THE BRACHIOPODA 



GENERAL CHARACTERS OF THE CLASS. 



The Brachiopoda are marine animals inclosed in bivalved, 

 equilateral and inequivalved shells. 



As a group they attained a most prolific and multiform devel- 

 opment during the Palaeozoic and Mesozoic eras in the history of 

 the earth, while their existing representatives are comparatively 

 few in number. A knowledge of their variations in form, struct- 

 ure and in time is, therefore, of highest importance to the student 

 of biology and geology. 



In brief, these animals are characterized by the unequal devel- 

 opment of the two lobes of the cuticular integument {mantle), 

 which results in the unequal size of the valves. The mouth is 

 situated subcentrally or posteriorly and is surrounded by elon- 

 gated, usually spiral cirrated expansions, which occupy the ante- 

 rior portion of the valves and serve as conveyancers of food-bearing 

 water-currents ; in part, also, subserving the function of respira- 

 tion. There is a simple oesophageal nerve-ring with an inferior 

 or sub-cesophageal ganglion, and various small accessory nerve- 

 knots. The sexes are distinct in all species in which these 

 characters have been studied. 



Habits of Life. From our knowledge of existing species it is 

 believed that all Brachiopoda, at some period of their existence, 

 are attached to extraneous objects. To this statement, there is 

 but one known exception among living forms, namely, the lin- 

 guloid, Glottidia Audebardi. In the fossil (Palaeozoic) genera 

 Pholidops and Pseudocrania there is no evidence of attachment 

 of any kind, either in the young condition (Pholidops), or in the 

 mature and senile stages of growth. 



Fixation is usually effected by means of a fleshy arm or pedicle 

 of greater or less length, protruded through an aperture at, or near 



7 



