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 This subclass, which includes the Chambered Nautilus and about 5000 species of fossil and extinct Ammonites, is not represented in American waters. The living species of Nautilus from the Indo-Pacific are characterized by a large, chambered, external shell, by two. pairs of gills, and by their numerous suckerless arms.

All of the living cephalopods with the exception of Nautilus belong to this subclass which is characterized by animals that have one pair of gills, 8 or 10 arms which bear rows of suckers, and whose shell is internal or entirely absent.

With 10 arms, 2 of which are the long tentacular arms; body long and cylinder-shaped. An internal shell is present in most cases, and nmv be calcareous (the cuttlebone) or thin and horny (squid pen). The small suckers on the arms are usually set on small stalks or peduncles and their apertures are armed with horny rings or hooks.

Spirula spirula Linne Common Spirula

Figure 98

Cape Cod to the West Indies. Worldwide.