300 



MOLLUSCOIDEA 



SUB-KINGDOM V 



the brachial supports, or bra'chidia, described above. There are no special 

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

 mantle and in the spiral arms, where it is brought into close osmotic relation 

 with the water. 



Muscular system. By means of muscles Brachiopods are enabled to open 

 and close their valves, and to a limited extent can protrude and retract the 

 pedicle. In the articulate 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 adjusters, which also by contraction 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 adductor, or closing muscles are attached on either side of the median line 

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



left of the median line, 

 enclosed between the 

 diductors (Fig. 498,5, a). 

 These muscles extend 

 almost directly from one 

 valve to the other, and 

 as each muscular band is 

 once divided, their inser- 

 tions on the dorsal valve 

 are quadruple. Their 

 impressions on this valve 

 are known as the anterior 

 and posterior adductors 

 (Figs. 498, A, B, a, a'}.. 

 The principal diduc- 

 tors, or opening muscles, 



Magellaniajlavescens, Val. Recent ; Australia (after Davidson). A, Dorsal originate at the anterior 

 valve. B, Ventral valve. D, Deltidial plates. F, Foramen. S, Loop, pr, ventral edge of the V1S- 

 Cardinal process; x, Hinge-plate; z, Hinge-teeth; a, a', Impressions of , -, .,-, 



adductors ; a, d', Didnctors : p, P ', Pedicle muscles. ceral area, and on either 



side of the median line ; 



the scars of 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. There are also inserted on 

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

 which are called the accessory diductors. Their attachment on the ventral valve is 

 represented by two feeble scars in the posterior part of the muscular region, but these 

 are rarely observable in fossil specimens (Fig. 498, A, B, d). 



When a functional pedicle is present, there are found in addition to the valvular 

 muscles two pairs (one to each valve), and a single unpaired muscle ; these are 

 attached to the pedicle, and are called the pedicle muscles (Fig. 498, B, p,p"). The pair 

 in the ventral valve originates immediately outside of and posterior to the adductors 

 and diductors ; the pair in the dorsal valve is attached behind the posterior adductors ; 

 and the unpaired muscle lies at the base of the pedicle in the ventral valve. Only 

 the latter unpaired band, as a rule, leaves a perceptible scar in fossil specimens. 



The entire muscular system in the Protremata and Telotremata works with the 

 utmost precision. The cardinal process is received between the teeth of the ventral 

 valve in such a manner as to allow the dorsal valve to swing freely in the median 



FIG. 493. 



