Zoological articles contributed to the "Encyclopaedia Britannica"

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ZOOLOGICAL AETICLES

ZOOLOGICAL ARTIC

LES

BY

E. RAY LANKESTER, M.A., LL.D., F.R.S.

DEPUTY LINACKE PROFESSOR IN THE UNIVERSITY OF OXFORD, AND HON. FELLOW OF EXETER COLLEGE PRESIDENT OF THE MARINE BIOLOGICAL ASSOCIATION OF THE UNITED KINGDOM ;

HON. MEMBER OF THE CAMBRIDGE PHILOSOPHICAL SOCIETY;

CORRESPONDING MEMBER OF THE ACADEMY OF SCIENCES OF PHILADELPHIA.

TO WHICH ARE ADDED KINDRED ARTICLES BY

PROFESSOR OF GEOLOGY IN TRINITY COLLEGE, DUBLIN.

LUDWIG VON GRAFF. Pii.D.

PROFESSOR OP ZOOLOGY IN THE UNIVERSITY OF GRAZ, AUSTRIA,

A. A. W. HUBRECHT, Ph.D., LL.D.

PROFESSOR OF ZOOLOGY IN THE UNIVERSITY OF UTRECHT.

A. G. BOURNE, D.Sc.

PROFESSOR OF BIOLOGY IN THE PRESIDENCY COLLEGE, MADRAS.

LIVERPOOL.

W. A. HERDMAN, D.Sc.

PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY COLLEGE,

LONDON:

ADAM & CHARLES BLACK

Coll.

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GU

PEE FACE.

T HAVE been anxious to render tbe articles on various groups of Animals written by me for tbe Encyclopcedia Britannica more readily accessible to the University student than they are when bound up in the large volumes of that great work. The Publishers have very kindly met my wishes in this respect by consenting to issue the present reprint. With my articles on Protozoa, Hydrozoa, Mollusca, Polyzoa, and Vertebrata, are here included, by the kind consent of the authors, the article on Sponges by Professor Sollas, that on Planarians by Professor von Graff, that on Nemertines by Professor Hubrecht, that on Eotifera by Professor Bourne, and that on Tunicata by Professor Herdman. The volume thus forms a treatise on a considerable section of the animal kingdom. Obviously it does not profess to be a complete handbook. Since the articles are reprinted from the original plates, and issued at a low price, it has not been possible to introduce any large additions into the text. Here and there an error, due to oversight, has been corrected, and one or two new figures have been added, rendering the work more complete. The chief additions are — the woodcut illustrating recent discoveries concerning the Dinoflagellata (p. 37); the note by Professor Sollas on the classification of Monaxonida (p. 39); the woodcut of Scyphomedusae from the Deep-Sea (p. 57); the woodcut fig. 19 on p. 107, which replaces a similar but incorrect figure in the original article, and the woodcut, fig. 1a on j3. 159, showing forms connecting the Eupolyzoa and other Gephyraea.

There are one or two matters, by way of addition to or correction of my own articles, which this preface gives me the opportunity of mentioning.

In regard to the Protozoa, the reader should note that Professor Btitschli’s treatise in Bronn’s Thierreich is now completed. He has rejected the classification of the Ciliata, which we owe to Stein, and adopts the following — Branch A. Gymnostoma ( = Holotricha with chitinised pharynx, Prorodon, Trachelius, &c.) ; Branch B. Trichostoma ( = the remaining Ciliata, all of which have the pharynx ciliated, if present). The Trichostoma are divided into two classes — the Aspirotricha, and the Spirotricha. The Aspirotricha are the rest of the Holotricha of Stein, not comprised in the Gymnostoma of this classifi-

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Preface

cation. The Spirotricha are characterised by all possessing the adoral “ heterotrichal ” band of large cilia ; they are divided into the sub-classes Heterotricha, Hypotricha, Peritricha, and Oligotricha. The two first of these groups correspond with Stein’s groups of the same names, whilst the Peritricha of Stein are now divided into Peritricha and Oligotricha, the latter sub-class being formed for such genera as Halteria, Strombidium, and Tintinnus. I consider Btitschli’s classification an improvement upon Stein’s, with the doubtful exception of the distinct position assigned to the Oligotricha.

In regard to the Hydrozoa, the most important additions to knowledge since the date of the article are to be found in the large and richly-illustrated monographs by Haeckel {System dev Medusen, Jena, 1879-1880 ; “ Eeport on the Deep-Sea Medusae,” Challenger Reports, vol. iv., 1882; “Reports on the Deep-Sea Siphonophora,” Challenger Reports, vol. xxviii., 1888), and in the remarkable researches of Weissman on the origin of the sexual products {Enstehung der Sexualzellen hei der Hydromedusen, Jena, 1883). The student who takes in hand the actual examination of a specimen of Aurelia aurita by aid of the description given of it in the article Hydrozoa, should also refer to the plates of Ehrenberg’s account of this animal {Physikalische Ahhandlungen der Konigl. Akad. d. Wissensch., Berlin, 1835), and Mr Minchin’s brief but valuable paper on the enclosure of the embryos in minute brood pouches formed by sacculation of the grooves of the oral lobes {Proc. Zool. Soc., 1889, No. xxxix.).

