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Volume 74, 1944-45
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The Orientation of Pre- and Post-metamorphic Forms.

Conklin (op. cit.) showed that the mantle is a dorso-ventral growth, dividing the posterior peduncle from the anterior lobe, and decided that “the valves which are formed by the mantle folds are dorsal and ventral, while the opening of the valves is anterior” (p. 61). Students of Brachiopoda—e.g., Schuchert and Cooper (9, p. 8) name the two valves dorsal and ventral, and seem to assume that the parts so arbitrarily named coincide with larval dorsum and ventrum. Conklin nowhere provides information which satisfactorily links the larval and adult dorsal and ventral surfaces, except that in his Figures 35, 61 and 62 he shows patches of cells which he interprets as rudiments of ventral sense plate and suboesophageal ganglion. This interpretation must have been based on an assumption that the parts did in fact pass over to the young adult and form portions of the nervous system, which could not be known since he was apparently unaware of the process of metamorphosis of Terebratella apart from what was provided by Morse's work. Morse's results give no clue to the relation between the fine structures of larva and adult.

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Fig. 1.—Optical section through part of equator of egg showing follicle cells.
Figs. 2, 3, 4.—First, second and third cleavages.
Figs. 5, 6.—Late gastrula, showing disappearance of posterior cilia, differentiation into regions of anterior lobe and mantle and stalk, and appearance of ventral groove from blastopore. Live material.
Figs. 7, 8.—Complete loss of posterior cilia; appearance of apical cilia. Live material.
Fig. 9.—Differentiation of anterior lobe, from blastoporal face. Live material.
All figures based on camera lucida drawings.

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Fig. 10.—Differentiation of anterior lobe, from left side. Live material.
Figs. 11, 12.—Closure of blastopore, appearance of mantle rudiment and of rudiment of peduncle. Fig. 11 from dorsal side (blastoporal site seen by transparency), Fig. 12 from left side. Based on live material and whole mounts.
Fig. 13.—As Fig. 11, from ventral side. Live material.
Fig. 14.—Later stage with enlarging mantle fold, and more slender peduncle. Live material, lateral view.
Fig. 15.—As Fig. 14, dorsal view.
All figures based on camera lucida drawings.

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Fig. 16.—Older larva, with longer mantle fold and more slender peduncle. Apical tutt absent. Live material.
Fig. 17.—Mature larva on emergence from parental mantle cavity, with eyespots and setae. Based on live material and whole mounts.
Fig. 18.—After two hours' attachment. Live material.
Fig. 19.—After attachment; mantle beginning to reverse. Live material.
All figures based on camera lucida drawings.

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Fig. 20.—Shortly after mantle reversal, pai tial enclosure of anterior lobe. Live material.
Figs. 21, 22.—Enclosure of anterior lobe, Fig. 22 with early calcified shell. Live material.
Figs. 23, 24, 25.—As 20, 21. 22. Fig. 23 showing left pedicle adjuster muscle.
Fig. 24 vential side, Fig. 25 dorsal side. Whole mounts.
All figures based on camera lucida drawings.

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Fig. 26.—Early appearance of stomodaeal invagination, before its union with rudimentary enteron. Ventral side. Whole mount.
Fig. 27.—Rudiments of first pair of lophophoral cirri. Ventral side. Whole mount.
Fig. 28.—Rudiments of first two pairs of lophophoral cirri. Ventral side. Whole mount.
Fig. 29.—Three pairs of lophophoral cirri, fifth and sixth appearing in succession. Ventral side. Whole mount.
Fig. 30.—Five pairs of lophophoral cirri. Whole mount.
Fig. 31.—Six pairs of lophophoral cirri. Gastrie caeca well formed. Whole mount.
All figures based on camera lucida drawings.

