In contradistinction to the preceding class these Molluscs are 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 differences in the arrangement of the visceral nerve-loop.
Section Streptoneura.
Cases 4–94.
The Molluscs of this section are bisexual and furnished with a 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.—Scutibranchia.
The Scutibranchia have a free bipectinate gill, or the gill may be absent (Lepeta, Helicina), and generally exhibit traces of bilateral symmetry.
Case 4.
The Acmæidæ are called False Limpets, because, although the 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 Patellæ 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) inhabits 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 spiny tongues. That of the common English Limpet (P. vulgata, Fig. 3) is longer than the shell itself, and armed with as many as 1920 glassy hooks in 160 rows of twelve teeth each. The Limpet is commonly used for bait in the sea-fishing off the Scottish coast, and vast quantities are consumed as food in some parts of Ireland. Some Limpets, such as P. compressa, P. mytilina, etc., are found on the stems of floating seaweeds, and have the shells usually thinner and smoother than the Rock-Limpets, which have to resist the fury of the breaking waves.
Fig. 3.
1. Radula of the Common British Rock-Limpet (Patella vulgata), natural size.
2. Two transverse series of teeth: a, median teeth; b, laterals; c, uncini or marginals.