28 



HEMICHORDATA 



would indicate a near relationship between Cephalodiscus and 

 Fhoronis. 



Phoronis is a small tubicolous animal, of gregarious habits, 

 which has usually been regarded as related to the G-ephyrea. 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 

 Ceplialodiscus, but a more striking simi- 

 larity is asserted by Masterman to exist 

 between the latter and Actinotrocha, the 

 larval stage of Plwronis. 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 

 gill-slits. 



The remainder of the body of Actino- 

 trocha corresponds with the trunk of 

 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 rino- of 

 strong cilia or membranellae ^ which surrounds the anus. 



Important memoirs on the structure of Actinotrocha have 



1 Cf. p. 19. 



Fig. 13. — Pharonis hiiskii, 

 M'liitosli, Philippine 

 Islands, x about 2. 

 (After M'Intosli, from 

 Sliipley.) 



