364 



MOLLUSCOIDEA 



PHYLUM V 



the brachial supports, or brachidia, described above. There are no special 

 respiratory organs, the blood being oxygenated in the inner surface of the 



I, a mantle and in the spiral 



Fig. 533. 



Liothyrina vitrea 

 (Linnaeus). Recent. 

 Fleshy brachia simply 

 recurved. 



: Fig. 534.1, 

 Magellania flavescens (Val.). Median vertical 

 section, slightly enlarged, d, Spiral brachia ; h, 

 Fringed brachial margin ; pr, Cardinal process ; 

 -, Alimentary canal ; v, Mouth ; ,f.s, Septum ; o, 

 Adductors ; c, c', Diductors (after Davidson). 



arms, where it is 

 brought into close os- 

 motic relation with the 

 water. 



Muscular system. — 

 By means of muscles 

 Brachiopods are en- 

 abled to open and close 

 their valves, and to a 

 limited extent can pro- 

 trude and retract the 

 pedicle. In the articu- 

 late forms (Protremata 

 and Telotremata) there are three sets of muscles — namely, the diductors, 

 which by contraction open the valves ; the adductors, which by contraction 

 close the valves ; and the pedicle muscles, or adjustors, which also by contrac- 

 tion withdraw the pedicle. The points of attachment of these muscles leave 

 more or less distinct impressions in the valves of both recent and fossil 

 Brachiopoda, and the subject is therefore worthy of careful examination. 



The adductors, or closing muscles, are attached on either side of the median line 

 in the dorsal valve, and leave two elongate scars lying immediately to the right and 

 left of the median line, enclosed between the diductors (Fig. 535, L', ft). These 

 muscles e.xtend almost 

 directly from one vah'^e 

 to the other, and as each 

 muscular band is once 

 divided, their insertions 

 on the dorsal valve are 

 quadruple. Their im- 

 pressions on this valve are 

 known as the anterior and 

 posterior adductors (Fig. 

 535, A, B, a, a'). 



The principal diduc- 

 tors, or opening muscles, 

 originate on the ventral 

 valve at the anterior edge 

 of the visceral area, and 

 on either side of the 



lnAr^i^n linp • tlip ^r-iTs; of ^^ooP- P''' Cardinal process ; x, Hinge-plate; z, Hinge-teeth ; a, «', Ini- 

 meaicUi niic , Lue ^uub ui pr^,ssions of adductors ; d, rf', Diductors ; p, p', Pedicle muscles. 



these muscles being usually 



the largest and deepest of any in the animal. They taper rapidly in crossing the 

 interior cavity, and their small extremities are attached to the anterior portion of 

 the cardinal process of the dorsal valve. There are also inserted on the cardinal 

 process, behind the principal diductors, two much smaller muscular bands, which 

 are called tlu; accessory diductors. Their attachment on the ventral valve is repre- 

 sented by two feelile scars in the posterior part of the muscular region, but these are 

 rarely observable in fossil specimens (Fig. 535, A, B, d). 



Fig. 635. 



MaiieUania flaresrcns (\'al.). Recent ; Australia (after Davidson). 

 Dorsal valve. B, Ventral valve. D, Deltidial plates. F, Foramen. 



A, 

 S, 



