BRACHIOPODA. 367 



of the circulating fluids, since the cilia which fringe the margin of 

 the central stem (fig. 173, k, k) present, under the microscope, a 

 horny texture, instead of being of a vascular character, and the 

 muscular stem itself contains no blood-vessels of sufficient size to 

 indicate that the brachia are at all efficient as respiratory organs. 



The course of the circulation has not been actually demonstrated, 

 but from analogy there is no room to doubt that the two hearts 

 are systemic, receiving the purified blood from the lobes of the 

 mantle, and distributing it through the body. 



The nervous system of the Brachiopoda is but imperfectly 

 known. Cuvier conceived the brain of Lingula to be represented by 

 some small ganglia visible near the mouth (Jig. 170, a), but was 

 unable to follow the nerves ; and Professor Owen, in dissecting Or- 

 bicula, detected two small ganglia on each side of the oesophagus. 



(401.) The muscular system in the class before us differs very 

 materially from that exhibited by any other bivalve Mollusca. 



In Terebratulct) two pairs of muscles arise from each valve :* 

 those of the imperforate valve arise at a distance from each other ; 

 the anterior pair (Jig. 173, /*, f) come off fleshy just behind the 

 middle of the valve (fig- 171, B, g, g) ; they soon diminish to 

 thin shining tendons, which converge and unite below the stomach ; 

 they then again separate, and pass through the foramen of the per- 

 forate valve to be inserted into the pedicle. 



The posterior pair are very short, and wholly carneous : they 

 arise from the lateral depressions in the base of the central portion 

 of the hinge (Jig- 171, B, A), and are inserted into the pedicle. 



The muscles of the perforated valve arise close together, so as 

 to leave only a single muscular impression on each side (fig. 

 171, A, c) ; the anterior pair soon diminish to slender tendons, 

 and are inserted into the base of the imperforate valve ; the pos- 

 terior pass exclusively into the pedicle. 



The pedicle itself consists of a peculiar tendinous-looking struc- 

 ture, enveloped in a tubular prolongation derived from the mantle. 



Little is known concerning the reproduction of the Brachiopoda. 

 The ova, when present, have invariably been found lodged be- 

 tween the layers of the two lobes of the mantle ; a position analo- 

 gous to that in which we have already seen them deposited in the 

 Cirripeds ( 395) preparatory to their expulsion. No internal 

 generative system has as yet been detected ; but, notwithstanding 

 this, we are by no means prepared to assume, as some writers do, 



* Professor Owen, loc. cit. 



