The Early Stages of Terebratulina septentrionalis

Abstract

MR. MORSE is one of the band of New England naturalists who have lately been making themselves known to us through that excellent periodical the American Naturalist, and who have shown themselves determined to take advantage of the opportunities offered to them by the presence on their sea-board of such zoological treasures as Limulus and Lingula. Mr. Morse obtained Terebratulina in abundance in the harbour of Eastport, Maine, and gives in this paper an account of the change in the form of the shell and the “arms” during development of this Brachiopod from a scarcely visible speck onwards. The changes are illustrated in two plates containing outline figures, and as far as Mr. Morse has observed consist firstly in the passage of the shell from a flat and shorter form to the elongated and convex shape with which we are familiar. Further, the arms were found to commence as a series of ciliated tentacles placed around the mouth, and as nearly as possible identical with the lophophor of such a Polyzoon as Pedicellina. At first but six of these tentacles are seen; these increase in number, whilst the lophophor takes on its horse-shoe shape; and finally by the development of the free ends of the two sides of the horse-shoe the great Brachiopodian arms are produced. This is very interesting, and confirms a priori notions. At the same time we must dissent from the stress which Mr. Morse lays on the affinities of structure of Brachiopoda and Polyzoa, in so far as he wishes to separate these two widely from the Mollusca, and join them to a group which he calls Vermes. The Vermes have never been accurately defined, and are in fact at present, as Carl Gegenbaur (whom Mr. Morse cites) fully admits, one of those classficatory lumber-rooms, which are so convenient from time to time in the progress of zoological science. Whilst we fully admit the close affinities of the Polyzoa and the Brachiopoda—now long recognised by all zoologists—we cannot overlook the very strong affinities of these to the true Mollusca. Even a hasty study of the embryology of the Mollusca is sufficient to bring under one's eyes larval forms of various classes bearing many of the characteristics of the Polyzoa on the one hand, and of certain Vermes on the other. The early condition of the gill-plates in some Lamellibranchs is only to be compared to the tentacula of the Molluscoidan lophophor, though presenting so large a shifting in some relations. Rather than detach the Molluscoida (with regard to the Tunicata there are a variety of new facts and considerations which require long discussion) from the Mollusca to place them in the lumber-room Vermes—we should prefer to put the whole of the Mollusca along with them there—a proceeding at present useless, but which would express a truth which Mr. Morse does not seem to admit, though it is indicated by Gegenbaur, and accepted also by Huxley, namely, that there are close genetic ties between the group Mollusca (including Molluscoida), and certain so-called Vermes, such as the Turbellaria, Archi-annelida, &c.

The Early Stages of Terebratulina septentrionalis.

By Edward S. Morse, Ph. D. (Boston Society of Natural History, vol. ii.)