If I were rewriting the article Mollusca, I should adopt the conclusion of my friend and former pupil. Dr Paul Pelseneer, of Ghent, and remove the Pteropoda from association with the Cephalopoda, not to maintain them as a distinct class, but to place them, as he has done, among the Palliate or Tectibranchiate Opisthobranchiate Gastropoda, to which, it seems, they bear the same relation as do the Natantia to the Azygobranchiate Streptoneura. It appears that the Thecosomate Pteropods are nearly related to the Bullidse and Tornatellidse, whilst the Gymnosomate forms are derivable from the Aplysiidse. A careful study of the nervous system convinced Dr Pelseneer that the sucker-bearing lobes of such Gymnosomate Pteropods as Pneumodermon are really cephalic in nature, and innervated from the cerebral ganglion, whilst the sucker-bearing lobes of the Cephalopoda are productions of the foot, and are convincingly demonstrated by Pelseneer (as maintained by me in the article “ Mollusca ”) to be innervated by the pedal ganglia. The remarkable coincidence in the Pteropoda and Cephalopoda of adoral appendages provided with suckers which had been, to my mind, the chief ground for supposing a genetic relationship between these two sets of forms, proves to be a case of homoplasy.^ It is, indeed, a very striking case of the parallelism of genetically distinct organs. The whole of this question is ably treated by Pelseneer in Part HI. of his “ Report on the Pteropoda,” published in vol. xxiii. of the Challenger Reports, 1888. The student of molluscan anatomy should not fail to read this

1 The reader is referred for an explanation of this term, and a discussion of the phenomena concerned, to my article “ On the use of the term Homology in Modern Zoology, and the distinction between Homogenetic and Homoplastic Agreements,” Arm. and Mag. Nat. Hist., 1870.

PREFACE.

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clear and well-illustrated discussion of the structure of the Pteropoda, and of the inferences which may be drawn therefrom as to their affinities.

In regard to the article Polyzoa, I may mention that I think it preferable to make use of the established term “Gephyrsea” in place of that introduced in this article, viz., “ Podaxonia.” The Gephyrsea, then, include the Sternaspidomorpha, Echiuromorpha, Sipunculomorpha, Phoronidomorpha, Polyzoa (Eupolyzoa of the article), Brachiopoda, and Pterobranchia. Concerning the affinities of the first four of these classes with one another, there is little doubt ; as to the affinities of the last three with one another, and with the first four we are still in a very uncertain state, and are likely to remain so for some time, owing to the absence of satisfactory embryological data and the difficulty of obtaining such.

The subject matter of the article Vertebrata is much more extensive than that of the other chapters, and, owing to limited space, is treated in a much more general way than is the case with the latter. In regard to the Craniata, the intention was to give only a sketch of leading features which should be supplemented by the study of such works as Gegenbaur’s Comparative Anatomy, Wiedersheim’s Anatomy of Vertehi'ates, and the special articles on Fishes, Eeptiles, Birds, and Mammals, written for the Encyclopcedia by eminent authorities on those groups. The treatment of the Cephalochorda (Amphioxus) and its relations to the Urochorda is a little more complete, and I therefore take occasion to refer the reader to recent publications, in which our knowledge of this most interesting member of the Vertebrate group has been largely extended. They are — Contributions to the Knowledge of Rhabdopleura and Amphioxus (ubique citata), by E. Ray Lankester (London : J. & A. Churchill, 1889); “The Development of the Atrial Chamber of Amphioxus,” by E. Ray Lankester and Arthur Willey, in the Quart. Jour, of Mic. Sci., vol. xxxi., 1890 ; “The Later Larval Development of Amphioxus,” by Arthur Willey, B.Sc., in the same Journal, vol. xxxii. ; and “The Excretory Organs of Amphioxus,” by F. E. Weiss, B.Sc., also in the Quart. Jour, of Mic. Sci., vol. xxxi.

The article “ Sponges,” by Professor Sollas, contains the only summary account of the Porifera written since the recent extraordinary advances in our knowledge of this group. Its incorporation in the present volume cannot fail to be welcome to students. In Professor Bourne’s article on Rotifera are given the only extant woodcuts of the important genus Pedalion. This most important form is not figured or discussed in any other general treatise accessible to students. The articles on Planarians and Nemertines, by Professor von GraflP and Professor Hubrecht respectively, are brief summaries of what is known, written by the chief living authority on each group.

E. RAY LANKESTER.

Oxford, December 1890.