DEVELOPMENT OF THE BRACHIOPODA 275 



Thus the cirri farthest removed from the median lobe are the 

 oldest. Tentacles are added rapidly until the first arc is ex- 

 tended to a semi-circle, and then progressively the whole disk 

 becomes surrounded by a circle of these organs. The further 

 introduction of cirri can take place only by the enlargement 

 of the oral disk or through the deformation of the circle by 

 lobes, loops, or extensions. In &lottidia, Lingula, Discinisca, 

 Crania, and Rhynchonella the two points of tentacular in- 

 crease, originally together and on opposite sides of a median 

 lobe, or tentacle, gradually separate, and the further multi- 

 plication of tentacles results in strap-shaped extensions on 

 each side, which finally assume a coiled form, due to the 

 limited space in which they grow. Therefore the arms 

 in adult individuals of these genera have a single cirrated 

 edge, extending from their free extremities to the sides of 

 the oral disk, and, continuing posteriorly, unite on the ven- 

 tral side of the disk behind the mouth. Each cirrated edge 

 in the adult lophophore apparently has two approximate rows 

 of alternating cirri (Hancock 9 ,), but as they were originally 

 a single row in early stages, this appearance is evidently 

 the result of a crowding of the cirri or a crumpling of the 

 edge. 



Kovalevski 10 has shown that in Cistella the tentacles also 

 originate in pairs on each side of the dorso-median line, with- 

 out a central tentacle or lobe. The same mode of increase 

 has been shown by the writer 2 to be present in Magellania 

 and Terebratalia. In young stages of Cistella, Terebratulina, 

 Magellania, and other terebratuloid genera, as well as in The- 

 cidea, after the circlet of tentacles is complete the two points at 

 which new ones are added do not separate, but remain close 

 together throughout the life of the animal. In this case the 

 cirrated margin is lengthened by means of lobation and loop- 

 ing, and often by the final growth of a single, median, coiled 

 arm, cirrated on both margins. G-wynia illustrates the com- 

 pleted circle of tentacles about the mouth. Adult Cistella 

 shows an advance in having the anterior margin of the lopho- 

 phore introverted, making it bilobed. Megathyris is slightly 



