XVII THE SOFT PARTS 469 



melen ^ was able to show that the spaces between the tubules 

 did not increase with asre. He therefore reasoned that the shells 

 of Brachiopoda do not increase by intussusception, and that 

 their increase in size must be entirely due to additions made 

 round their free edge. 



The function of the tubules has been a matter of some dis- 

 cussion. They have been regarded as respiratory organs, but it 

 would seem more reasonable to suppose that they serve as organs 

 to supply nourishment, etc., to the organic matrix of the shell. ^ 



With the exception of the genus Crania^ it is usual for 

 Brachiopods to bear round the edge of their mantle rows or 

 bundles of chitinous setae or bristles (Figs. 315 and 319). The 

 length and arrangement of these structures vary in the different 

 species ; they are secreted from little pits in the edge of the 

 mantle. It seems probable that they^ serve to some extent as 

 organs of defence, especially in the larva, Avhere they make their 

 appearance at an early stage ; possibly they also serve as a 

 filter, and prevent the entrance of foreign bodies into the shell. 

 Their presence has been taken to indicate a certain degree of 

 affinity between Brachiopods and Chaetopods, since setae are 

 very characteristic of the last-named group. 



The Body 



The shell of a Brachiopod is secreted partly by the general 

 surface of the body which is situated at the hinder end of the 

 shell, and partly by the two leaf-like extensions of the body, 

 which are termed the dorsal and ventral mantles. These are, 

 in fact, folds of the body wall, and into them the body cavity 

 and certain of its contents, such as the liver and generative 

 glands, etc., extend. The space between the two folds of the 

 mantle, which is limited behind by the anterior wall of the body, 

 IS termed the pallial or mantle cavity. On each side of the 

 middle line the anterior wall of the body is produced into two 

 "arms," which occupy as a rule a considerable part of the 

 mantle cavity. These arms may be but flattened portions of 

 the general body wall, which occupies a large part of what in 



1 Loc. cit. p. 465. 



2 Shipley, "On the Structure and Development of Argiope," Mitt, aiis d. Zool. 

 Stat, zu Neap. Bd. iv. 1883. 



