CHAPTER XV 



PHYLUM BRACHIOPODA 



BRACHIOPODS (Gr. /fyax&ov, the arm; TTOVS, 770805, the foot) are 

 true Coelomata and retain the coelom in a primitive and typical 

 condition. Like the Mollusca, they are not segmented, and the 

 only trace of a repetition of parts in the group is in a genus called 

 Rhynchonella in which the excretory organs are repeated, so that we 

 find two pairs. A similar repetition of the same organs occurs 

 amongst the Mollusca in Nautilus. 



Brachiopods are exclusively marine. They have a shell con- 

 sisting of two valves, so that at first sight they appear 



features** 1 * resem l e the Pelecypoda, but in Brachiopods the 

 shells are placed ventrally and dorsally, and not on 

 the two sides of the animal as in Pelecypoda. In a few cases such 

 as that of the primitive genus Lingula the two valves of the shell 

 are nearly alike in size and shape and consist largely of horny matter 

 or chitin. In most -cases however the shell is calcareous, and since 

 in Brachiopoda, as in most bilaterally symmetrical animals, the two 

 sides resemble one another whilst the back and front are unlike, 

 each valve of the shell is symmetrically shaped, but the dorsal valve 

 differs from the ventral, the latter being usually the larger. In a 

 few cases, such as that of Crania, a British form common in certain 

 localities, the ventral valve is flat and attached by its whole surface 

 to the substratum ; all that is then seen is the arched dorsal valve. 

 Since in the overwhelming majority of Pelecypoda the two valves of 

 the shell are similar in appearance, while each is asymmetrical in 

 shape, the umbo being situated near the anterior end, it is easy to 

 distinguish at a glance the shells of the Pelecypoda from those of 

 the Brachiopoda (Fig. 173). 



The posterior end of the body terminates in a stalk which in 

 Lingula helps to keep the animal in the holes of the sand in which 

 it lives. In other forms the stalk is shorter and it is firmly glued 



