Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/66

48 form of a spiral. They are represented in the present seas by very few genera, Lingula, Terebratula, etc. These spiral-shaped gills were supposed to be used as arms or feet: hence their common name of Arm-feet or Brachiopoda. The Conchifera are better named Lamellibranchiata, from the gills in this class being arranged in the shape of plates or lamellae. This class includes the oysters, clams, mussels. The remaining classes of the Mollusca, the Gasteropoda and Cephalopoda, differ from the Acephala not only in having heads, but in many other respects. The Gasteropoda, or Belly-feet, are so called from these creatures moving on that part of their body. This class is represented by the Whelks (Fig. 52), most of the shells on the sea-shore, and the Snails. The common garden-snail is remarkable on account of the great number of teeth which arise from the tongue: as many as twenty-five thousand are said, according to competent authority, to have been discovered. The Cephalopoda (Fig. 54) are distinguished by having long arms or feet radiating from the head; hence the name of this class, which includes the Cuttle-fish and the Pearly Nautilus of the Indian Ocean. Gasteropoda like the Dentalium are so rudimentary as regards the development of the head, that they may be looked upon as offering the transition from the Gasteropoda to the Lamellibranchiata. The Spirobranchiata, or Brachiopoda, in their development and structure are so closely allied to the Bryozoa that perhaps they ought to be considered rather as part of the Worms than as belonging to the Mollusca. The curious affinities of the Brachiopoda, Bryozoa, and Worms are .striking proofs for the view that the Mollusca have come from the Bryozoa, or animals allied to them, and they from the Worms. That the Brachiopoda were the first Mollusca that appeared on the earth is. at once suggested from the enormous number of fossil forms that are found in the oldest rocks. They are so numerous that the name "Age