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Studies on the Comparative Embryology of Nemerteans with Special Reference to Their Interrelationships

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Title: Studies on the Comparative Embryology of Nemerteans with Special Reference to Their Interrelationships
Authors: IWATA, Fumio Browse this author
Issue Date: Dec-1960
Publisher: Akkeshi Marine Biological Station
Journal Title: Publications from the Akkeshi Marine Biological Station
Volume: 10
Start Page: 2
End Page: 51
Abstract: Up to the present time the phylogeny of the nemerteans has been mainly discussed from the _morphological point of view (Bürger, 1895 ; Friedrich, 1935; Hylbom, 1957) . The most important characteristics of the nemerteans used for consideration of the problem lie in the relative position of the nervous system in the body Wall. Bürger's conception was partly revised by Bergendal (1903), Friedrich (1935) and Hylbom (1957) . The latter reported a so-called phylogenetic tree of the Palaeonemertea, in which the questionable family Cephalothricidae was placed near the bottom of the group. It has been generally accepted that the palaeonemerteans are closely related to the heteronemerteans, but the phylogeny of the Hoplonemertea and the Bdellonemertea remains unknown. The nemerteans show in general two types of development, namely direct and indirect. In the indirect type, the pilidium and Desor larva occur as seen in the species belonging to the order Heteronemertea. The pilidium larvae have been described in detail for Cerebratulus lacteus (Metschnikoff, 1869 ; Wilson, 1900), C. marginatus, Micrura leidyi, and M. caeca (Coe, 1899 and 1943), and Lineus torquatus (Iwata, 1957). The metamorphosis of the pilidium was observed by Bütschli (1873) and Salensky .(1912). On the other hand, the Desor larva of Lineus ruber (Desor, 1848) was studied by Barrois (1877), Hubrecht (1886), Arnold (1898), Nusbaum and Oxner (1913), and Schmidt (1934) . Iwata (1958) studied the development of Micrura akkeshiensis and reported that the species produces a larva, modified from Desor larva. These larvae above mentioned are all the same in the characteristic that the adult ectoderm was secondarily formed. On the other hand, direct development has been observed in Cephalothrix rufifrons (Smith, 1935) in the order Palaeonemertea and in five species, namely Prosorochmus viviparus (Salensky, 1914), Tetrastemma vermiculus and Drepanophorus spectabilis (Lebedin- sky, 1897), Prostoma graecense (Reisinger, 1926 ; Reinhard, 1941), and Malacobdella grossa (Hammarsten, 1918) in the orders Hoplonemertea and Bdellonemertea. The present paper deals with the interrelationships of the nemerteans from the viewpoint of their comparative development. The development of eight species of the nemerteans belonging to the three orders Palaeonemertea, Heteronemertea, and Hoplonemertea has been observed at the Akkeshi Marine Biological Station under the guidance of Professor Tohru Uchida since 1950. Out of the species used, the development of Lineus torquatus and Micrura akkeshiensis has been reported in detail in 1957 and 1958 respectively. Accordingly brief notes from those papers are incorporated in the present paper. As the result of these studies the writer proposes here a new revised system for Nemertea. Before going further, the writer wishes to express his sincere thanks to Professor Tohru Uchida at whose suggestions the present study was performed and by whose kind guidance this paper has taken its shape. Hearty thanks are also due to Professor Katsuma Dan of Tokyo Metropolitan University for his helpful advice and criticism on embryological study.
Type: bulletin (article)
Appears in Collections:Publications from the Akkeshi Marine Biological Station > No.10

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