BRACHIOPODA. 5 



Mr. Hancock is of opinion that "the brachial organs 

 subserve the function of gills, as well as that of respira- 

 tion." As far as I could judge from the examination of 

 living specimens of T. caput-serpentis, the inner folds of 

 the mantle appeared to have the same action as that of 

 the gills in many Lamellibranch Mollusca. The name 

 of Palliobranchiata was given to the class now under con- 

 sideration in consequence of a belief that their respiratory 

 system was dependent on the mantle. Mr. Macdonald 

 has shown, in the f Linnean Transactions ' (xxiii. p. 375), 

 that the pallial sinuses serve as organs of circulation. 

 Some of this class have calcareous spicula or plates in 

 the mantle, as well as in the arms and cirri. In Tere- 

 bratula caput-serpentis these spicula are very numerous, 

 large, and often branched like the antlers of a deer ; and 

 they form an extensive though incomplete network. 

 Their use is doubtless the same as that of the spicula in 

 sponges, viz. to strengthen and support the tissues of 

 the animal, and especially to protect the delicate canals 

 of the mantle from the pressure of the external fluid. 

 Under a microscope with polarized light these spicula 

 are remarkably beautiful objects. The pallial tentacles 

 resemble those ofAnomia. When the animal is dead and 

 dried up, they are stiff from contraction, and of a horny 

 texture, for which reason they have been called setae or 

 bristles. The muscular system is well developed, and 

 admirably adapted to the complicated machinery by which 

 the animal opens, closes, and moves the valves of its shell. 

 Dr. Carpenter was the first to point out and explain a 

 very singular apparatus of canals or cylindrical holes 

 observable in the shells of Terebratula and other allied 

 genera, which are occupied by tubular appendages of 

 the mantle and closed on the outside. These processes 

 penetrate every part of the shell, but their function has 



