. Comparative zoology, structural and systematic. For use in schools and colleges. Zoology. MOLLUSCA. 255 There are six classes of Mollusks. Of these, tlie first three form a group by themselves, called Molluscoidea, distinguished by having only one or two nervous ganglia, an imperfect circulatory apparatus, and generally no or- gans for prehension and locomotion. Of the other three, Lamellibranchs are headless, while Gasteropods and Ceph- alopods only have a distinct head, and an apparatus for mastication. Class I.—Poiyzoa. These minute Mollusks resemble the Polyps in appear- ance, living in
. Comparative zoology, structural and systematic. For use in schools and colleges. Zoology. MOLLUSCA. 255 There are six classes of Mollusks. Of these, tlie first three form a group by themselves, called Molluscoidea, distinguished by having only one or two nervous ganglia, an imperfect circulatory apparatus, and generally no or- gans for prehension and locomotion. Of the other three, Lamellibranchs are headless, while Gasteropods and Ceph- alopods only have a distinct head, and an apparatus for mastication. Class I.—Poiyzoa. These minute Mollusks resemble the Polyps in appear- ance, living in clusters, each individual inhabiting a del- icate cell, or tube, and having a simple mouth surrounded with ciliated tentacles. The colony often takes a plant- like form; sometimes spreads, like fairy-chains or lace- work, over other bodies; or covers rocks and sea-weeds in. Fio. : 1. Ilornera lichenoides; natural size. 2. Brauch of the same, magnified. 3. Discopora Shenei; greatly enlarged. patches with a delicate film. The majority secrete carbo- nate of lime. Tiiough an extremely low form of the Mol- luscan type, a Polyzoan shows its superiority to the Coral, which it imitates, in possessing a distinct alimentary canal. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Orton, James, 1830-1877. New York, Harper and brothers
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