INTRODUCTION 



CHAPTER I. 



ON THE ANATOMY OF THE TEREBRATULA. 

 By professor OWEN, F.R.S., F.G.S., &c. 



With the view of extending the knowledge of the organisation of the Brachiopoda, 

 the unfolding of which Pallas and Cuvier had so well begun — the one in the description of 

 his so-called Anomia,^ the other by his admirable anatomy of the Lingula anatina^ — I 

 communicated, in 1833, an account of my dissections of some species of Terehratula and 

 Orbicida to the Zoological Society of London,^ and have since availed myself of every 

 opportunity of completing the anatomy of this very beautiful and singular class of 

 Mollusks. 



Cuvier had shown, in Lingida, a condition of the respiratory organ, which might be 

 paralleled with one of the transitory states of that organ in the Lamellibranchs, that, viz. 

 in which the rudimental gills appear as processes from the inner surface of the pallial 

 lobes, and in which the distinction, whether morphological or physiological, of the gills and 

 mantle is not fully established : the modifications of the breathing organ in both Terehratula 

 and Orbicula exhibited a more interesting condition, comparable to a still earlier stage of 

 the respiratory system in the embryo Lamellibranch, that, viz. in which the vessels of the 

 pallial lobes have not begun to bud out in parallel rows of vascular loops, — the fii'st stage 

 in the formation of gills, and the one at which it is arrested in the Lingula. Notwithstand- 

 ing, therefore, the manifestation of many beautiful modifications of structure, which seemed 

 to render the organisation of the Brachiopod more complex than that of the ordinary 

 Bivalve, I was led, in 1833, to regard the latter as standing higher in the Acephalous 

 series, and to place the Brachiopoda between the Lamellihrancldata and the Tunicata : 

 subsequent investigations, especially those recorded in the present Memoir, have confirmed 

 me in that view. I am the more desirous to repeat these convictions in reference to the 



1 Miscellanea Zoologica, 4to, 1775, p. 182, {Anomiarum Biga.) 



2 Annales du Museum d'llist. Nat., 4to, torn, i, p. 69 (1802). M^moires pour servir a I'Histoire et 

 I'Anatomie des Mollusques, 4to, 1817. 



2 Transactions of the Zoological Society of London, 4to, vol. i, p. 145 (1835). 



