2 8 HEMICHORDATA 



would indicate a near relationship between Cephalodiscus and 

 Plwronis. 



Phoronis is a small tubicolous animal, of gregarious habits, 

 which has usually been regarded as related to the Gephyrea. Its 

 body ends in a plume of ciliated tentacles, which can be pro- 

 truded from its tube, and the anus is on the dorsal side, not 

 far from the mouth. In both these respects it agrees with 

 Ccphalodiscus, but a more striking simi- 

 larity is asserted by Masterman to exist 

 between the latter and Actinotrocha, the 

 larval stage of Phoronis. The prae-oral 

 ciliated hood (Fig. 14) of Actinotrocha is 

 regarded as the proboscis, and it contains 

 a median cavity, traversed, like that of 

 Balanoglossus, by muscular fibres. The 

 collar is the region between the con- 

 stricted neck and an oblique line, parallel 

 to and immediately behind the series of 

 tentacles, which thus belong to the collar. 

 This division has a collar-cavity which is 

 said to be distinct from the prae-oral 

 cavity, and is separated by a septum from 

 the posterior body - cavity. Its dorsal 

 epidermis contains the central nervous 

 system (n.s}, which is connected with 

 a system of nerves resembling those 

 of Balanoglossus. A median diverti- 

 culum of the alimentary canal of this 

 part may be compared with the noto- 

 chord of that animal, but there are no 



FIG. 13. Phoronis buskii, --i-i O i:f 

 M'liitosh, Philippine & U 



islands, x about 2. The remainder of the body of Actirio- 



( After M'Intosli, from -, > ,-, ,1 i f 



Shipley.) trocha corresponds with the trunk ot 



Balanoglossus. Its body-cavity is distinct 



from that of the collar, and is divided by a ventral mesentery, 

 though not by a dorsal mesentery. A noteworthy fact is that 

 both Actinotrocha and Tornaria swim by means of a ring of 

 strong cilia or membranellae l which surrounds the anus. 



Important memoirs on the structure of Actinotrocha have 



1 Cf. p. 19. 



