ON THE DIPLOOHORDA. 



409 



whole body- surface is divided into ti'opliophoral and hydro- 

 phoral areas, and the pinnsB or "arms" and processes may 

 be all, as I hope to show later, accounted for as contributing 

 to the efficiency of filtration. In Actinotrocha the line of 

 contact between the two areas is at first multiplied by a 

 simple row of tentacles, throughout the length of which the 

 line of contact passes (cf. figs. 59 — 62). 



After fixation the young Phoronis is provided in like 

 manner with a simple row of tentacles round the oral aper- 

 ture, and the subsequent history of the Phoronidea may be 

 traced as a successful attempt to increase the efficiency of 

 filtration by a gradual multiplication of the number of ten- 

 tacles. 



The known species may be placed in order from Cori's (4) 

 table : 



From this it will be seen that the increase in size corre- 

 sponds very closely with the number of the tentacles and 

 their thickness. Probably the comparative length of the 

 tentacles follows the same rule. As the tentacles seem 

 to serve the ingestion of food, the increase of ciliated sur- 

 face on the one hand, by increase in number, length, and 

 thickness of the tentacles, and the increase in bulk of the 

 body, on the other hand, are probably directly connected 

 with each other. 



The tentacles in transverse section show that their inner 

 or oral surface is covered with trophophoral epithelium and 

 their outer surface with hydrophoral, and bear a remarkable 

 structural resemblance to the ctenidia of Lamellibran- 



