1883.] in the anatomy of Brachiopods. 335 



all Brachiopods by far the most important concentration of 

 nervous elements is below the mouth. To what extent the 

 smaller Brachiopods possess a continuous nervous sheath repre- 

 senting the ancestral condition, I have not been able to determine. 

 It is possible that they are descended from larger species, and 

 really further removed from the ancestor than the latter. If this 

 be so, they present analogy to Polyzoa. The variation in the position 

 of the concentrations of the nervous system already mentioned, 

 proves that the ancestor of the present Brachiopods possessed 

 a continuous ectodermic nervous sheath. It is possible that 

 this ancestor descended from forms with the adult nervous system 

 inside the mesoblast. 



The fact that no evidence for this exists does not render 

 it improbable. If any organ is arrested in its development, 

 and this stage persists throughout life, there can be no ortogenetic 

 evidence of its more highly specialised ancestor. 



If the larvse of Brachiopoda possess a ganglion in the pra?oral 

 lobe, it is probable that this, as in Phoronis, disappears at the close 

 of free swimming life. 



I have this opinion on the fact, that no trace of a structure 

 resembling the brain of Chsetopods and Molluscs is present in adult 

 Brachiopoda. 



Body cavity and Mesenteries. 



The development of the mesoblast in Brachiopoda has un- 

 fortunately not been traced. A knowledge of the fate of the 

 archenteric diverticula of Argiope would probably settle many 

 problems. I therefore put forward the following interpretation 

 only tentatively. 



In Phoronis there is a ventral mesentery, attaching the gut to 

 body wall. The dorsal mesentery breaks clown as soon as it 

 is formed. This is probably secondarily induced by the adult 

 approximation of mouth and anus. 



In Brachiopoda there is a mesentery with dorsal relations. Its 

 existence may be due in part to the dorsal surface not being 

 so seduced. In crania the anus is almost terminal. 



The gastro- and ileo-parietal bands of Brachiopods have been 

 regarded as transverse septa, dividing the body into three segments. 

 They do not however divide the body cavity in the same way as 

 the septum cutting off the cavities in the arms from the throat in 

 the trunk does. They are lateral mesenteries passing from the 

 side of the first and second stomach to the walls of the body. 1 

 would suggest that they are parts of the same pair of lateral 

 mesenteries homologous with that of Phoronis. Their adult 

 position in two planes almost parallel to each other is explained 

 by the bending of the gut upon itself. 



