BEACHIOPODA. 509 



merits particular description. Above each external lamina of the gills 

 is a duct proceeding from the posterior part of the foot towards the anal 

 tube, long ago described as an oviduct by Oken, and having, on its 

 lower surface, a long row of openings placed transversely, and forming 

 the entrances to the cells or compartments of the gills themselves. 

 These compartments are all arranged vertically in the gill, and, separated 

 from each other by partitions, they appear as though they originated 

 from the mutual recession of the two membranous surfaces of the gill, 

 which remain connected only by the vertically-disposed vessels that give 

 rise to the septa ; they serve for the reception of the ova, which, coming 

 from the ovary placed within the foot, and not by any means in the gill 

 itself, are, however, lodged there, and there receive their further de- 

 velopment as in a uterus. 



CHAPTER XXI. 



BEACHIOPODA* (Cuvier). 



PALLIOBRANCHIATAf (Owen). 



(1338.) THE next class of MOLLUSCA which presents itself for our con- 

 sideration was named by Cuvier on account of the remarkable character 

 of the organs by means of which the animals composing it procure the 

 food destined for their support. These instruments consist of two long 

 spiral arms placed on each side of the mouth, that in many species can 

 be unrolled to a considerable length, and protruded to some distance in 

 search of aliment. The above character, however, taken by itself, 

 would scarcely warrant us in considering the creatures before us as 

 forming a separate class of Mollusca ; but when, in addition to this 

 remarkable feature in their organization, we find that they possess a 

 respiratory apparatus peculiar to themselves, and differ widely from all 

 other bivalves in almost every part of their structure, we feel little 

 hesitation in continuing to regard them as distinct, and devoting the 

 present chapter to an investigation of their anatomy. 



(1339.) The BKACHIOPODA inhabit bivalve shells, and for the most 

 part are suspended by a fleshy tubular pedicle, resembling that of the 

 Cirrhopods, to various submarine bodies. Such, at least, is the case in 

 Lingula and Terebratula ; but in the third genus belonging to this class, 



* (Spaj^iwv, an arm ; TTOVS, TTO^OS, a foot. 



f pallium, a mantle ; branchite, gills. This name, originally proposed by M. de 

 Blainville notwithstanding his belief that the spiral arms were the organs of respi- 

 ration, has since been proved by the researches of Professor Owen to be strictly appro- 

 priate to the class. 



