Book - The Pineal Organ (1940) 8
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Gladstone RJ. and Wakeley C. The Pineal Organ. (1940) Bailliere, Tindall & Cox, London. PDF
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The Pineal Organ - Eyes of Invertebrates
Chapter 8 The Eyes of Molluscoida
The Molluscoida are marine animals comprising three classes, the Polyzoa, Brachiopoda, and Phoronida. The Polyzoa form colonies known as " sea-mats " or " coral-lines." The Brachiopoda are characterized by the possession of bivalve shells which differ from those of the molluscs in being dorsal and ventral with regard to the animal instead of right and left as is the case in molluscs. A typical example of the class is the common " lamp shell." The Phoronida are worm-like animals enclosed in leathery tubes. They live in associations composed of numerous individuals each of which possesses a number of tentacles springing from a horseshoe-shaped pedicle, the lateral horns of which are often spirally coiled. The body is strongly flexed in a dorsal direction, so that the mouth and anus are brought near together at the anterior end of the body.
Fig. 58. — Trochopore Larva of Phascolosoma vulgare, showing the Typical Position of the Paired Eye-spots in the Podaxonia (Sipunculoidea). (From McBride, after Gerould.)
ap. : apical tuft of cilia. oc. : eye-spots.
head blastema. stom. : stomodajum.
metatroch. tb. : trunk blastema,
prototroch.
The dorsal surface or back of the animal is thus markedly reduced in length. The adult animals of all these three classes are fixed and no sensory organs are present, but the ova give rise to free-swimming larva; of the trochophore type which in some species have a typical apical plate, bearing a vertical tuft of cilia and lying over a cerebral ganglion. In close relation with this, e.g. in the Phoronida, Phascolosoma (Podaxonia) (Fig. 58), there are two pigmented eye-spots which lie right and left of the median plane ventral to the plate. Bilateral eye-spots are also present in the free-swimming larva of Cistella. When the larvae become fixed the eyespots disappear. A point of great interest with regard to the presence of paired eyes of simple type, in the larvae of the Brachiopods is that the more primitive forms of this class, judging from the characters of the shell, have existed almost unchanged from the palaeozoic period represented by the lower Cambrian rocks of America, e.g. the hinged bivalve brachiopod Paterina, which is regarded as the ancestor of the living brachiopods (Fig. 59).
Fig. 59. A — Pater ina. Simple ancestral fossil form of Brachiopod — enlarged ; lower Cambrian rocks, America. B — Rhynchonella. Dorsal aspect of shell, showing foramen, /., in beak which
transmitted the stalk. Fossil shell. Lower Cretaceous. C — Young larva of Cistella, showing two eye-spots, three segments, and two
bundles of sets. D — Interior of dorsal valve of Cistella, which closely resembles the fossil types of
Brachiopoda shown in A and B. The simpler species have presumably persisted with little change in their form or mode of development since the lower Cambrian period (see Fig. 324, p. 479).
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Cite this page: Hill, M.A. (2024, April 24) Embryology Book - The Pineal Organ (1940) 8. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Pineal_Organ_(1940)_8
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
