill, or the gill may 

 be absent {Lepeta, Helicina), and generally exhibit traces of bilateral 

 symmetry. 



The Acmmd(R are called False ]jimpets, because, although the Case 4. 

 shells are identical with the true Limpets, the animals differ in 

 having only a small gill on the left side of the neck, whilst the 

 PatellcB have the gills greatly developed all round the sides of the 

 foot. Both the true and the false Limpets are littoral and found 

 on rocks between tide-marks. They have the power of excavating 

 the surface to which they attach themselves, and adhere so firmly 

 that it is easier to break the shell than detach the animal. The 

 largest known Limpet {Patella (Ancistromesus) mexicana, case 7) in- 

 habits the west coast of Central America, its shell having sometimes 

 a length of 12 inches. The Limpets are vegetable feeders and fond 

 of seaweeds of various kinds, which they rasp with their remarkable 



