﻿T. Davidson — WJiat is a Brachiopod? 263 



affinities of the Brachiopoda, or the exact position the group should 

 occupy in the animal kingdom. The Tnvertehrata have been grouped 

 into five sub-kingdoms, namely, the Protozoa, Coelenterata, Annuloida, 

 Annulosa, and Mollusca, and for many years the Brachiopoda 

 have been considered to constitute a separate class in the sub- 

 kingdom Mollusca, a view still maintained by some distinguished 

 naturalists. Milne-Edwards, some years iDack, sep)arated the 

 Mollusca into two divisions, Mollusca and Molluscoida, and into the 

 last division he placed the Brachiopoda, Polyzoa, and Tunicata, an 

 arrangement that has been followed by many naturalists. Although 

 the greater number of zoologists have admitted the close connexion 

 existing between the Polyzoa and Brachiopoda, some considerable 

 doubt has been expressed with respect to the affinities and position 

 of the latter to the Tunicata. Moreover, a strenuous ^,'ffort has been 

 made by such excellent observers as Steenstrup, Morse, Kowalevsky, 

 and Alex. Agassiz to demonstrate that the affinities of the Brachio- 

 poda and Polyzoa are with the worms, and that they should form a 

 division, or two divisions, of the Annulosa, and be placed close to 

 the Annelids. 



In his review of Kowalevaky's admirable memoir on the embryo- 

 logy of Argiope, Thecidium, and Terehratula, Alex. Agassiz observes : 

 " The close relationship between the Brachiopoda and Bryozoa 

 (Polyozoa) cannot be more full}' demonstrated than by the beautiful 

 drawings upon plate v. of Kowalevsky's history of Thecidium. We 

 shall now have at least a rational explanation of the homologies of 

 Brachiopods ; and the transition between such types as Pedicellina to 

 Memhranipora and other incrusting Bryozoa is readily explained from 

 the embryology of Thecidium. In fact, all incrusting Bryozoa are 

 only communities of Brachiopods, the valves of which are continuous 

 and soldered together, the flat valve forming a united floor, while the 

 convex valve does not cover the ventral A^alve, but leaves an open- 

 ing more or less ornamented for the extension of the Lophophore." 



With respect to the Timicata we are reminded by Morse, that 

 Kowalevsky, Kupffer, Schultze, and others, assign to them a position 

 at the base of the Vertebrate series, through the affinities of some of 

 their forms to Amphioxus, as well as their singular embryological 

 relations to the Vertebrates. G-ratiolet states likewise that the Tuni- 

 cata are in no way related to the Brachiopoda, and Hancock, in 1870, 

 expressed himself to me as follows, " Of late years I have gradually 

 inclined to the opinion that the Brachiopoda are not so closely re- 

 lated to the Tunicata as we at one time thought. I am now busily 

 engaged in working out the Tunicata, and they seem to be very in- 

 timately connected with the Lamellibranchs. I am disposed to 

 consider that there is a considerable hiatus dividing the Tunicata 

 from the Brachiopoda and the Polyzoa or Bryozoa, and that these 

 two latter groups alone should be included in the Molluscoida. If 

 therefore Morse can establish his doctrine, it will relieve me of some 

 little difficulty, inasmuch, as it will militate against Huxley's view 

 that the branchial sac of the Ascidian is the homologue of the 

 pharynx of the Polyzoa. My idea being that the branchial sac is the 