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Fig. 32.—Nine pairs of large lophophoral cirri, and a nineteenth cirrus appearing. Inflection of anterior border of mouth producing a crescent-shaped slit. Whole mount.
Fig. 34.—Nearly horizontal section of slightly later stage than Fig. 33, showing mesoderm lying laterally to enterior, and posterior connexion between endodermal and mesodermal cavities.
All figures based on camera lucida drawings.

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Fig. 33.—Sagittal section of late gastrula, wedge-shaped, showing posterior enterocoelic pouch opening from archenteron, and showing absence of granular layer from endodermal and mesodermal cells.
Fig. 35.—Optical section of same stage as Fig. 34, where anterior lobe is marked off and blastopore closed, showing posterior continuity between enteron and coelomic cavity. Whole mount.
Fig. 36.—Dorsal view at stage of appearance of mantle rudiment, showing right and left anterior and posterior coelomic sacs and restricted anterior position of enteric cavity. Whole mount.
Fig. 37.—Ventral view of same stage as Fig. 36. Whole mount.
Fig. 38 a, b, c.—Stages in elaboration of right gastric diverticulum.
Fig. 39.—Transverse section of base of mantle of late larva after emergence, showing dorsal (pedicle) adjustor muscles connected to dorsal surface.
All figures based on camera lucida drawings.

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In the present study, it has been possible to follow one batch of material in development from early gastrula to fixation, but only one specimen survived to fix itself and it died before any further change took place. Two lots were killed and preserved daily and sketches were made similarly, so that a reliable set of records exists of that particular material. Another brood of larvae emerged naturally from a female and proceeded to fix, as has already been described. It was not possible in the circumstances to decide by direct external observation how the orientation of the newly-fixed larva was related to that of the young adult. There was no certainty that rotation did not take place, and the striking changes in the form of the anterior lobe precluded any following of the parts from one condition to another during a period of 24 hours or more. Thus, from that standpoint, there was little more hope of connecting larval and adult orientation than there was of linking Morse's and Conklin's observations.

The clue to the connexion lies, however, particularly in the presence in the late larva, while still free swimming, of a pair of muscles, right and left, passing from the peduncle forwards and upwards, each being attached dorso-laterally in the base of the mantle, the place of attachment being marked externally by a small indentation (Plate 7, Fig. 39). After mantle reversal, and for a short time later, these are the only muscles distinguishable. They are then connected with the larger mantle flap which produces the larger valve and are the so-called ventral adjustor muscles: they must now be renamed. In view of the fact that the adjectives “dorsal” and “ventral” have already been used on the understanding that the smaller and larger valves are dorsal and ventral, confusion would arise merely by changing their application. It seems therefore advisable to cease the usage of the words “dorsal” and “ventral” and to maintain the use of other terms such as are given by Thomson (11, p. 4). It seems that the words “pedicle” and “brachial” would well satisfy the need of the conchologist and palaeontologist as far as these are concerned with the Testicardines. Lingula, for reasons to be later considered, falls into another category.

The “ventral sense plate” of Conklin is, in Terebratella, represented also by a well-marked thickening along the mid-ventral line of the mantle of the late larva, and, in section, is densely nucleated. In the present study, it is not seen to form anything significant in the newly reversed animal, and cannot yet be said to contribute anything important to the structure of the adult nervous system.

It becomes obvious that only by examination of material in various stages of transformation the relations of the parts of pre- and post-metamorphic can be decided. Quite truly, we must accept the view that the ventral side of the late larva is the same as that of the early one, or we must reject it. If the former, then the identification of the dorsum and the ventrum of the sedentary animal is easily possible by means of the musculature: if the latter, then at present there is nothing which has been found to persist throughout early life which might conveniently be used as a datum of reference. There is no reason to doubt that the greater dorsal curvature of the early larva remains until metamorphosis, and this determination,

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based on a number of recorded steps, refers in the beginning to the position of the blastopore. Conklin's orientation, as far as it goes, agrees with the present one, except in the case of the gastrula when the enterocoelic sac is in formation.