MOLLUSCOIDA BRACHIOPODA. Ill 



siphons exposed at the surface. The siphons are some- 

 times separated from one another; in other instances they 

 are united along one side, forming a compound tube. The 

 animal has generally the power of withdrawing the si- 

 phons within the shell. The muscle by which this is 

 effected is termed the retractor muscle of the siphons. 

 The indentation produced in the pallial line by the action 

 of this muscle has already been described. 



154. The Nervous System consists of the three normal 

 pairs of ganglia the cerebral pair, placed near the 

 mouth ; the pedal pair in the foot ; the parieto-splanchnic 

 pair, which supplies filaments to the mantle, gills, and 

 other internal organs. These ganglia are connected by 

 nervous cords. 



Eyes are present in the form of minute coloured spots 

 which are " set at intervals like a row of sentinels," 

 round the borders of the mantle. 



Auditory sacs, containing otoliths, are placed near the 

 pedal ganglia. 



155. Development. The sexes are generally separate, 

 but are united, in some cases, in the same individual. 

 The ova are retained within the body of the parent until 

 they are hatched. When set free, they are able to swim, 

 being furnished with cilia. Thus, the fry or spat of the 

 oyster can move about, while the adult animal is sessile. 



SUB-KINGDOM V. Molluscoida. 

 CLASS 1. Brachiopoda. 



156. The Brachiopoda (Gr. brackia, arms ; poda, feet) 

 are so called, because they are provided with two long 

 arms which are coiled up within the shell. They are all 

 marine animals ; and, as most of them inhabit the deep 

 seas, they are less known than the shells of the true 

 molluscs are. They are often called lamp-shells, as 

 piany of them have a striking resemblance to an ancient 



