STRUCTURE OF BRACHIOPODS. 147 
composed of carbonate (Terebratulina) or largely (Lingula, 
Fig. 103) of phosphate of lime. It is really the thickened 
integument of the animal, the so-called mantle being the 
inner portion of the skin, containing minute tubular canals 
which do not open externally. 
The body of Brachiopods is divided into two parts, the 
anterior or thoracic, comprising the main body-cavity in 
which the arms and viscera are contained, and the caudal 
portion, i.e. the peduncle. The part of the body in which 
the viscera lodge is rather small in proportion to the entire 
animal, the interior of the shell being lined with two broad 
lobes, the free edges of which are thickened and bear sete, 
as seen distinctly in Linguwia. The body-cavity is closed 
anteriorly by a membrane which separates it from the space 
in which the arms are coiled up The ‘‘pallial cham- 
ber” is situated between the two lobes of the mantle (pal- 
dium) and in front of the membrane forming the anterior 
wall of the body-cavity. In the middle of this pallial 
chamber the mouth opens, bounded on each side by the 
base of the arms. The latter arise from a cartilaginous 
base, and bear ciliated tentacles, much as in the worm Sa- 
bella. In Lingula, Discina, and Rhynchonella, they are de- 
veloped, as stated by Morse, in a closely-wound spiral, as in 
the genuine worms (Amphitrite). In Lingula the arms can 
be partially unwound, while in Rhynchonella they can not 
only be unwound but protruded from the pallial chamber. 
In many recent and fossil forms the arms are supported by 
loop-like solid processes of the dorsal valve of the shell, but 
when these processes are present the arms cannot be pro- 
truded beyond the shell. The tentacles or cirri on the arms 
are used to convey to the mouth particles of food, and they 
also are respiratory in function, there being a rapid circula- 
tion of blood in each tentacle, which is hollow, communi- 
cating with the blood-sinus or hollow in each arm, the sinus 
ending in a sac on each side of the mouth. 
The digestive system consists of a mouth, cesophagus, 
stomach, with a liver-mass on each side, and an intestine. 
Fig. 98 shows the relation of the mouth and digestive canal 
to the head and arms, as seen in a longitudinal section of 
