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The Morphology of the Tunicata, and Its Bear- Ings on the Phylogeny of the Chordata

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The Morphology of the Tunicata, and Its Bear- Ings on the Phylogeny of the Chordata The Morphology of the Tunicata, and its bear- ings on the Phylogeny of the Chordata. By Walter Garstang, M.A., D.Sc. Oxou., Professor of Zoology in the University of Leeds. With 13 Text-figures. CONTENTS. 1. PHYLOGENY AND BIONOMICS ....... 58 2. THE METAMORPHOSIS OF ASCIDIANS ..... 63 3. LABVAL EVOLUTION AND THE OBIGIN OF THE ENDOSTYLE . .84 4. APPENDICTJLABIANS AS PAEDOMORPHIC DOLIOLIDS ... 93 5. EPICABDIAL VESTIGES IN APPENDICTJLABIANS AND AMPHIOXUS . 135 6. THE LAEVAL ASYMMETBY OF AMPHIOXTJS .... 147 7. BUDDING AND THE OKIGIN OF EPICARDIA .... 156 8. NEURAL CANAL, NEUBOPORES, AND HYPOPHYSIS. 9. THE ANCESTRY OF THE CHORDATA. (The last two sections are reserved for later publication.) SUMMARY OF ARGUMENT. As many special points have required detailed examination in the course of this essay, it may be convenient to the reader to be provided with a summary of the main argument. § 1. The ancestry of Chordata must be consistent with the systematic sequence : Echinoderm—Hemichordate — Proto- chordate—Vertebrate, which, at bottom, implies an evolu- tional progress of plankton-feeding organisms from a fixed condi- tion with external ciliated tentacles and food-grooves to an eventually free and motile state with endopharyngeal apparatus of gill-slits and endostyle. This implication is corroborated by the secondary character of the coelomic (locomotive) metamerism of Amphioxus, by E2 52 WALTER GARSTANG the association of an external lophophore with the simplest known condition of gill-slits (Cephalodiscus), and, nega- tively, by the absence of any proof that pelagic larvae necessi- tate pelagic ancestors. The alternative view that Protochordates and Hemichordates are degenerate Vertebrates is precluded by the existence in Vertebrate embryos of vestiges of the complete Protochordate organization of endostyle (=thyroid), epipharyngeal groove (=subnotochordal rod), and tongue-bars (=thymus). § 2. The metamorphosis of Ascidians conflicts at various points with the idea that Tunicata have been derived from Am- phioxus-like ancestors, especially in the absence of metamerism, the development of lateral atria before the gill-slits, and the independence of larval and adult nervous systems. The neuro- muscular relations in Ascidian larvae and Appendicularians are much more consistent with a theory of incipient than of vestigial metamerism. The development of atria before the gill-slits is in accordance with the phyletic history of the Protochordate type of gill-slit, while both the form of the atrium and its mode of development in Amphioxus are probably secondary modifications. The discontinuity between larval and adult nervous systems is unintelligible on the Amphioxus theory, and points to an early divergence between larva and adult in pre-Chordate ancestors, i. e. to a derivation of Tunicates from ancestors with a metamorphic life-history, before the typical Chordate nerve-tube had come into existence. On the other hand, the form of the gill-slits in Ascidians has undoubtedly undergone great elaboration from the original type, which, as in Amphioxus, was U-shaped, tongue- barred, and synapticulate. In the retention of this type Am- phioxus is more primitive than any Tunicate, and in showing the origin of this type from a simple pharyngeal pouch, Balanoglossus is more primitive than either. As the com- mon ancestor could not have possessed more than two to three pairs (the limit in Ascidians), the multiplication of gill-slits in Balanoglossus and in Amphioxus must be regarded as independently acquired since the separation of these forms from MORPHOLOGY OF THE TUNICATA 53 the common stock, presumably in correlation with a secondary- elongation of the body. These conclusions with regard to the original form and ancestral number of gill-slits, together with the points previously mentioned with regard to metamerism, atria, and nervous sys- tem, become coherent and intelligible on a theory of the deriva- tion of the Chordata from a line of fixed metamorphic ancestors leading from Pterobranchia to Tunicata, Balanoglossus and Amphioxus being derivable from this sequence at different evolutional levels by loss of fixation, retention of the larval symmetry, and independent adaptation to a burrowing life. § 3. The theory just outlined imposes the necessity of ex- plaining the Ascidian tadpole as an interpolation in the life- history. The author's Auricularia theory of 1894 is there- fore drawn upon to suggest a way in which the muscular Chordate larva may have been evolved from an original ciliated larva of the Dipleurula type, by the substitution of muscular for ciliary means of locomotion. A parallel to this is furnished by certain