THE SOFT PARTS 469 



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

 did not increase with age. 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, where 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 Brachio- 

 pods 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 other genera is the 



' Loc. cit. p. 465. 



- Shipley, "On the Structure and Development of Argiope," Mitt, aus d. Zool. 

 Stat, zu Neap. Bd. iv. 1883. 



