GASTROPODA. 9 
fourth that number has been found fossil from the Silurian age 
downwards. 
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, Chcetoderma, 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 
of the viscera and most of the shells. They may be divided into 
two sections, Streptoneura and Euthyneura, distinguished by differ- 
ences in the arrangement of the visceral nerve-loop. 
Section Stkeptoneuka. 
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 . — Scutibkanchia . 
The Scutibranchia have a free bipectinate gill, or the gill may 
be absent (Lepeta, Helicinu), and generally exhibit traces of bilateral 
symmetry. 
The Acmceidce are called False Limpets, 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 
Patellce 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 
