File:American malacological bulletin (1988) (17968485140).jpg

Title: American malacological bulletin
Identifier: americanmal6719881990amer (find matches)
Year: 1983 (1980s)
Authors: American Malacological Union
Subjects: Mollusks; Mollusks
Publisher: (Hattiesburg, Miss. ?) : (American Malacological Union)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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c Fig. 5. Oral shield of a Chaetodermomorpha: section through the mouth, pharynx, and oral shield of Scutopus megaradulatus Salvini- Plawen showing continuous cuticle of pharynx and oral shield (from 650 m off Cape Hatteras, North Carolina, U. S. A., 34°14.8'N, 75°46.7'W; fixed in formalin, preserved in alcohol, stained with haemotoxylin/Gray's double contrast, sectioned at 0.7 ^m.) (c, spiculose cuticle of integument; n, nerve fibers from precerebral ganglion; o, cuticle of oral shield; p, cuticle of pharynx). Small arrow indicates change from oral shield cuticle with a thickened outermost layer to homogeneous cuticle of pharynx. Scale bar = 0.05 mm. P
Text Appearing After Image:
I 1 Fig. 6. Oral shield of Scutopus megaradulatus. A. Anterior view of oral shield in situ surrounding darkened mouth in center. B. Semischematic drawing of area between large arrowheads in figure 5 showing histology of pharyngeal and oral shield cuticle (lettering and small arrow as in Fig. 5). Scale bars: A = 0.3 mm; B = 0.05 mm. oral shield, further refuting Hoffman's homology, which likened the lobes of mucous cells opening at the lateral edges of the oral shield in advanced Chaetodermatidae with the pedal gland of Neomeniomorpha. This important aspect of Hoff- man's homology linking lobed mucous cells of the oral shield and foot furrow was ignored by Salvini-Plawen (1980) while retaining the homology itself. Definitive evidence that the oral shield is a part of a vestigial ventral sole would require inner- vation from the ventral (= pedal) nerve cord rather than from the cerebral ganglia. Thus, the oral shield of the Chaetodermomorpha is considered here to be an autapomorphy, a cerebrally inner- vated external continuation of pharyngeal cuticle like a lip belonging to the head, not to a ventral sole. There is no con- vincing evidence that it is a remnant of an original creeping sole homologous to the ventral surface of a turbellarian flat- worm. The separation of the Aplacophora into two classes based on the supposed (1) plesiomorphy of ventral innerva- tion of the chaetoderm oral shield by the cerebral ganglia and (2) apomorphy of a head separate from the foot in the neomenioids and all other mollusks except chaetoderms is unsatisfactory. A head separate from the foot is considered here to be a plesiomorphy shared by mollusks generally but lost in the bivalves and, because of their burrowing habit, also in the chaetoderms.

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