4 INTRODUCTION. 



position and affinities of the acephalous Mollusks, because my statement as to the value of 

 the group Brachiopoda, in comparison with the group Lamellibranchiata^ has been misunder- 

 stood by some esteemed contemporaries on the continent. I believe the Brachiopoda and 

 Lamellibranckiata to have equal claims to be considered as distinct groups, whether called 

 classes, sub-classes, or orders of Acephala ; but, I regard the group Brachiopoda as 

 exemplifying a less advanced grade of organisation than the Lamellibranchiata, in which 

 the lowest forms, as, e. g. Anomia and Ostrea, show the nearest affinities to the 

 Brachiopoda. Those monomyary or unimuscular bivalves become fixed, like the Orhicula 

 and Terebratula, whilst the higher Lamellibranchs exhibit, with a progressive development 

 and increased complexity of the muscular system, powers of locomotion more or less active 

 and varied ; some, e. g, the Solens, burrowing ; others, e. g. the Cardiums, progressing by 

 short leaps, whence the name subsilentia, given to the group by Poli ; others, again, e. g. 

 the Pectens, are said to swim by violent flapping movements of their valves, whence 

 these light and richly-painted bivalves have been called " Sea-butterflies." 



The Brachiopoda, being deprived of the power of locomotion, have the development of 

 their respiratory system arrested at a corresponding low grade ; the Lingidce, which have 

 the largest and most flexible peduncle, being the only forms that show distinct rudiments 

 of gills. In the Terebratulm, the mantle-lobes, besides their ordinary office, perform by 

 their rich vascularity the breathing function. The admirable microscopical researches of 

 Dr. Carpenter and Prof. Quekett have demonstrated that close and intimate adhesion of 

 the mantle to the shell, which was noticed in my earlier dissections of the Terebratula^ 

 to be due to the penetration of the pores of the shell by minute tubular membranous 

 processes, which they believe to be glandular caeca.^ These processes may perform an 

 excretory function, and be associated in that action with the depurative respiratory office 

 of the mantle, the probable condition of their development being the low grade of the 

 proper branchial organisation, or they may take some share in the formation of the shell 

 itself, necessitated by the modification of the mantle to subserve respiration. 



^ " M. Owen a ete le premier a faire remarquer que les Brachiopodes ne doivent pas etre envisages 

 corame une classe a part, mais qu'ils peuvent etre convenablement ranges sur la meme ligne que les 

 Monomyaires et les Dimyaires." (Agassiz, in 'Poissons Fossiles' of the "Old Red," 4to, 1845, p. I.t.) 

 Compare, however, 'Zool. Trans.,' vol. i, 1835, p. 159 : — 



" In all the essential points, the Brachiopoda closely correspond with the Acephalous Mollusca, and I 

 consider them as being intermediate to the Lamellibranchiate and Tunicate orders ; not, however, possess- 

 ing, so far as they are at present known, distinctive cnaracters of sufficient importance to justify their being 

 regarded as a distinct class of Mollusks, but forming a separate group of equal value with the Lamelli- 

 branchiata.^^ 



2 Trans. Zool. Soc, vol. i, p. 147. 



3 Carpenter, Report on the Microscopic Structure of Shells, Part II, Trans. Brit. Association, 1847, p. 93. 

 " In these tubes, as will hereafter be shown, certain csecal appendages of animal membrane are situated." — 

 Quekett, 'Histological Catalogue,' vol. i, 1850, p. 270. In the same work it is shown that "each 

 perforation has a series of radiating lines or tubes on its outer margin," (p. 270). The corresponding 

 parts of the membranous tubes would resemble a terminal brush of vibratile cilia. 