starfish larvae in which muscular flappings of one or more body-processes have replaced the original ciliary locomo- tion. A primitive Chordate tadpole may well have arisen from an elongated Auricularia-like larva which acquired the power of undulating from side to side. The organization of such a larva would provide the rudiments of characteristic Chordate features, the circumoral band and its underlying nervous system being regarded as predecessors of the medullary folds, and the adoral band with its ventral loop as the predecessor of the peri- pharyngeal band and endostyle. By postponement of fixation, the rudiments of the gill-slits, which were primarily organs of the adult stage, may have come into working relations with the predecessor of the endostyle, leading to a synthesis of the full Chordate combination of organs. The loss of the larval tail and nervous system at the metamorphosis of an Ascidian would thus correspond to the loss of muscular appendages and larval nervous system at the metamorphosis of a starfish larva. On the other hand, the persistence of these structures in other 54 WALTER GARSTANG Chordata would be attributable to the loss of fixation and meta- morphosis and consequent survival of the larval type of organi- zation (paedomorphosis). This chapter closes with a reinterpretation of the develop- ment of the endostyle in Amphioxus, which is confirmatory of its origin from the ventral loop of the Echinoderm adoral band. § 4. The prevalent view of the primitive nature of Appendicu- larians is here examined in detail. The horizontal tail is a com- mon feature of Synascidian larvae, but the dextral twist of the intestinal loop, in opposition to the sinistral twist in fixed Ascidians, militates against an origin of Appendicularians directly from that source. On the other hand, the passage of the nerve-cord across the right side of the oesophagus is found again in the larva of D o 1 i o 1 u m, together with a straightening out of the intestinal loop and even a dextral twist in many species. This indication of affinity is corroborated by agreement between the two types in what appears at first sight to be a primitive feature, the position of the endostyle in front of the gill-slits; but this, in Doliolids, is shown to be a secondary modification, caused by the backward rotation of the cloaca and the withdrawal of the peribranchial cavities from the front part of the pharynx. From the mode of working of the pharyngeal apparatus, and from the structure of the endostyle, it is shown that the Appendicularian pharynx has been derived from that of Doliolidae, which is itself derivable from that of normal Ascidians. The oozooid of Doliolum possesses the equivalent of only two pairs of gill-slits, so that the single pair of Appendi- cularians represents no great reduction. The sequence is not reversible. In addition to these points the differentiation of the Appendi- cularian ectoderm into oikoplastic and non-oikoplastic areas is a unique and extremely modified character. As the non- oikoplastic area lies posteriorly, and receives the apertures of gill-slits, intestine, and gonad's, it is claimed as the equivalent of a shallow atrio-cloacal cavity, like that of Doliolum, from which it has probably been derived by suppression of the MORPHOLOGY OF THE TUNICATA 55 cavity and eversion of the epithelial lining (cf. gastrozooids of Doliolum). The dorsal hood of various Appendicularian genera is regarded as a modification of the dorsal outgrowth of the cloacal rim which carries the creeping stolon in A n c h i n i a and the free buds in Doliolum. On this theory, the ' Haus ' of Appendicularians, formed from the oikoplastic area, becomes the exact equivalent of a com- plete Doliolid test, which has been shown by Uljanin to be subject to exuviation and renewal like the Appendicularian ' Haus '. The primitive form of house is shown to correspond with the requirements of this theory by possessing two opposite apertures, one oral and the other posterior. The mixture of larval and adult characters which is so characteristic of the young Doliolum, and is a result of the abandonment of fixation, provides the key to Appendicularian structure, which combines the simplified body of an adult Doliolum with retention and specialization of the larval tail. The alleged metamerism of the Appendicularian muscle-cells is examined, and Martini's theory of ' Eutely ', as a substitute for ' partial Neoteny ', is criticized. § 5. This chapter deals primarily with a number of anomalous and puzzling endodermal glands, derived from the pharynx im- mediately behind the endostyle, and either retaining that con- nexion or acquiring a secondary communication with the exterior by fusion with the ectoderm. These are the ' pharyngeal pockets ' of Pritillaria, the' oral glands 'of Oikopleura, and the ' club-shaped gland' of Amphioxus. All are claimed
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