300 MOLLUSCOIDEA SUB-KINGDOM V 
the brachial supports, or brachidia, 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 adjustors, 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 dorsal valve, and leave two elongate scars lying immediately to the right and 
left of the median line, 
K enclosed between the 
diductors (Fig. 498, B, a). 
These muscles extend 
almost directly from one 
valve to the other, and 
-p 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- 
Fic, 498. tors, or opening muscles, 

(inn 

Magellania flavescens, 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 vis- 
Cardinal process; x, Hinge-plate; z, Hinge-teeth; «a, a’, Impressions of = ; 
adduetors ; d, d', Diductors: 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 
