TYPE ANNELIDA. 



249 



The only sense-organs whicli have been described are a pair 

 of ciliated depressions lying one on each side in the concavity 

 of the lophophore ; no definite statement can be made as to 

 their function. 



A single pair of nephridia is present, opening into the 

 posterior chamber of the coelom by funnel-like mouths, and ,to 

 the exterior on each side of the anus. They serve not only 

 for excretion, but also as ducts for the reproductive elements. 

 The various species of PTioronis, with the possible exception 

 of P. architecta, are hermaphrodite, the ova and spermatozoa 

 developing from cells of the peritoneum lying in the vicinity 

 of the pouched longitudinal blood-vessel. They are shed 

 from their place of formation into the coelomic cavity and 

 thence pass to the exterior through the nephridia. 



Fig. 113. — Metamorphosis of Actinotroaha (after Metschnikofp from Balfour). 

 in = invagination. 



Development of the PJioronidce. — In their development the 

 various species of Phoronis so far as known undergo a very 

 remarkable metamorphosis. The larva which develops from 

 the ovum is known as Actinotrocha (Fig. 113, A) and is a some- 

 what elongated structure possessing at the anterior end a 

 large hood which overhangs the mouth, its edge bearing 



