Tetrabranchiata
Tetrabranchiata names 2 different subjects across the 6 encyclopedias on this shelf. Each is set out below.
Tetrabranchiata in zoölogy, an order of Cephalopoda
Collier's New Encyclopedia (1921)
in zoölogy, an order of Cephalopoda, comprising three families; Nautildæ, Orthoceratidœ, and Ammonitide, though in some recent classifications classificatio the second family is th merged in the first. Animal creeping, protected by an external shell; head retractile within the mantle; eyes pedunculated; mandibles calcareous; arms very numerous; body attached to shell by adductor muscles and by a continuous horny girdle; branchiæ four; funnel formed by the union of two lobes which do not constitute a distinct tube. Shell external, in the form of an extremely elongated cone, either straight or vari- Cyc-Vol. IX ously folded or coiled, many-chambered, keeled and striated; first dorsal of nufeeble spines, and continuous with siphuncled; the inner layers and septa merous nacreous, the outer layers porcellanous. In palæontology, they attained their maximum in the Palæozoic period, decreasing from that time onward, and being represented at the present by the single genus Nautilus. The Nautilidæ proper and Orthoceratidæ are pre-eminently Palæozoic, while the Ammonitide are almost exclusively Mesozoic.
Tetrabranchiata in zoölogy, an order of Cephalopoda
Collier's New Encyclopedia (1921)
in zoölogy, an order of Cephalopoda, comprising three families; Nautilde, Orthoceratidœ, and Ammonitide, though in some recent classifications the second family is merged in the first. Animal creeping, protected by an external shell; head retractile within the mantle; eyes pedunculated; mandibles calcareous; arms very numerous; body attached to shell by adductor muscles and by a continuous horny girdle; branchiæ four; funnel formed by the union of two lobes which do not constitute a distinct tube. Shell external, in the form of an extremely elongated cone, either straight or vari- Cyc-Vol. IX acid, and extracting with ether. It crystallizes in rhombic tables, soluble in alcohol and ether, melts at 76.5°, and boils
What Has Changed in the Last Century
When Collier's wrote this in 1921, "Tetrabranchiata" — the four-gilled cephalopods — was a standard order lumping Nautilidae, Orthoceratidae and the ammonites together. The name has since been dropped: gill count is no longer treated as a primary division, and the ammonites (Ammonoidea) are now generally reckoned closer to squid and octopus than to nautiluses, so the ammonites' gills were probably never four at all. The living survivors are still very few, but Collier's "single genus Nautilus" became two when Allonautilus was recognised in 1997; both are now commercially threatened enough that Nautilus shells were placed under CITES trade control in 2016.
Written for this edition, 2026.