182 



BracMopoda. 



and that the presence of the shells does not denote a relationship is 

 evident from the circumstance that those of the Brachiopoda are 

 dorsal and ventral, whilst in the Lamellibranchs they are right and 

 left. 



d h 



rig. 143. Diagrammaiio longitudinal section of a Brachiopod. d digestive tract, 

 e excretory organ, h heart, n nerve ganglion, o mouth, s shell, vfith the mantle lying within 

 it, st peduncle, t tentacle. — Orig. 



Compared with the whole extent of the animal, the actual 

 body is of very small size, and very short. Two large mantle- 

 f olds, lining the inside of the shell, spring from it. The shells 

 are secreted by the mantle, and are to be regarded as cuticular 

 structures. Unlike the Lamellibranch valves they are not connected 

 by a ligament ; but in some forms they are attached by a hinge 

 posteriorly. Chitinous bristles, implanted in pits in the skin 

 are often present along the edge of the mantle. From the posterior 

 end of the body there usually springs a process, the peduncle, 

 which projects from between the valves, or from a hole in the hinder 

 part of the dorsal shell : in some species it is longer than the rest 

 of the body, in others it is very short. Most of these animals are 

 fixed to foreign objects by means, of the peduncle, but some are free. 

 In young Brachiopods a circle of tentacles surrounds the 

 mouth, but during development, an in-pushing of the wreath occurs, 

 which results in its becoming kidney- or horseshoe-shaped and 

 gradually both branches of the horseshoe are drawn out into long 

 arms beset with a double row of tentacles : the arms are usually 

 spirally coiled, and lie between the mantle-lobes j they serve as a 

 respiratory organ, and also waft food (minute organisms) into the 

 mouth with their cilia ; frequently they are supported internally by a 

 variously shaped {e.g., ribbon-like) calcareous structure, which is 

 connected with the dorsal valve. The alimentary canal may 

 be short or long; curiously enough, in most Brachiopods an anus is 

 wanting, when present it is on the right side of the body. There is 

 a well-developed liver. The central nervous system is repre- 

 sented by a nerve-collar surrounding the oesophagus, swelling out on 



