Though the larval Brachiopod resembles a mesotrochal Chætopod larva, it does not appear to resemble the trochosphere larvæ so far described, or the more typical larvæ of the Chætopoda, in that the ring of tentacles, which is probably, as already mentioned, derived from the ciliated ring shewn in fig. 137, is _post-oral_, and not _præ-oral_. The ring of tentacles is like the ring in Actinotrocha (the larva of Phoronis) amongst the Gephyrea. Although there is no doubt a striking resemblance between the tentacular disc of a larval Brachiopod and the lophophore of a Polyzoon, which has been pointed out by Lankester, Morse, Brooks, etc., their homology is rendered, to my mind, very doubtful (1) by the fact that the lophophore is præ-oral in Polyzoa[133] and post-oral in Brachiopoda; and (2) by the fact that the concave side of the lophophore is turned in nearly opposite directions in the two forms. In Brachiopods it is turned dorsalwards, and in phylactolæmatous Polyzoa ventralwards. [133] For the ectoproctous Polyzoa it might be held that the ciliated ring of tentacles is post-oral, but the facts of development recorded in the previous chapter appear to me to shew that this view is untenable. The view of Morse, that the Brachiopoda are degraded tubicolous Chætopods, is not so far supported by any definite embryological facts. The development of the tentacular ring as well as its innervation from the sub-oesophageal ganglion prohibit us, as has been pointed out by Gegenbaur, from comparing it with the tentacles of tubicolous Chætopoda. BIBLIOGRAPHY. (325) W. K. Brooks. "Development of Lingula." _Chesapeake Zoological Laboratory, Scientific Results of the Session of 1878._ Baltimore, J. Murphy and Co. (326) A. Kowalevsky. "Development of the Brachiopoda." _Protocol of the First Session of the United Sections of Anatomy, Physiology, and Comparative Anatomy at the Meeting of Russian Naturalists in Kasan_, 1873. (Russian.) (327) H. Lacaze Duthiers. "Histoire de la Thécidie." _Ann. Scien. Nat. etc._ Ser. 4, Vol. XV. 1861. (328) Morse. "On the Early Stages of Terebratulina septentrionalis." _Mem. Boston Soc. Nat. History_, Vol. II. 1869, also _Ann. & Mag. of Nat. Hist._, Series 4, Vol. VIII. 1871. (329) ---- "On the Embryology of Terebratulina." _Mem. Boston Soc. Nat. History_, Vol. III., 1873. (330) ---- "On the Systematic Position of the Brachiopoda." _Proceedings of the Boston Soc. of Nat. Hist._, 1873. (331) Fritz Müller. "Beschreibung einer Brachiopoden Larve." Müller's _Archiv_, 1860. CHAPTER XII. CHÆTOPODA[134]. [134] The following classification of the Chætopoda is adopted in the present section: I. Achæta. (_Polygordius_). II. Polychæta. { Sedentaria. { Errantia. III. Oligochæta. _Formation of the Germinal Layers._